Yargk

I'm wondering, has anyone here run computational fluid dynamics on the rear wing height on the GT4? There is some open source software available. I was thinking I could plug one of the GT4 3d models available on the web and see what happens.

The reason I wonder is that it actually seems empirically wrong that a higher wing is always better. I've seen a few research papers showing that the optimal height for a sedan-like body is equal to about the chord of the wing (cleaner air isn't always better, sometimes there's a benefit from using air near the body). Of course this depends on the exact car body used. The leading edge of the GT4 wing is actually at a height less than the chord of the wing, so raising it could help, but it's not obvious that that's the case.

4carl

Interesting but way above my pay grade! carl

ajw45

Agreed. Air flow from under the car and off the raised spoiler under the wing could also impact performance of the wing and change with wing height. The GT4 wing also has angled end pieces that follow the flow of the rear fenders vs say big straight end pieces like on an RS wing. I'm not saying I know anything about anything but it would seem like the surest thing to do is go high downforce mode and/or addd the cs gurney flap to the wing if you want more df vs raising the wing height.

I think dundon is doing some analysis and look forward to see what they come up with.

Yargk

Exactly my thoughts. Gurney and the higher angle of attack at stock height is guaranteed downforce, it will take CFD or a wind tunnel to tell if raising the wing actually helps.

GAZZ

It will be better as the air is less distrurbed, but more importantly, its looks way better higher up!

Bill Lehman

I added a Gurney flap last winter and generally my lap times have suffered. Studying my data, I'm losing speed in every straight and not seeing more speed in fast corners. Could be the driver but I'm planning to remove it before my next event at Watkins Glen and compare data with last weekend at Watkins Glen. I would love to see CFD on wing height. My friend, a former race engineer, with a GT4 setup just about the same as mine has the higher wing supports but has not installed them.

RennKit-Dave

PorscheAddict

That's really interesting, I wonder if it is due to front lacking sufficient downforce now as a result, as Dave said. Stock front bumper/splitter?

plucas

I have done CFD on a stockish GT4. It was a scan of Dundon's GT4 that we are using for an aero package. Dundon's car has/had Salter Aero Risers and their gurney so that was the baseline for us to improve on.

We used ANSYS Fluent for the analysis and can give details on that if wanted. I cannot comment on if the gurney or the risers help increase downforce or not as I have not run it without them. I would venture to say though that the gurney shouldn't hurt aero balance as the shift rearward would be pretty small. This setup below does make downforce overall (which is rare among factory cars in general) and is decently balanced. Aero balance has a mathematical target based on the CG of the car, but it is adjusted based on drivers feel.

Bill Lehman

I'm running stock front bumper/splitter. At the limit, the car understeers. I'm running -3.0 F and -2.5 R camber on Hoosiers with rear bar full hard and front bar full soft. Front ride height lowered to get stock rake with my small diameter rear tire. On my 987 I fixed it with springs and shocks. I don't want to ruin the GT4 "street ability".

4carl

On my first GT4 i played with the rear wing angle .With it in the high setting it cost about 3-4mph on the straight 117 vs 114 @ WS streets. Didn't feel any difference on the corners but i don't drive @ 10/10ths . carl

GAZZ

Dont forget to open the front flaps for more front end downforce.

ShakeNBake

Awesome data plucas!! Thanks for that pic.

I couple things I've noticed when playing around with the wing.

1) The higher angle of attack + front channel removal
- reduced my top speed T11-T12 at COTA by 3-4mph, which is a big deal.
- Allowed me to stay flat foot through T3 with zero worries.
- Could exit T19 3-4mph faster
- Lap times overall came down a bit, maybe a second.

2) Gurney Flap
- I didn't notice anything to be honest, but I didn't try reducing the angle of attack to gain speed back.
- The right thing to do would be to see if the effect is the same as raising angle of attack, and leaving the car in this configuration - probably optimal.

3) BBi uprights.
- The car is completely out of balance now. With no other changes, the car understeers so badly in high speed corners, that I can't go any faster. T16-T19 is a good example at COTA.
- No reduction in top speed, though hard to tell with the temps.

Bill Lehman

After a little internet seaching I found this: https://nasaspeed.news/tech/aero/air...e-whole-point/ Posted by Neil Roberts who wrote "Think Fast" which is a book I like and recommend. I'm going to try ShakeNBakes's suggestion and run the wing flat with the Gurney flap.

Yargk

This is a good idea, but in practice it's not always true. The plot below shows that for an example GT/sedan type car the optimal wing height is equal to the front to back length of the wing (distance from leading edge to trailing edge, called the chord), which actually isn't that high above the body. CL is lift/downforce, h is wing height, c is chord. The plot shows that h/c = 1 is optimal so the height should be equal to the length of the wing, not higher. For other cars higher might be better, but without testing, we don't know.






Interesting, do you think you're at the limit in the high speed corners?

Yes, but with the right rake I bet the lift on the front would be minimal. Both the front and rear have net downforce with the front tunnels open and the rear at the higher angle of attack. With a gurney the rear will have more downforce and the front a little less, but the front should still be net positive downforce.

The 997 cup car had very little front downforce and a lot of rear downforce. In fact, the numbers I'm looking up say 72 kg of downforce in the rear and 18 kg of lift in the front at 100 mph! Yet they don't take the rear wing off the 997. Of course balanced is better, with the same total downforce. However, I think more downforce might be better than less, even if it's rear biased. Just my feeling now based on what I see done on competitive race cars. Perhaps the driver might need to adjust how he/she takes high speed corners. I bet one could get on the gas a lot sooner with more rear downforce and the right line.

By the way, the GT4 Clubsport MR has a different rear wing than the normal clubsport that makes a lot more downforce. The normal clubsport has the same wing as the GT4 street car, but with a gurney flap. So in terms of rear downforce:

GT4 clubsport MR >> GT4 clubsport > GT4 street

Yet even though these race cars have a lot more rear downforce, the front is the same. And they are faster. (also note the S2000s at my local track days with huge rear wings that also run times similar to GT4s, with less power)

Awesome, thanks!!! Would you share the model?

Thanks for the info, that's very interesting. I'm surprised the BBi uprights make such a huge difference, but I believe it, the wing could be very sensitive to height. So you're understeering from turn entry to exit? Above what speed is it most noticable? What's your rake? Do you have dive planes?