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914World.com _ 914World Garage _ Lifting rear of roof
Posted by: retrotech Jan 1 2006, 09:24 PM
I remember a thread a long time ago. Can some one tell me the benefits of raising the rear of the btoom. Do you think it would have any effect on air flow over engine lid, in a positive way?
Opinions?
Posted by: Aaron Cox Jan 1 2006, 09:33 PM
search this subject with john rogers......
he did this on his race car
Posted by: retrotech Jan 1 2006, 09:37 PM
Can't find it. Do you know the answer?
Posted by: Aaron Cox Jan 1 2006, 09:38 PM
| QUOTE (retrotech @ Jan 1 2006, 08:37 PM) |
| Can't find it. Do you know the answer? |
something about airflow....
not sure. PM him and advise him of this thread?
Posted by: john rogers Jan 1 2006, 10:12 PM
I just sent a long PM explaining why I ( and some others ) have dome it.
Posted by: Aaron Cox Jan 1 2006, 10:13 PM
| QUOTE (john rogers @ Jan 1 2006, 09:12 PM) |
| I just sent a long PM explaining why I ( and some others ) have dome it. |
mind posting it here john
Posted by: john rogers Jan 2 2006, 10:52 AM
Okay, here is what I sent in the PM. Has this been backed by scientific data, not yet as NASA and DEI have ignored my requests to use their wind tunnels but it seems to work.
Hi Stephen, I am the one that mentioned that. One of the reasons a 914 gets squirrely at high speed is the rear tends to lift, so the IMSA and POC and PCA cars run a big rear wing to eliminate the lift. The factory did not do that and with racing the vintage 2L series now days, a wing is not allowed either. So how do you cancel the lift, which is a large part due to the vacuum created behind the rear window, over the engine cover. At 7K RPM the fan is sucking a huge amount of air down through that opening being blown out the bottom and the air needs to come from some place, so many racers just cut the roof off and eliminated the rear window. I have seen several cars from the 70's that kept the roof but had it raised at the back edge and when I tried it I noted a large increase in the wind in my face coming in the side window opening, so it has to be going out the roof opeing at the top. I also noted the car was really stable at full throttle when taking turn 1 and 2 at the CA Speedway at full throttle so I am thinking the rear has less lift now as it was pretty twitchy before. Since my 914, like most of the cars have the rear of the targa top bolted down, raising it was just a matter of making some spacers and using longer bolts. Is the rear lift being canceled, I think so do to the increased stability of the car at high speed as the 911s I race with float all over the track at full speed, but they all have 4 to 6 holes in the lexan rear windows to try to eliminate the rear lift some? Hope this helps some.
Posted by: North Bay 914 Jan 2 2006, 11:19 AM
So does that explain the vortex generators on Wayne Baker's 2.0 4 cyl. car?
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Posted by: john rogers Jan 2 2006, 12:35 PM
Hummm, most people don't ever notice things like that. I have also seen those on many new PCA GT-3 race cars too. I think his roof is lifted some too?
Posted by: lincoln Jan 2 2006, 12:58 PM
this is actually interesting- seems like years ago i read something about race drivers modifying there own cars with a slight bump , or ramp , amost like a TA spoiler on the roofs , to assist in the airflow over the car and while i cant recall where i read it i seem to recall that it also did just this same trick
Posted by: North Bay 914 Jan 2 2006, 01:15 PM
Quick definition of Vortex generator at this link...here is the rest of the car, it decimated the 2.0 liter field that day with Waynye behind the wheel. He told me he would let the 2.0 six cars by so he could re-pass them.
http://www.aerospaceweb.org/question/aerodynamics/q0009.shtml
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Posted by: North Bay 914 Jan 2 2006, 01:26 PM
Another...
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Posted by: alpha434 Jan 2 2006, 01:27 PM
have you seen the ferrari racers? The ones with the exotic panelwork use little aluminum tabs to disperse the air randomly infront of those ugly panels. It seems that having the air broken is more predictable than having to calculate the downforce on those tabs.
What is the proper term for the really long flare things that stick out and look cool anyway. That one california company makes a kit for the 914 to make it like that.
Anyway, some ricer; the WRX or a toyota, uses dispersion tabs on the roof too. They are very uniform up down nubs. Like one side of a zipper. Only like 9 tabs on the roof though. so they're big. The article i read on it paid reference to audi doing something similar in the old days........
Posted by: john rogers Jan 2 2006, 01:30 PM
Wayne is a pretty fast driver, IMSA and Sebring champion remember. At the Palm Springs Revival race in November he was holding off Tommy Thompson's 2.8L RS based 911 with that same 4 cylinder car!!!
Posted by: alpha434 Jan 2 2006, 01:32 PM
that link was very informative. I bookmarked it. 
Posted by: ChrisNPDrider Jan 2 2006, 01:34 PM
I've seen vortex generators marketed/used to increase vehicle fuel efficiency - there was an ad for a goofy looking twin turbine looking wing in an electric car newlsetter I got a while back. I checked thier website, can't remember the site now, and it claimed that the vortexes created by their wing actually pull the car forward. Some physics about how the vortexes created behind the wing generate force forward. The experimental mini-van had a 15% gain in fuel efficiency.
I'd be interested in learning more about the 914 vortex generator on the race car-like was it custom? Who makes such a thing?
Posted by: North Bay 914 Jan 2 2006, 01:37 PM
Wayne is also a very nice guy. It was a joy to speak with him. That roof panel was 100% custom. It was also very thin. Definitaly not just added to an original roof. You can see it has DZUS fasteners on it.
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Posted by: alpha434 Jan 2 2006, 01:57 PM
chris- a fuel effifiency increase of 15% indicates an incredible reduction in drag!!! Please track down that hyper link!!!
And i think the proper term for ferrari fins is "canards"
Posted by: TimT Jan 2 2006, 02:23 PM
Try http://www.airtab.com they sell stick on vortex generators
Posted by: brant Jan 2 2006, 02:33 PM
We did this...
I can't really tell you if there is a reduction in lift.
I wouldn't doubt it, but its just not one of those things I can absolutely confirm that I'm feeling.
