l2yeP

Searched and didn't find anything. I've been thinking a lot about selling my Z06 and finding an FRC and heading to STU. Mostly because I like to tinker and mod.

The boost buggies will all have intake, tune, exhaust, 9-9.5 inch wheels with 245s, sways, and coilovers.

What will it take to make the C5 competitive?
Throw street tires on your SS setup, adjust the alignment and go?
Coil overs aren't allowed on the c5.
Stiffer front spring?
Sways front and rear plus a stiffer front spring?
Wheel and tire size? 285 rear on a 10.5? What for front?
Don't do any power mods because you wont get it to the ground anyway?
Seats are obviously a no brainer!

thoughts?

hklvette

I'm also building for STU ('00 coupe).

I'd use T1 suspension + poly bushings. Any stiffer would probably be wasted effort.

Tires: This is a tough one, and depends on if you want to run a staggered height setup or not. 265+ width on all four corners for sure. I'm using base C5 rears all around because I'm cheap, but C5Z rears would be better. I'm also looking at AD08Rs in 265/40 and 265/35. If you're willing to spend the coin, I'm sure there's a better setup than that.

Power: You can still put full power down when going straight, especially with the MN6's long 2nd gear. You can always give it less pedal. See also: Mark Donohue

Alignment: Less camber than used with R-comps. On a firm suspension, less than 2 degrees on both ends should be enough, but I don't have data to support that theory.

Ramo7769

I have an FRC that is stock now but I have I have a few different sway bars and springs I'll be messing around with. I'm going to tune springs and sways with my stock dampers and then upgrade damping from there. I'm not in a rush to swap control arm bushings.

For wheels and tires, I believe there is something to be had from wider front wheels than what is allowed in SS. I have a set of 4 C5 Z06 rears that I'll be putting 285/30-18 Dunlop Direzza ZIIs on. Compared to track w/slicks setup, I believe one would benefit from less static negative camber and more negative front toe (out) to simulate ackermann for low speed cornering. Correct me if I'm wrong if someone has different thoughts on this. I'll probably leave some performance on the table with alignment because I'll have a compromise-setup for running the track.

I know I'm leaving power on the table. We can run long tubes w/cats and intake pre-TB in STU. I don't doubt for a second that more power will improve performance until you're leaving black marks between all corners. My FRC doesn't even do that in 1st gear with all seasons on it. I have to say that I believe people underestimate the grip of today's street tires. I have to believe that those who say a Corvette on street tires is like a pig on roller skates haven't driven on competitive street tires since the 1980s.

The very hardcore guys may raise rev limiter to get more speed in 1st. We don't have that lovely M12 2nd gear which is a big part of why we're allowed in STU now, I believe. I'm not hardcore enough to risk my engine for more parking lot performance, however.

The car is new to me so I'm going to see how much fun running it in STU is compared to track days and TTs. I know it will be cheaper . But I'm still deciding on a direction for my car.

brkntrxn

You guys must be talking SCCA?

STU in NASA is a run-what-you-brung class.

hklvette

Yes, this is for SCCA Auto-X.

strano@stranoparts.com

Having driven an A-stock car back to back on Hoosier's and Rivals I have some take on this. All the normal things apply. I disagree with less camber. Tires are still subject to load and flex. Yes, there is a bit less load, but also these aren't R-comps type construction tires either.

I have really no idea how things will go in STU with the C5. It should do well as it turns better than anything else there. However, it doesn't have all wheel drive. And street tires suffer more than R's in getting power down. Which leads me to the notion of the "terrible" gearing I hear time and time again...

Is the car geared tall? Yes. But you can only get down what you can get down. Example, the day I tested back to back, a section I could run flat on A6's I was nowhere near flat in on the Rivals. The car doesn't stop as well, go as well, or turn as well. Ironically it suffered LEAST in the turning department.

The cars I've been working with I've done to a slightly stiffer front spring, and gone more moderate on the front bars vs. what I've been doing on the SS and AS cars the last few years. And the fastest ones have been running my setups/parts.

As for what to do exactly? No definitive answer, it's very if/then. Different tires like different sized rims. For instance where a ZII likes a little squeeze, the Hankooks don't, so that will effect your rim size (or it would for me). A bigger rear bar? Maybe... not sure it's required and might be ok on some tires, but not on others. Despite my hate of how "street" happened and such, I am quite familiar with the tires overall, and C5 and C6's both too. Trouble is this is a whole new game, and when you add so many variables like multiple tires on multiple rims with multiple spring and bar options with multiple drivers on different surfaces it's gonna take some testing to work it out.

