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Posted by: john rogers Sep 15 2004, 12:02 PM

Has anyone tried using a spacer under the rear of the Targa top, say about an inch or two to make a vent? Since my top has to be and is bolted down in the rear I thought this might help break the vacuum that forms behind the rear window? I have seen a couple with plexiglass/lexan rear windows with several holes across the width which would do the same thing I think?

Posted by: brant Sep 15 2004, 12:29 PM

No track miles on mine yet, but I did it.
Mine is a vintage car also, and will have a roof.

I Dzus fasten my roof on to the roll cage, so I just made the mounts on the cage tall enough to give it the 1.5 (or so) inch of clearance in the back...

I'm doing holes in the rear window too... but only on the passenger side. I get nervous about pulling or cutting the rear window in half since I suspect the most likely fire location will be on top of the carbs, right behind my head.

brant

Posted by: J P Stein Sep 15 2004, 12:35 PM

The only "fer sure" solution I've seen was an airfoil section (cruved about 90 deg) bolted to the roof , directing the air down to the grill. The guy that had it said it dropped the engine (4 banger) temps 20 deg at track days......the high drag solution

Is rear glass required by your sanctioning body? If not...
then you could *really* hear them Webers sing

Posted by: brant Sep 15 2004, 12:59 PM

JP,

I won't speak for John and his reasons for the mod..

but my reasoning was to reduce drag, and really had nothing to do with forcing air into the intakes....

I'm running no windows and no front triangle window, so there is some amount of air forced into the cabin that I wanted to let out.

The big/serious cars are 1/2 deleting or fully deleting the rear window like you suggest... again to reduce the drag.
My only fear is that I've seen a few fires and they almost always start on top of the carbs (fittings coming loose, etc.) A friend of mine's 911 had one of these fires just last summer.... I was there watching it and it was an awfully big ball of flame. Her engine lid was closed and it was still an awfully big ball of flame. Took the paint off of both rear fenders...

It was put out in 3 seconds or so and the car was driveable, but I was surprised at how much damage to the paint had occured in 3 seconds.... Got me to thinking that I might want something (even if it was just a piece of lexan) between my head and a fire like that...

I think the scoop Idea is a good solution for intake air, but at the cost of creating drag.... I just wanted to reduce drag.

brant

Posted by: john rogers Sep 15 2004, 02:18 PM

Thanks for the input. The reason is just to break the vacuum behind the window. I was the one that JP probably was thinking of with the curved air deflector as my stroker four ran a tad warm. The 2L six on the other hand runs really cool. At the race in Tecate, with 95 degree air temps the oil got to 200 max. I did notice that when I blew the four last year at the CA Speedway at full speed, the cockpit filled with oil smoke immediately (really hard to see) and I imagine a fire would do the same. I somehow think this is related to the low pressure area we have and if it could be eliminated it would be nice. As Brant does, I plan to keep the windshield and roof and cut it off to make a roadster.

Posted by: J P Stein Sep 15 2004, 02:38 PM

I can't think of a way to eliminate the low pressure area short of turning the thing into a roadster. Any type of ducting or airfoil is only a partial solution, me thinks.

John:

The guy with the air foil was a local guy....great minds & all. The cooling he got was what he was after and served to indicate that he also got more air back to the lid.... but prolly not to the rear window. As to smoke filling the pilot house.....It seems most all racin' cars fill that spot first when sumthin' goes bang....even open cockpit cars. Some kinda Darwin thing maybe

Posted by: Jeroen Sep 15 2004, 05:58 PM

How about a Gurney flap at the end of the roof?

Posted by: bernbomb914 Sep 15 2004, 06:55 PM

I saw this at Ventura


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Posted by: bob91403 Sep 15 2004, 07:36 PM

I saw a device on "This old house" you might be interested in. It's made to drill holes in wood at a 45' angle, so you can use screws to do butt corner joints. I don't remember what it's called, but it gives you a constant repeatable angled hole. Drill the hell out of your roof. It will still give you shade. And with the holes angled toward the back of the car, the air flow over the top of the roof will draw air from the cockpit. I have no idea how much unwanted upforce it would generate. Just a thought.