The reason we did this was to reduce drag.
most of the race cars around here do.
if fact most of the GT type car either run no windshield and no rear window, or else a slopped windshiedl and only half of a rear window
here is our rear window.
we figure all the air coming in the side windows needs to go somewhere or else the cockpit is a huge sail:
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Posted by: brant Jan 2 2006, 02:39 PM
So technically we didn't "Raise" our roof.
what we did do was cut out all of the structure in order to open up the same gap that would be there if you raised one.
I used a hand grinder with 35grit poly-sanding paper on the inside of the roof.
cut through it like butter.
I cut out all of the inner structure on the back of the roof panel that normally attatches/anchors the latches. By cutting this out and attaching the roof into its normal position there is over a 1 inch gap between the targa bar and roof panel:
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Posted by: brant Jan 2 2006, 02:47 PM
We didn't end up buying the glass top, because we thinned our own.
I can't remember how much Sheridan's top was, but I think it was outside of our $50 per pound rule.
plus I like that the stock top has the stock texture, and stock color and appearance.
We used the same 35 grit grinding/sanding wheel to thin the stock top.
After grinding away all of the structure, nuts, reinforcement, and anything that weighed anything... it was really very light.
I sanded the inside of the whole top.
In fact right in the middle I went through a tiny bit and had to put a tiny patch on it to seal it back up.
you can take off a couple of layers and take off about 10+ lbs.
have to use different attachements obviously.
and its thin enough that I can take my pinky finger and easily "flex" the top now from the outside with very little pressure
here is that rear area sanded away... that creates the gap for air:
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Posted by: john rogers Jan 2 2006, 04:04 PM
Jeeez Brant has put in a tad more work in the rear roof gap than I did......As he noted, any increase in stability would only be noticable at a track like the CA Speedway which is so smooth and fast that small changes are very apparent. We found that by putting in 1/8 inch of toe in instead of straight ahead like I usually run made the car feel much more stable on the front straight and when riding the yellow line in turns 1 and 2 flat out.
Posted by: kdfoust Jan 2 2006, 04:05 PM
Hey Brant that's pretty cool. With the conversion I've been considering what I need to do to deal with the aerodynamics of running 120 with the windows down.
When you made the change to the top did you notice any difference in your speeds at the end of straights or the wind inside the cockpit? OR did this go in in concert with the rear window holes?
Regards,
Kevin
Posted by: brant Jan 2 2006, 04:44 PM
sorry Kevin,
when I made the change there were just too many changes at once to notice the difference. My before car was a PCA club racing production class car with differnent top, driveline, weight, etc.
Posted by: race914 Jan 2 2006, 05:27 PM
What a great thread!
I'm going to mod my lid for rear venting, to copy what Brant did, and also just ordered a set of vortex generators from the site recommended by TimT (www.airtab.com) so I can copy the Wayne Baker setup!
Thanks guys!
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Posted by: Dead Air Jan 2 2006, 06:09 PM
| QUOTE (race914 @ Jan 2 2006, 03:27 PM) |
| What a great thread! :clap: I'm going to mod my lid for rear venting, to copy what Brant did, and also just ordered a set of vortex generators from the site recommended by TimT (www.airtab.com) so I can copy the Wayne Baker setup! Thanks guys! |
[QUOTE]
What about putting those window louvers over the engine cover. Does the car have to have origonal profile? This pic isn't quite right...
You remember... the ones they used on the 240Z's, etc!
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Posted by: alpha434 Jan 2 2006, 07:48 PM
I like the rear louvers for the engine bay...
Al Lager, my guy in the know, says that the vortex can actually push the car forward, by creating a greater suction behind it than the wind resistance in front.
He says that the same effect made VW busses more aerodynamic than damn near anything out there at the time. Including ferraris.
But putting holes in the windows ruins the "catching" part of the effect.
On the other hand- he also says that putting the holes in is the best way to reduce drag on these cars.
SOOOOO.... I guess it depends on what effect you're going for. And it would be nice to know wich offers the most performance.
Al says that they always let him race with a passenger window in place and he used a small tab on the body work in front of the driver window in combination with a "safety net"- but not there for safety.
Then you could reduce the drag on the rear window by reducing the amount of air driven hard into the cockpit and maintain the unique area behind the window for the vortex.
I spent all afternoon pumping him and another guy for info- and trying to fit 7" wide phone dials under stock fenders..... 
Posted by: bd1308 Jan 2 2006, 07:52 PM
what was mentioned, the air pocket...was mentioned oddly enough on MythBusters....apparently the micro air-pocket will correct the air flow over a truck-bed, reducing the drag effects caused by the tailgate being up. It's actually more fuel efficient to leave the tailgate up.
b
Posted by: alpha434 Jan 2 2006, 08:06 PM
Al knows. He was a national level crew chief. Won Sebring and some other stuff. He offered to have his rocket scientist give me all the data he'd already collected on it....
Posted by: john rogers Jan 2 2006, 10:16 PM
If you leave the tailgate up on a pickup, the air in the bed will reach some equilibrium, but with a 914 we have a huge air pump sucking the air down into the engine bay and then out the bottom, in effect trying to lift the rear of the car off the ground. Leaving the windows up would probably be best for less drag, but we can't race that way, so we need a good way to get the air out. On the Wayne Baker #22 car, they had air inlet openings in the bottom of the side longitudenals and the engine cooling fans sucked air out the top rear sections of them. Acted sort of like a "sucker car", which we know how that one work so well.
Posted by: porsha916 Jan 2 2006, 10:18 PM
I wish you 914 racers luck with these aerodynamic changes, these generators use energy(drag) to create the vortex's, the question is do they direct air flow enough to reduce drag in other places, or reduce lift (perhaps increasing drag). The problems you need to overcome or use to your advantage: 1. The 914 does not have a very aerodynamic shape. 2. the engine cooling uses air from behind the passenger compartment and exhausts that air under the car. The key is to direct the airflow to minimize lift and or drag. Most likely it will be a compromise between the two. There is alot of things you can do to a 914 racer to decrease drag, but many of would seem to be quite odd, look at the history of aircraft design.
Take Care
Bill
Posted by: Eric_Shea Jan 2 2006, 10:31 PM
| QUOTE |
| On the Wayne Baker #22 car, they had air inlet openings in the bottom of the side longitudenals |
So that's what those are. I've had quite a few 914's with that proble... errrrrrrr... feature.
Posted by: alpha434 Jan 2 2006, 11:29 PM
bump...
the fan isn't that big a deal. It's soft air- i.e. controlled. and its going to be really hard to do anything VTOL because of the fan. Take off your rear bumper, and its all good- nothing more you can do.
