RX-8 in NASA ST5 / TT5 Thread - RX8Club.com



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Old 12-12-2017, 06:00 AM   #1  
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RX-8 in NASA ST5 / TT5 Thread

Hey All,


I want to open discussion on people's approaches to NASA's new ST5 / TT5 class. As opposed to the previous points-based TTD / TTC classes, this class doesn't use points but uses power to weight ratio modifiers. There are also fewer discreet penalties, although there are restrictions in place to keep budgets in check.


https://www.nasaproracing.com/rules/...ng_5_rules.pdf


Overall the rules feel a lot more open, but the 0.7 A-arm penalty will give BMWs a distinct advantage. I'll share my anticipated setup below, and include some comments and questions.
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Old 12-12-2017, 06:17 AM   #2  
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So my car has a few options from where it currently sits (NASA TTD): drop weight, or add aero (I can't really gain much power, because... well... rotary!). The two options are below (note that i'm using the opposite sign [+ / -] that NASA uses in it's rules because I think credits should be minus, they allow you to have a better power to weight ratio):

Lower weight option:
Weight 2885
Avg Power 199
AWPR 14.50
-----------------
Tires 0.0
Brakes 0.2
Suspension 0.0
Aero -0.4
Weight 0.0
A-arm 0.7
(so the calculation is 14.0+0.2-0.4+0.7 = 14.5 adjusted weight to power ratio)


Aero option:
Weight 2965
Avg Power 199
Raw WPR 14.90
-----------------
Tires 0.0
Brakes 0.2
Suspension 0.0
Aero 0.0
Weight 0.0
A-arm 0.7


Either way, I think 40TW race tires are the way to go. -0.3 is not nearly a big enough modifier for running >100TW (even though I would absolutely love if they restricted it to >100TW).

So basically, on my car:
100TW tires are only worth a 60# credit
Brakes are only a 40# cost
Aero is an 80# cost

But I don't want to gut my car (I have full, comfy interior because I drive to and from events), so I can't shave much weight anyways. Figured i'd try to spend it on aero. Stock brakes would be easy to run, but my fastbrakes kit shaves a bunch of unsprung weight off the front end and the pads are cheap.


So, my questions for the group:


- Is adding aero or dropping 80# likely a faster option?
- I'm currently running the OEM exhaust manifold. Is it worth the squeeze to consider a header? My 199 Avg WHP estimate above is optimistic; i'm likely below that value.

Last edited by hufflepuff; 12-12-2017 at 06:21 AM.
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Old 12-12-2017, 06:23 AM   #3  
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Some resources on Aero. The S2000 guys seem to find a LOT of time with a rear wing. My car is on the looser side, so I think the extra high speed stability would be a big help.


https://robrobinette.com/S2000Aerodynamics.htm


https://robrobinette.com/S2000RaceCar.htm



http://949racing.com/apr-gtc-200-wing.aspx
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Old 12-12-2017, 09:16 AM   #4  
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It will help a lot, can’t be understated. I’d recommend the APR GTC200 with Gurney lip rather than 80lbs

Last edited by TeamRX8; 12-12-2017 at 09:19 AM.
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Old 12-12-2017, 09:21 AM   #5  
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Man, I would love it if some more durable tires were an attractive option. I'll be moving to Hoosiers for 2018. No point in fighting that one any longer.
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Old 12-12-2017, 10:07 AM   #6  
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I'll be doing 5 in 2018. I'll be similar to you huff in running full interior. I decided for aero on a pig of a GT. I planned for a strong rebuild so hopefully that generates good numbers.

