Phi19

Again, TOP explanations and illustrations Paul It's fascinating, and best sound modelization I have seen so far. Many thanks.
I was asking about the rear diffuser, but would not refuse explanation on both rear and front diffusers actions and contribution to L/D.

Verus-Paul

Rear diffuser:
We do not target a specific angle or anything like that because in reality, the angle doesn't matter at all. There is a lot more going on than just the angle of the diffuser and I cannot give away all the secrets to how we design. The goal with the GT4/981 diffuser was twofold. First we want to make downforce, and second, we wanted to improve the flow field behind the car to help with drag. Downforce was made by using 3 separate tunnels, 1 center, and the 2 outer tunnels. The outer tunnels are symmetrical to each other while the center tunnel is unique. The tunnels are separated by polyurethane strakes. The polyurethane strakes are so they can hit the ground or parking curb without damaging the diffuser.

On the theory side, you want the throat of the diffuser to have a large peak low pressure which will help feed more air to the diffuser. More air = more downforce because you have more energy available. However, this is a fine balance between making downforce and drag. The larger the angle of the diffuser is, the more induced drag hurts. Hopefully, that explains most of what we were targeting on the rear diffuser. Actually, how the diffuser works full, we keep more tightly lipped. It took us a good long while and lots of testing to fully grasp since they are very complex devices.

Front diffuser:
Very similar to the rear diffuser on many counts. The main difference is our front diffuser is a single tunnel. The goal is to create a large pressure drop at the throat and that pulls more air under the car. This increase in airflow or velocity created a larger pressure drop and gradient from the top and bottom side. This will work the flat section much harder than just a flat section alone.

Bill Lehman

I just got back from Summit Point, other than a little contact with a deer, I found that running max wing angle without Gurney flap averaged about 2.5 mph more speed at the end of the straight then same angle with Gurney flap. Lap time was 1+ seconds faster. I tweaked the suspension and that may have helped the time. Flattened the wing Sunday but rain prevented a fair test. My first lap at speed was exciting in the brake zone for turn 1. Probably done for the year but plan to continue experiments with flat wing.

Yargk

One thought I had was that unless one is really within a 1 second or 2 from a pro time and consistently hitting those lap times (and this person is used to taking advantage of aero), these tests aren't really too informative. One thing that might help though is if there is some sort of data acquisition that allows for sector times and a "best theoretical lap" to be calculated. That might be a much better thing to compare because then you wouldn't have to really hit your absolute best on each of your test days, just make sure you nail each section individually sometime during the day, which is easier to do.

Bill Lehman

I'm not a pro but I can consistently run several laps within a few tenths and ran within 1 second of Randy Pobst's time at Summit Point when he was driving the TPC Racing prepped GT4. I do have the data and will take a look at "Theoretical Best" and report here.

Bill Lehman

I finally got a chance to look at my "Theoretical Best" @ Watkins Glen with various wing angle and Gurney flap combinations. High Wing Angle, No Gurney = 2:04.435, High Wing Angle with Gurney = 2:04.702, Flat Wing with Gurney = 2:04.027. Laps were done on different days and all on Hoosiers, non new.

ShakeNBake

Interesting. Thanks Bill.......and you are basically crushing my PB at WGI....

Bill Lehman

The problem with "Theoretical Best" is that you have to drive a perfect lap with no mistakes which I've never done.

pvgolfer

Thanks Bill for sharing your findings. Are you also able to look at sector times and compare top speed on long straights? What about high speed sweepers? It seems like those sections/sectors would exhibit the biggest differences contributing to the overall lap time differences.

Bill Lehman

Check my AiM data graph on post #31.

dfin23

Hey Bill, anything specific about how the car feels with wing up, no flap v wing flat, with flap?

Br,
Don

acey81

I'm quite disappointed that the GT4 doesn't produce any downforce from the diffuser. Seems like this is one area where there is a lot of gains to be made if you don't want to stick huge cup-like wings on the car.

Excellent thread, a lot of myths being debunked right here.

DR.S

The 981 GT4 does not have a diffusor, just a bumper cover which looks like, but in total the car produces downforce.
I guess, many points and there interactions are mixed upped in this thread: chord length, wing height, angel of attack, rake of the car (downforce does effect this, especially with "soft" springs), ...

However, if someone would provide a 981 GT4 CAD model (consisting at least of "real" surfaces, not just triangles) in .stp, .igs, .x_t format, I would perform some CFD analysis and share the results.
So I could bring more light into the dark
Calculations would also consider "moving" street and rotating wheels (I don´t know if this was considered in previous calculations).