Some 356 guys use air passages down the body through the headlight holes.
+HP
-Drag
Back to the aero-
Air in low pressure is "soft". Soft air bends more easily when faced with high pressure air and solid objects.
here's a picture...
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Posted by: john rogers Jan 2 2006, 11:34 PM
If you ever get a chance to take a close look at it you'll find that the floor has been cut out and is a large aluminum sheet with a passle of small chrome molly tubes here and there that take the place of all the heavy duty sheetmetal that Porsche has installed. Same with the front and rear trunk areas and many cars now duplicate this sort of "original metal removal" and replace with tubes. Here's a couple of picts that illustrate some of the mods.
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Posted by: alpha434 Jan 2 2006, 11:35 PM
whoops. try this
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Posted by: john rogers Jan 2 2006, 11:35 PM
Another, notice the extra sheetmetal beside the original side sections.
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Posted by: alpha434 Jan 2 2006, 11:37 PM
Nice dash... The yellow doesn't distract you from the task at hand? 
Posted by: maf914 Jan 3 2006, 09:54 AM
| QUOTE (john rogers @ Jan 2 2006, 08:16 PM) |
| If you leave the tailgate up on a pickup, the air in the bed will reach some equilibrium |
Last year in Road & Track magazine there was a brief article in which the writer indicated that Ford had done wind tunnel testing with pickups to determine if drag increased and fuel mileage decreased with the tailgate in the up position. Ford claimed the tests indicated that drag was less and fuel mileage increased with the tailgate up, as opposed to with it down. But I don't think Ford would recommend that people drive with the tailgate down in any case. In the past I've tried driving my old Nissan pickup with the gate down, but could tell no difference.
Posted by: brant Jan 3 2006, 09:58 AM
John,
got any more pictures.
I don't think it would pass for our local "vintage" club...
but it sure is interesting.
brant
Posted by: john rogers Jan 3 2006, 10:42 AM
If you mean pictures of the yellow #22 car, I have some someplace that I can scare up and put in a seperate thread as this one has wandered off the original topic. You statement about not being legal made me chuckle since that little 4 cylinder car was pretty famous in it's day and you are correct that there are many clubs won't let something like that in to race. Even VARA makes it run in CP-X since it was an IMSA car, not an SCCA class car. Ron will probably have it at the June HSR West race at the CA Speedway, maybe it is time to take a road trip to So Cal???!!!
Posted by: brant Jan 3 2006, 12:11 PM
John,
cool I'll look for it
would love to see more pictures eventually.
I know that my local vintage group is much more strict than most of the nation... (no aero at all, no fiberglass, no fiberglass fenders, etc)
but I can understand their decision too.
Their rule is 1972 and how it would have been presented legally in 72. (or could have been presented legally in 1972)
brant
Posted by: dinomium Jan 3 2006, 05:55 PM
how about a Gurny flap on the trailing edge?
Also, doens't the high speed instability be exsaperated by the two aero dirty areas (front supension and rear engine bay) intersecting with the relitively areo clean pan area? You have turbulance then lift back to tubulance... And in the stock rear valance and you might as well fly a gollywobbler!
Posted by: TimT Jan 3 2006, 06:55 PM
A gurney flap make a wing more efficient, I think this the polar opposite of what is trying to be accomplished here. I could be wrong though of course.
allow as little air as possible to get under the car
We punched some holes in the rear window of our 935 clone to allow the air an easy way out
Our scientific methods at the track are flawed though. Usually because of time constraints... ie we change more than one thing at a time.
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Posted by: john rogers Jan 3 2006, 07:15 PM
Since you mentioned the pan area, this should get all the engineers to thinking. On my race car the front spoiler covers the front suspension area and the pan is nearly the lowest part of the body (except for the headers) so I an thinking there is a tad bit of the nozzle effect since the air going under the pan is squeezed slightly, making it's velocity higher so to keep things in order the pressure will go down some. Some air will come out the sides I imagine but the car is moving. Bournolli thought up that stuff years ago and it is why when two ships are refueling at sea side by side they tend to get sucked together or like two NASCAR stock cars side by side.
Posted by: Jeroen Jan 3 2006, 07:56 PM
| QUOTE (TimT @ Jan 4 2006, 01:55 AM) |
| A gurney flap make a wing more efficient, I think this the polar opposite of what is trying to be accomplished here. I could be wrong though of course. |
as far as I understood, a gurney reduces drag from a wing by pulling the airflow back to the bottom of the wing
(excuse my poor description, the pic should clearify it)
Not sure it would work on the roof, but I have been thinking about it
Posted by: TimT Jan 3 2006, 08:01 PM
good pics Jeroen The gurney does help to pull the airflow back together, making the wing more efficient. Im not sure this is what you want on the roof of a 914, where lift may be generated by the roof itself?
Posted by: brant Jan 3 2006, 08:53 PM
John,
can I ask a clarification question about your front spoiler....
when you say it covers your suspension, do you mean that you have a flat bottom covering some of the suspension....
I ask because I've seen more than a few vintage cars (and am working this myself right now)... Where the front spoiler or valance has a sheet of aluminum attatched to the bottom edge of the valance and then covering all of the leading edge of the torsion bars etc. Thus making the front underside smooth.
brant
Posted by: Air_Cooled_Nut Jan 3 2006, 10:09 PM
I read the aerodynamic web site...good info. Not sure if I believe that those vortex generators are of any real help. There's still the fundimental issue of the car creating a low pressure area on top and higher below. So you make little tumbly areas of air aft of the roof; the engine is still sucking air from above itself and creating a region of lower pressure. And those vortex generators are used to help a wing with lift.
Pulling air from the sides or -- better yet -- the bottom of the vehicle makes more sense. Granted, things in aerodynamics may not always make sense but I use the term with regard to aerodynamics...from my arm chair 
Posted by: john rogers Jan 3 2006, 10:37 PM
No, the front spoiler only sits about 1/2 inch off the ground and wraps around to the wheel openings. I have never gotten the time or ambition actually to do the bottom aluminum work like Jim Patrick has done with his car.
