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?

search this subject with john rogers......

he did this on his race car

Can't find it. Do you know the answer?

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?

I just sent a long PM explaining why I ( and some others ) have dome it.

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

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.

So does that explain the vortex generators on Wayne Baker's 2.0 4 cyl. car?

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?

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

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/aerod...ics/q0009.shtml

Another...

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........

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!!!

that link was very informative. I bookmarked it.

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?

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.

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"

Try this site they sell stick on vortex generators

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:

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:

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:

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.

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

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.

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!

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!

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.....

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

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....

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.

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

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.

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...

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.

whoops. try this

Another, notice the extra sheetmetal beside the original side sections.

Nice dash... The yellow doesn't distract you from the task at hand?

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.

John,

got any more pictures.
I don't think it would pass for our local "vintage" club...
but it sure is interesting.

brant

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???!!!

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

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!

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.

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.

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

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?

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

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

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.