We're running GTA's Limited class. Same restriction against flat bottoms. We are not running a diffuser (despite having one already developed for the car from back when we had a flat bottom). Without a lot of CFD-based design for your car, a diffuser without a flat floor to feed it is going to be nothing but ballast. I suspect that with the budget and resources to really develop a device for that under-trunk area that made the most benefit of turbulent airflow would end up not looking anything like a traditional diffuser.

Thanks. I was leaning that direction, but then I hear about guys like Hartman who've done it and had positive results in classes like AIX and it throws me for a loop. I'll probably give it a go at first with no diffuser to get the car dialed in (and most importantly, finished and out on the track), and then if I decide to fart around with the dibond sheet sitting in my garage, I will.

Rule changes for my class also have just allowed a diffuser to be used, and of course no flat floor is permitted. From a rough memory the start points etc sound about the same.

Because I love all things rice, I plan on making one, very simple with a few strakes to test as who knows it might make me think I have more downforces and I will simply trick myself into going faster.

All stupidity aside, I will post results around march if I get it made.

Your time would be better served making tire spats outside and underneath the car, checking fit on body panels, removing any extraneous stuff on the body that might cause drag, fitting a high efficiency wing/end plate and spending time on track tuning it.
But I guess you already know that.

I would have to agree with Emilio on the diffuser without a flat floor. Tim happy to drop off my previous diffuser for you to get some ideas.

A decent bumper cut reduces drag and gives more benefit than a diffuser, however a diffuser with the bumper cut may give you more rear downforce and reduced drag :)

Then again a different wing will give you more downforce for less drag.

Initial testing of the wing.

Agreed. I actually sold my R theory diffuser and just run no bumper now haha

A better guess would be that I know nothing, so advice is always welcome :)

Thanks for the diffuers offer, but I have 2 here to get ideas off already, and I'm familiar with the jetstream one.

Couple questions, if the increased acceleration was due to reduced drag, wouldn't you see the slopes of the two "lines" diverge as the speed increased?

Also am I looking at that right, is it roughly 15%, maybe even 20% more acceleration? Doesn't that seem like too large a difference?

Just trying to do a little rough math on this, if I've goofed it up or misunderstood, I apologize.

Oh, and m^2 seems like odd units for acceleration.

Epigam >> Am I reading it correctly when I see that you lost df rear with the gtc200 mount spacers? Or do "mounts" stand for something else?

Correct. The modeling for the GTC200 was based on data available, however it would appear that the data on the GTC200 is sans mounts. When the effect of the mounts are factored in the downforce is reduced.

1. No the lines would not diverge if the increase is linear, which it should be. The data is when the car is at WOT, the dips in the data are gear changes (130 into 4th, 160 into 5th), hence why acceleration starts to taper. Acceleration is the rate of change and relies upon velocity and time (A = Δv/Δt ). What could also impact the acceleration would be a change in gearing, power, mass, drag or weight. F=ma.
2. A 15%-20% increase in acceleration is significant if you are looking to get faster lap times. Bang for buck this is zero cost and gives a benefit (allegedly).
3. Hopefully point 1 explains some of the maths. Remember G's already is a product of acceleration (G=a/9.80665), hence instead of using the inbuilt ACCEL G function I created a maths Chanel called ACCEL = 'Accel G'*9.80665. This converts the Accel G into acceleration m/sec², then I smoothed it to 100.
4. m/s² is metric and is the rate of change :) Hence as the car goes faster drag increases and acceleration decreases this will eventually reach a point where the cars power meets the point where it can no longer overcome drag and acceleration stops. This will happen at about 240kmh for me based on power and gearing, however I need a really long straight to do this. To date the max is 202 (8,200 rpm in 5th, I still had 6th gear to go but had to brake).

What I can do is look at the declaration plot and if it is the same then ceteris paribus acceleration has changed, or I have changed the brake bias or stuffed up the braking :)

Hope this helps.

https://wikimedia.org/api/rest_v1/me...eaad7e3848824c
Aka aero drag = quadratic with speed. Not linear. You should see an increasing difference in acceleration with increasing speed. Your graphs show the opposite. There could be something very tricky going on with speed related drag changes (like maybe a rake change with speed), but the graphs look a LOT like a change in HP or tire drag to me.

15-20% drag changes due to bumper removal are way out of the ballpark. You'd be lucky to see a tenth of that.

Damir130 i’ll Export the data to some statistical software and do an analysis. Apologies for the linear comment, I kept the explanation simple. The racepak software is pretty limited in what it can do and to do what I have done requires custom pages to be written, it can be very clunky.

i do have sensors on the rea4 suspension, but these are not hooked up to the Racepak to be logged, rather they are hooked up to the Vbox camera.

I know the correct units for acceleration, your chart doesn't say m/s^2, it says m^2.

And as damir helped say, if aero, the force difference will increase with increased speed resulting in a larger difference in acceleration.

And my other point, even if we say that at the speeds shown in your graph that aero drag is virtually all of the limiting factor for acceleration 15% seems like too much. You are effectively saying that the bumper cut reduced TOTAL drag by 15%, I need more/better proof for such a large gain.

I think there is something else going on with your data.

If those are real plates, testing both could be as simple as getting on the highway with the data logger running and measuring full throttle acceleration, pulling the bumpers, and repeating. Same tires, same day, same conditions.

Again apologies for the chart saying m^2 instead oh m/s^2, will fix it up. I can no longer do Coast down testing as the car is not road registered.

i will look at the data again and see what else was different, I know that there was about 20 deg C difference in air intake temps (remember the car is NA not boosted). The wing angle was different by 2 deg, 12 deg with the bumper cut, 10 without. AOA is measured including the gurney flap. When I corner weight it, I can check the weights not to mention I can check the fuel level.

if I want to statistically prove this I would have gone about this a whole different way, what I use this for is to assist with the evaluation of changes made to the car.

Short of having a back to back test on same day, same tires, same conditions, and the only difference being the bumper style, I don't see all this data having a lot of benefit to prove or disprove a bumper cut being a benefit.

I have it done on my car.. its free.. I like how it looks.. even if it has a miniscule benefit to aero, its worth the free mod regardless to me.

Epigam - that is disheartening. Turbulence? Delamination over the ht? You do run a ht right?
I got the spacers as they where supposed to increase, not decrease df..

I believe you mis-read. His tests show the difference between a GTC-200 with no uprights at all just floating in free air (which is how APR provides their test data for that wing) vs. a more real world scenario of having uprights attached to the bottom of the wing. As expected, there is a loss in downforce due to the flow seperation and turbulence created by the presence of the uprights. Risers/spacers are not addressed here.

We already know the lowest drag and least lift configuration is the factory bumper, its less than the cut bumper and speed holes. Someone already tested it.

The Pass - oh, yeah that makes sense, and is expected. Thanks for the clarification.