Posted by: race914 Jan 4 2006, 02:21 PM
| QUOTE (brant @ Jan 3 2006, 06:53 PM) |
| John, can I ask a clarification question about your front spoiler.... when you say it covers your suspension, do you mean that you have a flat bottom covering some of the suspension.... I ask because I've seen more than a few vintage cars (and am working this myself right now)... Where the front spoiler or valance has a sheet of aluminum attatched to the bottom edge of the valance and then covering all of the leading edge of the torsion bars etc. Thus making the front underside smooth. brant |
Hi Brant,
Is this what you are thinking of? My front splitter extends back the depth of the spoiler. Not a full belly pan, but it helps.
Greg
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Posted by: brant Jan 4 2006, 02:25 PM
Greg,
yep..
nice pic
thats one of the things I'm working on
brant
Posted by: TimT Jan 4 2006, 05:13 PM
We did a similar thing on our 935, The splitter extends back to the front crossmember. I dont have any pictures of it though. the rest of the 911 pan is pretty smooth, but even so we may get some ABS sheet and continue the belly pan all the way to the back of the car
Posted by: Aaron Cox Jan 4 2006, 05:14 PM
greg, got a front shot?
AA
Posted by: race914 Jan 7 2006, 01:03 PM
I do! I went for the 'stealth' look on the entire front spoiler and duct work so it's hard to see. Sure is easy to touch up all the rock chips though!
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Posted by: J P Stein Jan 7 2006, 01:18 PM
Brant:
Does the rule book require a rear window?
Posted by: brant Jan 7 2006, 01:22 PM
| QUOTE (J P Stein @ Jan 7 2006, 12:18 PM) |
| Brant: Does the rule book require a rear window? |
I don't technically think so...
I know some spyder 914's in vintage that run no rear window.
but when I was contemplating this... I had a picture in my head of a carb fire I once witnessed..... thats when I decided to go with the rear window. I could imagine a big ball of flame with my head and neck 12inches away.
I do think it would be less wind drag with the rear window out though.
brant
Posted by: J P Stein Jan 7 2006, 01:28 PM
| QUOTE (brant @ Jan 7 2006, 11:22 AM) |
| but when I was contemplating this... I had a picture in my head of a carb fire I once witnessed..... thats when I decided to go with the rear window. |
So........ what kinda chump has carb fires?
I run without a top. My biggest fear is goose shit. The gaggle at PIR takes off about 7ish......bout a thousand of em'.
Posted by: brant Jan 7 2006, 01:34 PM
I don't think I'd like the goose issue...
but I remember a fire ball on a 911 where they had mounted a fuel guage on the carb itself....
all that vibration caused the brass fitting on the fuel guage to crack and spray fuel onto the headers...
poooffff!
memory kind of stuck in my head.
and then I realized that on a 911 there was a lot more between the driver and that fire then I would have.
brant
Posted by: dmenche914 Jan 7 2006, 01:42 PM
what about the 914's that have the raked back windshild modifcation, this tilts the roof at an angle. Has anyone knowledge on what that does to air flow?
I have seen some photos of this, even on race 914's My gut says that it would help hold the car down at speed, what do you all think?
Posted by: J P Stein Jan 7 2006, 01:48 PM
Dunno, I'm fighting the Ginther wind screen urge.....
My windshield is starting to piss me off. 
Posted by: brant Jan 7 2006, 10:14 PM
I would think it would be the least drag, and a tiny bit of weight savings....
then a tonneau over the passenger side.
get a belaclava thou
brant
Posted by: race914 Jan 10 2006, 05:15 PM
Received my airtabs today and just set them on to mockup the spacing.
Plan is to paint to match the top & stripe.
Going to test with this first before lifting the top.
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Posted by: SirAndy Jan 10 2006, 05:48 PM
| QUOTE (race914 @ Jan 10 2006, 03:15 PM) |
| Received my airtabs today and just set them on to mockup the spacing. |
shouldn't they be mounted 180 deg. rotated?
Andy
Posted by: race914 Jan 10 2006, 06:05 PM
| QUOTE (SirAndy @ Jan 10 2006, 03:48 PM) | ||
shouldn't they be mounted 180 deg. rotated? Andy |
Hi Andy,
I thought so too at first. Good thing I read the directions from the manufacturer www.airtab.com
Here is their diagram of the concept of operation
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Posted by: TimT Jan 10 2006, 06:08 PM
No those are mounted in the correct direction.
http://www.airtab.com
Posted by: gregrobbins Jan 17 2006, 12:40 AM
| QUOTE (john rogers @ Jan 2 2006, 10:34 PM) |
| If you ever get a chance to take a close look at it you'll find that the floor has been cut out and is a large aluminum sheet with a passle of small chrome molly tubes here and there that take the place of all the heavy duty sheetmetal that Porsche has installed. Same with the front and rear trunk areas and many cars now duplicate this sort of "original metal removal" and replace with tubes. Here's a couple of picts that illustrate some of the mods. |
I had a chance to see #22 run at the HSR Palm Springs event. Afterwards I talked to the driver and crew. I am pretty sure they told me there was no VIN#, its a tube frame and 914 style body. Has a 2.0L four that is driven by a BMW mechanical fuel injection system and the car hauls ass.
Posted by: retrotech Mar 3 2006, 11:08 PM
Any results, or opinions on the Vortex generators after mounted?
Posted by: race914 Mar 4 2006, 08:46 AM
| QUOTE (retrotech @ Mar 3 2006, 09:08 PM) |
| Any results, or opinions on the Vortex generators after mounted? |
Will be going to http://www.buttonwillowraceway.com/trackmaps.htm in April
Good test for me because I'm in 5th on the 'riverside' turn and the rear end has always been light and dances around.
I'll report my results.
I saw a mention in the 'Phoenix Track Day' thread that Wayne Baker was there and commented on the vortex generators and something about 'stable at 160'. I'm hoping to be stable at 130!
Posted by: retrotech Mar 4 2006, 09:46 AM
Any pictures of them mounted? Painted to match yet?
Posted by: retrotech Mar 4 2006, 09:51 AM
Where can I find that info from Wayne baker?
Posted by: alpha434 Mar 4 2006, 10:01 AM
Hey Racer914!
Don't forget that your leaving really bad turbulence behind you, too. That could make it really hard your someone to follow you down the straight.
Just seeing if I can help with some strategy....