Thanks for starting the thread
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Old 12-12-2017, 10:22 AM   #7  
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my first track car was s2k, drove it for 3 years. short wheel base and snap oversteer like crazy. can hardly push it without a rear wing. i just got my rx8 last month and only did one track event with it fully stock on all season tires. but the longer wheel base definitely makes it feel more neutral than an s2k. i'll have to wait till the build is done this winter and drive it on rcomp to decide if i want to run with rear wing or not. but i ill try to set it up to run without rear wing if possible. i'd rather take the 80# less weight.
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Old 12-12-2017, 10:25 AM   #8  
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i might set the car up to jump between ST5 and ST4 by changing tune, add or remove ballast, and or remove aero, change tires size between the two class base on entries at each event.
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Old 12-12-2017, 02:10 PM   #9  
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Quote:
Originally Posted by hufflepuff View Post
So, my questions for the group:

- Is adding aero or dropping 80# likely a faster option?
- I'm currently running the OEM exhaust manifold. Is it worth the squeeze to consider a header? My 199 Avg WHP estimate above is optimistic; i'm likely below that value.
I've got the GTC-200 (no gurney), and I like it but can't say how much it's really adding (haven't done proper tests/tuning to know yet). The car feels very planted, not nearly as loose as before, but a lot of that is in the better alignment I did.

One thing I should note, the car was damn fast when it ran in PWC in their TCA class (2:37 at Road America, 1:36 at Mid-O). This would be dominant in TT5, and it did so without a rear wing...and some mighty fine slicks. I'm not saying that's a slam dunk nor doubting Team's observations, just noting the car's potential.

If it came down to choices: wing, weight, tire - go with tire all day long.

Of course, I need to man up and drive it that well...and for some of us (read: me) that may mean the added stability of the rear wing. What someone else can do in the car doesn't matter if I'm the one competing in it.

What I don't get is your anticipated weight. How are you pulling 2885 with a full interior and streetable car? Remember, your curb weight isn't what is used, it's your competition weight - that's with you in it rolling across the scales. My car is gutted, caged, lexan fr/rr windows, hacked up doors, trunk, hood...and still it comes in around 2840 with me in it and a couple gallons of fuel. I'm adding a passenger seat in lieu of ballast, since I'll get more use out of the seat. Just amazed if you're rolling across scales at that weight w/driver and fuel.

Last, thanks for starting this thread! Looking forward to seeing how folks are approaching the new class.
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Old 12-12-2017, 02:58 PM   #10  
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Take it off and you’ll find out. It may depend on your angle and spring rates to get the most out of it; planted high speed, rotates easily low speed. They have some good data posted (or used to) about the most effective angle for max df and min drag.
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Old 12-12-2017, 07:04 PM   #11  
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Old 12-12-2017, 07:36 PM   #12  
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No worries actually doing/understanding the testing, I just haven't gotten around to it yet. Been busy blowing up engines and all...

=)

For point of reference, the wing is currently set at 0deg AOA measured on the center of the wing. Since the rear window is in reasonably close proximity, the effective AOA is greater than what's measured. I don't have a wind-tunnel, so I couldn't say what it is exactly. But the point of a twisted element wing like the GTC-200 is to accommodate the airflow's true angle as it comes off the roof-line and down the rear glass w/o stalling the wing, all the while allowing the sides to run at their set/optimal angle relative to the free flowing air stream they see. So currently, no worries of it stalling or needing a gurney to really cheat its onset.

I figure starting at 0deg AOA is a pretty benign starting point while I get used to the car. It's not causing the car to push at speed right now, so I'm just trying to learn to drive the thing in this modest state. I'll start cranking up the wing to test once I grow more confident in my read of the car's basic performance characteristics...really is so much different than the Mustang.
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Old 12-12-2017, 07:54 PM   #13  
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Well the first step of a good ST5 build will be selling your RX-8 and buying a 4 door E36 sedan with McFailson strut suspension. Lol...
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Old 12-12-2017, 08:18 PM   #14  
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ouch ...
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Old 12-12-2017, 08:59 PM   #15  
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I would again encourage you to review the modeling data in detail and do the analysis. Running at 0 is essentially taking the majority of the drag hit without the df benefit. More so running without the gurney flap. It might make your car understeer, but that’s exactly what happened the first time it was ever used. You have to figure out how to use that to your advantage. I hinted at it a bit earlier. Aero is another level of engineering. You have to rethink chassis balance from a static 1/4 corner process to a dynamic/variable level.
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Old 12-13-2017, 12:27 AM   #16  
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Originally Posted by Arca_ex View Post
Well the first step of a good ST5 build will be selling your RX-8 and buying a 4 door E36 sedan with McFailson strut suspension. Lol...
Looks like I'm going to need a bigger rear view mirror in 2018 just to keep track of all those BMWs behind me.