Posted by: JmuRiz Sep 15 2004, 08:24 PM

You might be on to something there. Anyone have a good book on vehicle aerodynamics. I know there was an aero discussion on the bird-board's 911 bbs...maybe post there?

Posted by: Aaron Cox Sep 15 2004, 08:31 PM

now that looks like a good idea (this version is quite ugly) if it went all the way across and somehow blended into the roof better (more subtle) why would you use something straight on a convex surface??

i have an extra targa

Posted by: TimT Sep 15 2004, 08:37 PM

"New Directions in Race Car Aerodynamics" Auth: Joseph Katz

Check Amazon

Posted by: maf914 Sep 16 2004, 07:32 AM

Gurney flap, spoiler...all the same thing. It will generate a little downforce over the surface of the roof and add a bit more drag to the car.

Posted by: brant Sep 16 2004, 08:32 AM

I wonder how much turbulence that would create...

but most of the vintage orgs, wouldn't allow it anyways as the Look of the car and all exterior panels has to remain fairly stock...

In colorado they are so anal about the Look of the car that they only allow so many square inches of stickers on the surface of the car. (basically they argue that in 196x... there was no advertising)

brant

Posted by: maf914 Sep 16 2004, 08:44 AM

For some reason I don't think there is going to be much difference in a hole drilled at a 45 degree angle and one drilled at a 90 degree angle when we're talking about a 1/16 inch thick roof panel. About the ony difference will be that one hole is oblong and one is round!

I think that you would relieve cockpit pressure with almost any type of hole in the roof. The air flowing over the windshield and roof is moving quite fast, which creates a low pressure area (the old Bernoulli equation). Ever notice when driving with the top off how much more air is drawn through the fresh air or heater vents?

Posted by: Cloudbuster Sep 16 2004, 08:53 AM

How about a vortex generator on the roof? Its a series of vertical wings, or triangular ramps that are higher at the back. These create miniature disturbances that cause airflow to stay closer to a surface, especially if there is a big notch in the back, like a sedan.

See page 3 of the PDF
http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf

The picture is an aircraft VG
http://www.avweb.com/news/reviews/182564-1.html


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Posted by: Jeroen Sep 16 2004, 08:58 AM

I guess I misread or misunderstood the original post

Are you running without side windows?
Close them... problem solved

Posted by: seanery Sep 16 2004, 09:29 AM

some orgs make you run with windows down

Posted by: john rogers Sep 16 2004, 10:59 AM

As Seanery noted, I have to have both windows open. I think most organizations in the US require that they be fully down.

Posted by: JmuRiz Sep 16 2004, 11:14 AM

Here is a picture of the Mitsubishi Evo MR's vortex generators:

Posted by: brant Sep 16 2004, 04:00 PM

Yeah... no side windows and no triangle front wing type windows.... (too much weight)

brant

Posted by: Jeroen Sep 16 2004, 04:10 PM

How much drag do you thing you're adding by leaving the windows out?
The weight of the windows doesn't even come close to that

I think the open window rule is weird...

Anyway, how about a couple of naca ducts at the end of the roof (in a 180deg position)

Posted by: brant Sep 16 2004, 04:23 PM

In my case as a vintage car:
I'm allowed no naca ducts,
no spoilers (not even a front special edition valance)
no cut outs in my hood
no cut outs in my roof...
no nonstock apperance items.
nothing that looks or helps aero.

I am allowed to lighten the car, remove glass and improve driver comfort...

Removing all the glass, mechanisms, and max lightening my doors did save #60lbs...... quite a bit actually.

I'm sure that I added drag by having the windows out, but it would be an oven in there to replace them with plexi....

thus the reason I'm exploring methods of venting some of that air.

I heavily modified my top so it is more of a skin than a full roof panel. It no longer has the thick mounting area on the back, so it already is stood up from the chassis that amount (about 1.5 inches)

brant