Posted by: Al Meredith Mar 4 2006, 11:17 AM
Mythbusters on Discovery did a comprehensive show on tailgate up vs tailgate down on a pickup. Up is better because there is a "rolling" ball of air that builds up behind the flat rear window and the air comming over the top skips over that rolling ball and is less drag overall.
Posted by: race914 Mar 4 2006, 12:23 PM
| QUOTE (alpha434 @ Mar 4 2006, 08:01 AM) |
| Hey Racer914! Don't forget that your leaving really bad turbulence behind you, too. That could make it really hard your someone to follow you down the straight. Just seeing if I can help with some strategy.... |
I Love it!
I need to add that to my bag of tricks.
Actually the rear end twitching around keeps people back now through that turn!
At a previous event I was watching a 911 turbo come on me and then it suddenly backed off. Found out one of my brake cooling tubes (3" aluminum dryer venting from Home Depot) had come off and dropped onto the track. Turned out it was Hank Watts (Secrets of Solo Racing) behind me and I told him I was using the same strategy as WWII U-boats and occasionaly jetison parts. That did make him laugh.
Posted by: race914 Mar 4 2006, 12:30 PM
| QUOTE (retrotech @ Mar 4 2006, 07:51 AM) |
| Where can I find that info from Wayne baker? |
It was in the http://www.914world.com/bbs2/index.php?act=ST&f=2&t=46011 thread.
1st page. Response by Greg Robbins
"Wayne is running a four that he told me puts out 165 hp. Running a SCAT balanced crank, Cam (he wouldn't say which one), euro pistons, and 44mm Webers. I am sure he is running high compression as he runs race fuel. He's not a six, but he sure is fast.
I asked him about the airfoils on the back of his targa top. He said they don't do anything and smiled. Said they were on the car when he got it. Then he added there is no way to test if they work or not. Told me the car had run 160+ at Daytona and was very stable. Many find the cars get light on the rear at 130+. HSR does not allow spoilers or wings which is why you don't see them on any of the cars."
Posted by: Dan (Almaden Valley) Mar 4 2006, 12:31 PM
I'll probably see you at BW, Greg.
Be there with the new whip.
Hope to get everything buttoned down by then.
On my last items on my Punch List.
Oil cooler and plumbing.
Mine will be a virgin....no spoilers or airdams, no vortex generators 
But it will be fun!
See you and Tricia there.
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Posted by: gregrobbins Mar 5 2006, 10:27 PM
| QUOTE (retrotech @ Mar 4 2006, 08:51 AM) |
| Where can I find that info from Wayne baker? |
Me.
I met and visited with him at the HSR West event at Phoenix. I asked him about the vortex tabs on the roof of his car. He told me he didn't know if they worked or not, then smiled. He added that he didn't know of any "scientific" test to prove that they worked or not. Then added the car ran 160 at Daytona and was stable.
Can it do 160? Not sure, but it is one "kick butt" 914 and Wayne is one hell of a driver.
Posted by: gregrobbins Mar 5 2006, 10:29 PM
I sent an email to the company who makes Airtabs. I got a reply from the inventor, Gary.
| QUOTE |
| I had a llittle 914 back in 1971 so am quite familiar with it. Airtabs should be mounted across the trailing edge of the roofline. That vertical rear window type of "step", results in Airtabs increasing the turn angle of the main flow, so more mass-flow hits the trunk deck spoiler. He'll be very lucky if they're accepted as legal. Sanctioning bodies are usually notoriously against things like vortex generators. For example, I had a very bad time with NHRA over them - when installed on the world-record-holding Top Fuel Funny Car. They lived to regret it because I definately got my revenge and old timers still talk about it to this day. But that's another story for another time. Gary |
Posted by: brant Mar 6 2006, 11:28 AM
So the company basically confirmed that If your rules say: "no modifications for the purpose of Aerodynamics"
then you would be cheating to use these.
imagine that.
(some questions you shouldn't ask)
Posted by: maf914 Mar 6 2006, 12:04 PM
| QUOTE (Al Meredith @ Mar 4 2006, 09:17 AM) |
| Mythbusters on Discovery did a comprehensive show on tailgate up vs tailgate down on a pickup. Up is better because there is a "rolling" ball of air that builds up behind the flat rear window and the air comming over the top skips over that rolling ball and is less drag overall. |
Last year in Road & Track magazine there was a brief article about the tailgate up or down question. Ford indicated that wind tunnel tests showed there was less drag with the tailgate up.
Posted by: porschenut Mar 6 2006, 12:12 PM
How about reversing the airflowin the engine compartment. Pull from the bottom and exhaust up. Create a lot of downforce then
Posted by: brant Mar 6 2006, 01:59 PM
| QUOTE (porschenut @ Mar 6 2006, 11:12 AM) |
| How about reversing the airflowin the engine compartment. Pull from the bottom and exhaust up. Create a lot of downforce then |
and the underbody scoops that would be necessary to do this would also create a lot of drag...
the simpliest thing to do would be to just install a big honking wing...
but if your vintage class does not allow wings, then focus on reducing drag...
brant
Posted by: alpha434 Mar 6 2006, 05:08 PM
I don't think there is a problem with the stock fan configuration. It's "soft" air that is moving through that system. Air that is fairly slow moving and low volume, compared to the stuff going on around you at 60. But if you were to reverse the system, it would pick up dirt off of the track, and the hot air off the track, and hot air from the brakes. And I don't believe that it would provide any significant downforce from the mild suction going on there.
But on the other hand, I don't believe that the fan would starve down there without scoops. The top has no scoops. I think you would just need a screen to allow air to freely enter the engine bay when it's sucked in.
Either way, Brant's definately right about going for drag reduction. An exotic fan system isn't likey to help or hurt your aero at all. And so it would be a lot of extra work to get to the same place you've been.
Maybe an underbody kit? But this is a little of topic. I want to hear whether or not the Airtabs worked well.
Posted by: drew365 Mar 13 2006, 08:18 PM
I had idle hands this weekend so I decided to modify my targa top. I put together what Brent did and what John did. I probably took it a little too far but only a track day will tell.
I added 1" of spacers to the stock latch and used a grinder to get rid of the reinforcement and rear seal area. I now have over 2" of clear space between the targa and the top. I'm a little afraid with that much air passing through that I might start creating lift under the top. If it seems like that is the case I can always remove the spacers and drop it down so I have about !" of clear space.
The first pic shows a side view. Looks kind of weird.