On the aero data, it actually shows quite favorable results at 0AOA with the highest L/D ratios of any tested angle. That's typical of most airfoils; they become increasingly less efficient at a higher AOA up to the point they stall, at which time they're just producing drag.

I hadn't really paid much attention to the gurney CFD results. With the gurney in place the wing still has very good L/D ratios at 0AOA, and you can even run it with negative AOA (for very impressive efficiency). What really surprised me is just how much more df is produced with the gurney for very little drag penalty. So...definitely something not to ignore. Thanks for the heads up.

I think I still have a "baseline-then-at-speed" mentality with regard to aero. I've always thought the ideal would be to tune a car for mechanical handling, then develop an aero package that evenly grows df (for the car as a whole) in the same proportion fr/rr so as to maintain handling characteristics (i.e. maintain the overall center of pressure close to position of the car's center of gravity). Since we're not allowed active aero, the only shifts in the center of pressure will come from pitches in the car under braking and acceleration - and since we're talking about the mighty torque of a rotary, we're really only talking about pitches under braking. My springs are stiff enough to greatly reduce that angle, so I'm (poss. naively) writing that off as inducing minor shifts in the center of pressure.

To be honest, I hadn't thought about purposefully aiming for (or begrudgingly accepting) increasing imbalance as speeds grow as a way to optimize an aero package. I don't doubt that this might result in an overall higher performance envelope, but I shy away from the notion because...well, it requires greater driving acumen. IOW, as you point out, the driver has to adjust to a changing dynamic balance. I'd rather drive a more steady car that doesn't change under me as speeds increase. Slower? Maybe, but I have greater confidence in the car, and that makes me faster.

Keep preaching though brother, I'm open to new ideas (esp. those that reveal my errors).
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Old 12-13-2017, 03:25 AM   #17  
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I’m not trying to tell you how to setup your car. This is just my personal viewpoint that anyone can consider or toss. Certainly feel free to express your own.

If you just look at the ratio it appears as you stated. However, the magnitude of the drag number is very low. That’s the shape of a relatively efficient wing. So it doesn’t take much of a change in drag to influence the ratio. If you instead look at the actual numbers and compare how much df is gained relative to how much actual drag is. Yes the drag is increased with steeper angles, but what is the actual magnitude of gain vs loss? Ultimately it boils down to cornering speed vs straight acceleration; if you can go in deeper and faster into the corner and then further ahead coming out than your competitor is it enough to not be re-passed by same going down the straight before the next turn? Obviously that will vary by track;DF is less a factor on a grippy track than a slick one, few corners and long straights vs the opposite, and so on.

I also wasn’t referring to active aero devices. Aero in itself is active in that it varies by speed and a few other factors. It influences spring hz factos that don’t get considered in the static 1/4 corner model. Rake needs to be considered, ride height static vs aero loaded, and so on. It’s a bunch of work, but the chance of stumbling across what’s most effective is not likely without going through the effort. I’ll leave it at that.

Not sure why you’re going through engines but that obviously would matter more at the moment.

Last edited by TeamRX8; 12-13-2017 at 03:30 AM.
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Old 12-14-2017, 06:56 AM   #18  
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Quote:
Originally Posted by R(ace)X-8 View Post
I've got the GTC-200 (no gurney), and I like it but can't say how much it's really adding (haven't done proper tests/tuning to know yet). The car feels very planted, not nearly as loose as before, but a lot of that is in the better alignment I did.

