What Wheels/Tire set up on your GT body race car? Options

[koozy]

I have seen Trekkor's running his car at Thunderhill in the rain and could see the difference in the air movement behind his car (with duckbill) and another similarly setup car but without the duckbill. The interesting thing I noted was the swirl of air behind the cars (the rain and mist behind the cars was very visible). No duckbill resulted in a swirl of air providing lift to the rear of the car. The one with the duckbill showed that same swirl of air but nearly 3 feet behind the car, thus not contributing to lift at the rear of the car. I wish I could find the video.

[brant]

Regarding the spoilers, splitters, spoiler and wings....

My on-track experience leads me to believe:
1) The duckbill spoiler is of no tangible performance value but it does greatly help the aesthetics of the rear on a 914. That air is dead as dead as can be...
2) The spoiler is a decent place to mount brake cooling ducts but not much else..it increases drag vs a stock valance. The increased stability is probably a function of the car being slowed down due to the increased vertical surface area. I run this air-dam too...
3) A wing needs to be way the hell up there to grab clean air - the bottom of my wing is 1" (and should be higher) above the highest point of the roof line.
4) An actual splitter must be able to support at least 250 pounds of direct vertical force to remain stable at speed. My first effort fractured at 160 mph (mid-corner T9 RA) and I thought I had lost a wheel!!! IF you install one - bring it back all the way to the front suspension cross-member - there is more to aero than just the leading edge reveal.

Not meant to offend and step on toes - just sharing what I have learned...

T

I'm no aero expert
but Tony is!!!!

I think the car designers only had rudimentary aero in mind when they designed the car and the dead space is hard to over come with the roof line.

most classes will not allow the change
but the old PCA classes did....
this is how the aero was improved by a local 914 race shop (AJRS) on many of the PCA club race cars 15 years ago....

they changed the roof line to improve the high speed aero
they used clear lexan to build a square duct to get air into the electric fan shrouds for engine cooling....

notice the windshield is layed back... and the base of the windshield covers the cowl vents as it is moved forward all the way to the edge of the front trunk.

[stownsen914]

I'd agree that the small spoilers on the rear don't do a whole lot. It should make some difference, but you won't notice a large difference.

As for raking the windshield ... I make that mod to my car when I first built it. I can tell you it's a complete pain in the butt. There is a lot more work than you'd think. I eventually put a low windscreen on the car.

One comment on the car pictured above. The work appears nicely done, but I'd venture to say that the way it was done - raking back the windshield and lowering the front of the roof but leaving the rear of the roof at the regular height - probably doesn't yield much aero gain if any. You need to lower the rear of the roof too. In fact, the way this car was done may well make the turbulence over the engine and rear decklid worse.

Ideally there would be a hatch of some kind over the rear deck area to smooth the airflow. But then I guess it wouldn't be a 914 anymore!

[brant]

I think he cleaned up the cooling air and still has good downforce front and rear

They built approximately 5 like this with good result and many class records in the day

[tjtryon]

I am replying to this 5 year old post, as I think it may still do others some good with their aero. Aside from access to a wind tunnel, this is by far the best way to find out where to tune your aero goodies.

Get some of your wife's favorite yarn and cut it into 3" pieces. Use scotch tape to tape a hundred or so all over your car. Set up as many GoPro cameras as possible. Use your GoPros, use your buddies, bug your third cousin, just get several of them. Attach to outside car, pointing to the main areas you have taped the yarn (you can always repeat this with the camera looking at other areas), to find out what the air at body panels is doing, and use that info to tune your aero goodies.

Check this video: Video of Aero with yarn

Regarding the spoilers, splitters, spoiler and wings....

My on-track experience leads me to believe:
1) The duckbill spoiler is of no tangible performance value but it does greatly help the aesthetics of the rear on a 914. That air is dead as dead as can be...
2) The spoiler is a decent place to mount brake cooling ducts but not much else..it increases drag vs a stock valance. The increased stability is probably a function of the car being slowed down due to the increased vertical surface area. I run this air-dam too...
3) A wing needs to be way the hell up there to grab clean air - the bottom of my wing is 1" (and should be higher) above the highest point of the roof line.
4) An actual splitter must be able to support at least 250 pounds of direct vertical force to remain stable at speed. My first effort fractured at 160 mph (mid-corner T9 RA) and I thought I had lost a wheel!!! IF you install one - bring it back all the way to the front suspension cross-member - there is more to aero than just the leading edge reveal.

Not meant to offend and step on toes - just sharing what I have learned...

T

[Charles Freeborn]

Regarding the spoilers, splitters, spoiler and wings....

My on-track experience leads me to believe:
1) The duckbill spoiler is of no tangible performance value but it does greatly help the aesthetics of the rear on a 914. That air is dead as dead as can be...
2) The spoiler is a decent place to mount brake cooling ducts but not much else..it increases drag vs a stock valance. The increased stability is probably a function of the car being slowed down due to the increased vertical surface area. I run this air-dam too...
3) A wing needs to be way the hell up there to grab clean air - the bottom of my wing is 1" (and should be higher) above the highest point of the roof line.
4) An actual splitter must be able to support at least 250 pounds of direct vertical force to remain stable at speed. My first effort fractured at 160 mph (mid-corner T9 RA) and I thought I had lost a wheel!!! IF you install one - bring it back all the way to the front suspension cross-member - there is more to aero than just the leading edge reveal.

Not meant to offend and step on toes - just sharing what I have learned...

T

I'm no aero expert
but Tony is!!!!

I think the car designers only had rudimentary aero in mind when they designed the car and the dead space is hard to over come with the roof line.

most classes will not allow the change
but the old PCA classes did....
this is how the aero was improved by a local 914 race shop (AJRS) on many of the PCA club race cars 15 years ago....

they changed the roof line to improve the high speed aero
they used clear lexan to build a square duct to get air into the electric fan shrouds for engine cooling....

notice the windshield is layed back... and the base of the windshield covers the cowl vents as it is moved forward all the way to the edge of the front trunk.

I have the little brother to this car. Same builder, same roof mods. Haven't had it out enough to comment on it' performance, but according to the logbooks it took a couple of podiums in the day. Ran GT5S class.