Posted by: drew365 Mar 13 2006, 08:21 PM
Like I said, pic 1
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Posted by: drew365 Mar 13 2006, 08:22 PM
This one shows the spacers added to the stock latch.
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Posted by: drew365 Mar 13 2006, 08:24 PM
Here's one from the rear.
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Posted by: race914 Mar 13 2006, 08:28 PM
Great Job! Spacers are a great idea, since as you mentioned, you can tune the height of the gap.
I'm anxious to hear how your test day goes.
Any plans to come up to the Buttonwillow event April 15-16?
Posted by: drew365 Mar 13 2006, 08:53 PM
Probably won't get to that Buttonwillow event. I'm hoping to run the big track at Willow Springs April 22. We leave for a trip to England a few days after so I might be wishfull thinking. The big track at Willow would be the best test for me since the car gets light while turning in to T8 from the back straight. I'm really only expecting this mod to aid cooling in the cockpit more than changing the aero of the car. So any added benefit will be a welcome addition.
Posted by: alpha434 Mar 14 2006, 12:45 AM
I thiink it looks great. Even as just an accent to the chalon sail panel.
Posted by: Mueller Mar 14 2006, 01:17 AM
| QUOTE (drew365 @ Mar 13 2006, 07:53 PM) |
| Probably won't get to that Buttonwillow event. I'm hoping to run the big track at Willow Springs April 22. We leave for a trip to England a few days after so I might be wishfull thinking. The big track at Willow would be the best test for me since the car gets light while turning in to T8 from the back straight. I'm really only expecting this mod to aid cooling in the cockpit more than changing the aero of the car. So any added benefit will be a welcome addition. |
whatever you do, don't spin...that roof will last up there for about .05seconds before getting launched into the air
Posted by: alpha434 Mar 14 2006, 01:21 AM
| QUOTE (Mueller @ Mar 13 2006, 11:17 PM) | ||
whatever you do, don't spin...that roof will last up there for about .05seconds before getting launched into the air |
Hinge the front. And tie a "safety wire" to the back. Or a few safety wires.
Don't want to lose it and scratch the paint.
Posted by: Mueller Mar 14 2006, 01:26 AM
| QUOTE (alpha434 @ Mar 14 2006, 12:21 AM) |
| Hinge the front. And tie a "safety wire" to the back. Or a few safety wires. Don't want to lose it and scratch the paint. |
I'd rather have it fly off, if you hinge it, it'll take out the windshield and hood and possibly trash the windshield frame...of course I wouldn't want to be around the place if it becomes a flying saucer
Posted by: alpha434 Mar 14 2006, 01:30 AM
Yeah. Forget the hinging. That was a bad idea. But put about 10 inches of safetey wire loosely coiled and attached to all four corners. Then when it comes loose, It will pick up enough to break the front latch, and then starighten out and fall back down.
Or put 8 inches on the rear and 10 or 12 on front.
Engineers at work. Everyone stand back!
Posted by: alpha434 Mar 14 2006, 01:32 AM
And the less you use, the shorter the distance will be when it lands back on the roof. So then it would not hit and damage the windshield.
And you could safety wire the sides, in the midle too, so that it won't come crashing into the cockpit.
Posted by: Jeroen Mar 14 2006, 06:21 AM
just use bonnet pins to hold the roof down...
Posted by: brant Mar 14 2006, 09:20 AM
| QUOTE (Jeroen @ Mar 14 2006, 05:21 AM) |
| just use bonnet pins to hold the roof down... |
hmmmmmm
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Posted by: drew365 Mar 14 2006, 09:45 AM
| QUOTE (brant @ Mar 14 2006, 08:20 AM) | ||
hmmmmmm |
Could you elatorate a little?
Any pics?Since I rarely spin
I shouldn't be worried. But in the slight off chance that it happens you guys have a good point. One good thing is the stock latches locked very firmly. I could attach a bracket to make sure the latch can't move off the pin after it's set? The weak point will be the bolts into the top which are the stock holes and seem pretty solid. You guys got me worried.
Posted by: kart54 Mar 14 2006, 12:57 PM
Drew,
I just did almost the same thing but I did it the easy way and what is required by the SCCA rule book; lag bolts all the way through the roof and the targa bar. Very adjustable and no way to have the top come off without taking off the bolts. (sorry, no digital camera, no pics)
Randy
Car 54 Here I Am, G prod Porsche
Posted by: maf914 Mar 14 2006, 01:30 PM
| QUOTE (Mueller @ Mar 13 2006, 11:17 PM) |
| whatever you do, don't spin...that roof will last up there for about .05seconds before getting launched into the air |
Sort of like the roof flaps that NASCAR added a couple of years ago. When a car spins at speed and starts going backwards the flaps open and break up the airflow over the roof to keep the car from launching upward. Of course their flaps are built a little different and are of course "NASCAR Approved"! These were actually one of NASCAR's best innovations, IMO.
Posted by: race914 Mar 18 2006, 06:52 PM
Got inspired watching Sebring and since the weather was finally cooperating I painted the airtabs today in between Speedchannel coverage.
Hope to mount them on the roof tomorrow. Test day is April 15th at Buttonwillow! See unpainted mockup on page 4 of this thread for the color scheme.
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Posted by: retrotech Mar 18 2006, 07:07 PM
Anxiously waiting to hear. I am trying to decide between Air Tabs, and the Carbon WRX units.
Posted by: race914 Mar 19 2006, 04:26 PM
Mounted the Airtabs today. All ready for testing!
D-Day April 15th @ Buttonwillow. More to come...
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Posted by: SirAndy Mar 19 2006, 05:13 PM
| QUOTE (retrotech @ Mar 18 2006, 05:07 PM) |
| Anxiously waiting to hear. |
Posted by: retrotech Mar 19 2006, 06:18 PM
I have read the research papers from Japnese engineers that designed the one for the Mitsubishi WRX. Certainly is technical, and claims to achieve the same results, but they are triangular. The pointed end, into the wind, opposite of the air tabs? Air tabs will certainly create a bit more drag, but I can imagine that being an issue. I just wonder if they are over kill for 914 application, versus semi trailer. I thik they will work, but will something less also be adequate?
Inquiring minds want to know.