One thing I should note, the car was damn fast when it ran in PWC in their TCA class (2:37 at Road America, 1:36 at Mid-O). This would be dominant in TT5, and it did so without a rear wing...and some mighty fine slicks. I'm not saying that's a slam dunk nor doubting Team's observations, just noting the car's potential.

If it came down to choices: wing, weight, tire - go with tire all day long.

Of course, I need to man up and drive it that well...and for some of us (read: me) that may mean the added stability of the rear wing. What someone else can do in the car doesn't matter if I'm the one competing in it.

What I don't get is your anticipated weight. How are you pulling 2885 with a full interior and streetable car? Remember, your curb weight isn't what is used, it's your competition weight - that's with you in it rolling across the scales. My car is gutted, caged, lexan fr/rr windows, hacked up doors, trunk, hood...and still it comes in around 2840 with me in it and a couple gallons of fuel. I'm adding a passenger seat in lieu of ballast, since I'll get more use out of the seat. Just amazed if you're rolling across scales at that weight w/driver and fuel.

Last, thanks for starting this thread! Looking forward to seeing how folks are approaching the new class.

I've got something of a lightweight special: no TCS/DCS, no sport package extras. absolute base model 6MT. I have full interior, stereo, A/C, etc. I cross the scales at 2960-ish at 1/4 tank with 190+ pound driver. I guess it's a combination of a low starting weight and intelligent weight savings along the way without compromising comfort.


I still have the heavy OEM flywheel and a rather heavy RB DR midpipe.


But you're exactly right, which is why I think i'd prefer to go the aero route: no way I can get down to 2885 from 2965 without compromising comfort / streetability. I'd have to remove stereo or A/C or interior bits, and i'm not looking to do that at this time.


Agree with you that tire trumps all (except good driving...).
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Old 12-14-2017, 06:59 AM   #19  
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Do you guys have experience with the BHR longtube? I've reached out to a guy who "helped develop it" and uses it for autocross, and he said to believe the claims. But really, 15 HP? seems too good to be true.

it's my impression that shorty headers are only worth about 4HP. this is that last easy area I have to gain power and want to be intelligent about my choice.
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Old 12-14-2017, 08:11 AM   #20  
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I don’t want to go there in this thread, but my own experience and analysis are well documented on that subject

The one point I’ll make, but not respond further on, is that the Renesis is quite unique for exhaust flow modeling theory. Unlike almost any other IC engine there is no overlap between intake and timing port events (6* negative overlap timing actually). This is significantly different than what typical theory is “based” on.

There is some performance to be had, but not for the reasons most people use traditional theory to justify it with





.

Last edited by TeamRX8; 12-22-2017 at 05:49 AM.
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Old 12-15-2017, 08:40 AM   #21  
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Since it was just brought up, what ended up happening to Charles? I've noticed that he deleted a lot of his posts and is banned now?
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Old 12-21-2017, 04:05 AM   #22  
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Quote:
Originally Posted by TeamRX8 View Post
I don’t want to go there in this thread, but my own experience and analysis are well documented on that subject

The one point I’ll make, but not respond further on ...
I did mention more here in response to another thread, pretty much sums up what I learned back in early 2006 and have put into practice ever since:

https://www.rx8club.com/rx-8-racing-...0/#post4846510



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Old 01-11-2018, 02:27 PM   #23  
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Might be participating in TT5 w/ our 8 this year. What tire/wheel sizes are you guys planning on running?
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Old 01-11-2018, 02:34 PM   #24  
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Might be participating in TT5 w/ our 8 this year. What tire/wheel sizes are you guys planning on running?
With the new nasa section width measurement of 266, I think it has been confirmed from multiple people that hoosier r7 245/40/17 on 9" wheel will definitely pass. Some 9.5" wheels has also been confirmed but the Gap is much smaller and not all 9.5" wheels are the same
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Old 02-22-2018, 06:54 AM   #25  
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for TT, a 225 tire might be quicker than a 245. probably 245 for ST due to greater heat capacity.
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