Posted by: alpha434 Mar 19 2006, 06:23 PM
| QUOTE (retrotech @ Mar 19 2006, 04:18 PM) |
| I have read the research papers from Japnese engineers that designed the one for the Mitsubishi WRX. Certainly is technical, and claims to achieve the same results, but they are triangular. The pointed end, into the wind, opposite of the air tabs? Air tabs will certainly create a bit more drag, but I can imagine that being an issue. I just wonder if they are over kill for 914 application, versus semi trailer. I thik they will work, but will something less also be adequate? Inquiring minds want to know. |
The WRX ones are just square, straight up and down. Sorta. They are triangular shaped, but they look square from the front.
The "airtabs" look to be MUCH more refined and developed. I wonder if Subaru just did the vortex research as an "afterthought" when they had a little instability at high speeds.
Here's another $.02
Posted by: turboman808 Mar 23 2006, 08:11 AM
instead of going with the vortex what about adding a gurney flap to the back of the lid? No doubt those will probably work really well but man they are ugly. 
Posted by: Joe Ricard Mar 23 2006, 08:44 AM
Not if you make it out of CF or Lexan. 
Either one will make the car faster just by making the ashtray out of it.
Posted by: retrotech Mar 23 2006, 10:50 AM
Picture of Gurnet flap?
Posted by: Matt Meyer Mar 23 2006, 11:45 AM
I am anxiously waiting your reports also.
My $.02, and you are being overcharged.....
I think vortex generators are mostly a gimmick on these modern cars, but might work on a 914 because the stock aerodynamics suck.
The vortex generator on the Lancer and Subaru would probably not be effective on the 914. But I think it is worth a look anyway.
The "shark tooth" type vortex generators try to "re-energize the boundary layer" to get more airflow and keep the airflow attached as it travels over the car. These type of vortex generators are well researched and proven in the aerospace industry.
What is allegedly happening on the Subaru and Lancer is that the angle of the rear window is too great and the airflow starts to separate at the rear window. This creates excess drag and makes the rear wing or “spoiler” ineffective. The vortex generator takes energy from the flow and moves it into the (not moving) boundary layer to get the boundary layer moving. The airflow stays attached along the rear window, over the rear deck where a spoiler detaches it. In theory the drag being reduced is greater than the drag to create the vortices so there is a net gain. I would expect these would work well on an older 911.
The airflow will separate over the top of the 914, the undercut rear window insures that. It appears to me that “AirTabs” allegedly create (I assume larger) vortices that separate the airflow in a controlled manner and reduce drag. Like a spoiler on the rear of a car. This type vortex is notoriously hard to maintain, and I could find almost no research on the effectiveness of a vortex generator used to separate airflow (very quick search).
A small spoiler (¼ to 1 inch) on the back of the targa top might be effective as it would separate the airflow.
I possess no unique knowledge about the 914s aerodynamics but I doubt anyone can do much worse than what was provided originally at the trailing edge of the targa on a 914.
One cheap (crude) way to test these would be to do coast down tests. Take a car with a stock top up to a significant speed (80-100 mph) and then let it coast to 0mph. Time or measure how long it takes. Repeat in the opposite direction to negate any wind. Do the same with the aerodynamic aid installed and determine the net difference. If it coast farther/longer with the aerodynamic aid the aid works.
Posted by: brant Mar 23 2006, 11:54 AM
I don't want to sound like a pessimist...
so I'm anxiously awaiting the seat of the pants impression too....
but I seriously doubt that without a wind tunnel or computer calibrated equiptment that it will be possible to "feel" any difference.
If you need stability add a wing.
if rules don't allow a wing, then they don't really allow vortex generators either.
This thread has split into 2 seperate discussions...
#1 about creating downforce
and
#2nd about reducing drag.
Lifting the roof is really just letting cockpit air escape in the hope that it will reduce drag... (see item #1)
I still think that in regards to downforce a Wing is the most effective technique
just my 1.5cents.
brant
Posted by: retrotech Mar 23 2006, 12:13 PM
I didn't know how to attach this file, but if anyone is interested. I hope this link will lead to the in depth report by engineers in Japan, about the funtion of Vortex Generators on the Mitsubishi.
www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf
Its 6 pages and very convincing, that it is not a useless add on.
I am optomistic the VG will really help attach air to rear spoiler on 914.
The Arpil 15th test drive on the track will tell us the most.
Posted by: Matt Meyer Mar 23 2006, 01:02 PM
I couldn't get your link to work but I think I have been to that site.
I did not mean to seem too pessismistic. And all ideas need to be tried and tested. There have been huge advancements in the past 30 years and the 914 was not at the leading edge then. With enough time and money I have no doubt that small barely noticible changes could produce some significant aerodynamic improvements. I wish we had the money to invest where there is a benefit that Mitusubushi
The type of vortex generators on the Mitusubishi work. The Aerospace industry has proven that and they have extensive use there (where they usually increase lift BTW). I acutally have more confidence that these shark tooth generators will do something positive than the AirTabs. I just don't know if it will help the 914.
I think they are a gimmik on the modern cars, because on a car for the street, 70mph max legally, the rear wing is a gimmik. The generators produce drag, the wing produces drag and at below 100mph have no real benefit. Furthmore, if they are really worried about seperation they put a little more angle on the rear window and you have the same effect with less drag.
IIRC the early 911 just missed the proper angle on the rear window to keep airflow attached. I would recommend the shark toothed generators first in this application.
On a 914 the air will seperate at the rear of the targa. The trick is to get it to reattach or virtually stay attached without braking up. AirTabs claim they do this with semis, so I see more potential there than on the "shark tooth" type on inspection. But making desisions by inspection leads notoriously to failure (and gloriously to success by those who do not know better) in the speciality of aerodynamics.
I am just predicting in order of likely success AirTabs, small spoiler (which the lifting of the back might be effectivelly doing), and then the shark toothed generators. I hope someone steps up and tests them all. See my above post for the best meaningfull test that is not NASA funding type money that I can think of. Anyone want to apply for a grant?
Posted by: brant Mar 23 2006, 01:37 PM
most of the time when I see manufacture's claims about aerodynamics.. the numbers are relatively small. Possibly because the package is already so refined..
but here is an example:
the new lotus exige..
They reportedly changed nearly every single body panel, added a roof, added a splitter, the list goes on.
Now granted the "before" car was undoubtably pretty well refined. But my (failing) memory about the Aero tests for the "after" body work resulted in something like 15lbs of increased down force....
was it more? I could be wrong.. maybe it was more. But as I read about this vehicle and Wind tunnel proven numbers.. it shocked me that the amount of down force gained was so small.
I doubt that I could "feel" by the seat of my pants a 15lb down force gain at 100mph. In fact, I doubt that I could feel a 150lb down force gain at 100mph.
maybe my buttocks isn't sensative enough...
I'm not saying that small things can't be a step forward, I just doubt that anyone is going to gain 1 second a lap.. or even .1 of a second a lap.
The driver might "feel" more comfortable, but its going to be tough to assign a value to feelings without a stop watch to verify them.
and I hope I'm wrong..
anxiously awaiting the trial by fire.
brant
Posted by: Joe Ricard Mar 23 2006, 01:44 PM
I don't know But I think them air flappy things are cool. I could benefit from them just by gaining 1 mile per gallon. Having to travel 380 miles just to race is probably worth it.
And the look cool Gear heads love stuff that looks cool.
Posted by: alpha434 Mar 23 2006, 01:53 PM
Vortex generators fall into the "advanced" category of aero devices. Even though they increase profile, they don't increase drag. Well.. They do, but then they make up for it so the end result is less. If you look at the promotional website for the airtabs, they create a vacuum directly behind them, so that air behind the car will try to rush in and fill the void. This helps to propel the car forward. Theoretically, based on the info from their website.
The mitsubishi one was always just ment to turbulate the air for the rear spoiler. Has nothing to do with holding air against the bodywork.
I don't think that the advantages will be that huge. Even at speed. You may pick up some fuel efficiency, but who here runs 300 lap races? I've already mentioned the strategic advantages. Which won't help you go faster, but will make it harder for the car behind you to go faster.
And Brant.... If it's not specifically illegal, then it's legal. Until they MAKE it specifically illegal.
Posted by: alpha434 Mar 23 2006, 01:54 PM
Also anxiously awaiting the trial by fire....
Posted by: tdsmoonchild Mar 23 2006, 01:54 PM
| QUOTE (retrotech @ Mar 18 2006, 07:07 PM) |
| Anxiously waiting to hear. I am trying to decide between Air Tabs, and the Carbon WRX units. |
anyone have pics of the WRX ones?
Posted by: brant Mar 23 2006, 02:00 PM
Chris,
most of the amateur racing that I do have rule books that specifically say:
"if its not listed as something that you can legally do, then it is illegal"
PCA-CR for example has this rule in print.
kinda the opposite of the SCCA, or Pro type racing.
supposedly to keep things more affordable.
gives the scrutineers an open back door to say nearly anything is illegal after the fact.
Plus in Vintage they clearly state that "No aerodynamic devices of any kind are allowed"
my holes in my rear window are technically for driver comfort. By letting the air in the cockpit escape I am promoting a cooler temperature cockpit and driver comfort. The rule book says that I can modify things to do that.... I am clearly motivated to do it for the benefit in reduction of drag. That is my 2ndary gain, as I achieve my primary intent of promoting driver comfort....
if you don't believe me ask the tech.
I told him too
Posted by: retrotech Mar 23 2006, 02:00 PM
Some seats are more sensitive than others. With a passenger I can definately feel the difference in acceleration. I posted some time back a wierd custom spoiler I had on my 914. It was about 10". It was super stable, but I "could" feel the drag on the straights, no spoiler, higher straight speed, but not confidence building over 100 mph. Now I made a custom 4" duck tail/spoiler. No seat of pants drag, and solid as a rock 100+. I still believe there is cheap speed to be attained from reducing drag on rear as air foil wants to wrap under rear end, and more stabilty cornering at 60+ with atached air to spoiler. I agree with the Lotus info, but the 914 is starting from such a disadvantage, I think even the little things will translate to big improvements.
Posted by: retrotech Mar 23 2006, 02:09 PM
Here is a picture. If you are interested, Google Mitsubishi motors review vortex, and you will be able to down load. Its more factual than you might expect.
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Posted by: retrotech Mar 23 2006, 02:10 PM
another
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Posted by: Matt Meyer Mar 23 2006, 02:18 PM
Can't find pix of the WRX wing. (Damn, I am slow)
Fixed the link (I hope). http://http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf It is a great article on vortex generators for the car. But most importantly shows I am right. 
Turblent air is "bad" air. Mitsubishi designed the generators to get clean air to the wing and reduce drag by keeping the air flow attached "moving the seperation point rearward".
Is it time to start a seperate thread on general aerodynamics?
Posted by: J P Stein Mar 23 2006, 02:20 PM
Ah, at last....a trick piece I don't need. There's hope for me yet 
Posted by: Joe Ricard Mar 23 2006, 03:03 PM
| QUOTE (J P Stein @ Mar 23 2006, 12:20 PM) |
| Ah, at alast....a trick piece I don't need. There's hope for me yet |
I doubt it.
Posted by: byndbad914 Mar 23 2006, 08:33 PM
| QUOTE (brant @ Mar 23 2006, 11:37 AM) |
| I doubt that I could "feel" by the seat of my pants a 15lb down force gain at 100mph. In fact, I doubt that I could feel a 150lb down force gain at 100mph. maybe my buttocks isn't sensative enough... The driver might "feel" more comfortable, but its going to be tough to assign a value to feelings without a stop watch to verify them. and I hope I'm wrong.. anxiously awaiting the trial by fire. brant |
I am a bit with Brant on this one - though I don't want nor have first hand experience with how sensitive his buttocks are
Figured I will just get that joke outta the way...I don't think it will be generally "felt". My guess would be this - if the car can get to it's top speed at the track before, and this time out can go a bit faster (negating the concept that wind speeds that day might/will be different and have an effect) then the drag was decreased.
If the car could only go say 100mph through a specific corner before and was limited because the rear couldn't hang on, and now goes through at a higher speed as the rear is better planted, then downforce was increased.
A general track day where extremes can't be recalled/compared against, I doubt any feeling would be more than mental, or desire to have spent money wisely (and I am uber guilty of that one!). Not with what I would expect the differences to be (not huge). Add 150lbs of rear downforce or drag and that will be noticeable for sure. The fine tuning I suspect will be noticeable at the outer limits.
Just my .02 and not out to downplay the concept. I hope it works - just another item to throw apparently unwanted cash at
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