Many won't want to hear this, but good shocks will be even more key on street tires because those tires are more sensitive to how they take loadings, and response isn't as good either on streets vs. R's. The streets are getting better and better which is part of the joke of trying to mandate the speed of a tire, but they aren't R's. Another test we ran involving 4 drivers of different levels on a Z0K Solstice non-turbo with 3 street tires and Hoosier's showed definitively that the 4 drivers were closest on the Hoosier's. The less grip the tires had the further ahead I was vs. the other guys. And two of them are trophies winners at Nationals the 4th has been close but hasn't done it. They aren't terrible drivers at all, even the slowest of them.

I'm around to help, keep it in mind.

talon95

The best setup has to be 285's all around. I suspect stock C5Z rears would be close to the best width. For example the 285 RS3 is fairly well stretched on a 10.5" rim. I had those year before last.

If you're in a warm climate, I'd go with the RS3. It's a 35 series, so not so crazy short as the Dunlop. Keep in mind though that the RS3 V2 is coming out in the next couple of months supposedly, so buying now might/might not be the best way to go. Not particularly cheap though.

Otherwise you're pretty much stuck with a 275 as I don't see another 285/35/18 on the TR site. A 275/35/18 is a pretty short tire too. I have a pair of the Dunlop ZII's in that size. They look small on the 18x10.5.

Dave G.

RAREBLU1

Been thinkin about trying STU myself this year. Been running SSM, so Ill have to add catalytic converters add put the stock throttle body back on. The one thing I wasnt sure about the the crossbar I've got for seat belts, if its not legal what a good alternative to help hold yourself in the seat.

ltborg

A harness bar would be legal in ST.

Matt_27

I've been playing around with the idea of an STU C5 for a while now, just haven't seen this thread yet. Some background, I've been autocrossing a Solstice GXP on street tires the past two years so I've tried RS-3s, Rivals, and Z2s and have some idea of how well each works for autocrossing and how well each puts down power.

So far I think I have a vague picture of what an STU C5 setup would look like, any input would be greatly appreciated:

-First question is Coupe vs FRC, FRC is 80 lbs lighter but doesn't have the LS6 intake and some other engine updates, probably wouldn't have a problem lighting up the rear tires anyways though. Undecided on this, I'll probably just go with whichever once I find thats a good deal and in good shape first.
-Penskes/Ohlins/MCS/Koni... any preferences? I live about 30 minutes from Ohlins' NA headquarters so that would be convenient for me.
-Stiffer front spring, T1? Would Z06 be enough? Aftermarket?
- Z51/Z06 rear spring?
-Strano or other adjustable front bar
-long tubes/cats/cat-back, more so for weight reduction than power increase (what's the lightest exhaust?)
-race seats, light battery, lighter crank pulleys?
-4 rear Z06 wheels with 285 Z2s or 275 Rivals
-Wilwood rotors w/ aluminum hats for weight reduction
-T1 FUCAs, or are poly bushings on the stock ones just as good? Worth it to add poly bushings to any of the other control arms?
-better shifter
-I have my Solstice set to 3-3.5 deg front camber, 1.5 rear, so I'd do approximately the same thing. That seems to give good wear and even use of the tread for autocross.

Now that I've asked so many questions, I thought I'd share some work I've done looking at the thrust capability of the C5 and putting down power compared to some other STU cars. I used Jason Rhoades' excel sheet, found here: http://www.rhoadescamaro.com/build/?page_id=591

I can provide more info on the assumptions for this chart if requested. The C5 torque curve is from a stock LS1, and the STI curve assumes a 100 octane tune (talking to some top STU guys it sounds like they are making about 350 wheel torque). I imagine with bolt ons the C5 curve would look more like the STX camaro torque curve.


As you can see, at 30 mph the C5 has the same thrust as my Solstice. I am running 245/40-18s square and while you do have to be careful with the throttle on corner exit it is by no means difficult to get a good exit, especially on Z2s. I think with 285s we can still be fairly aggressive with the throttle. There's nothing like the thrust available to an STI coming out of a slow corner, but we should be able to carry a lot more speed through the turns, and once above 50ish mph they lose their thrust advantage. Also, once 93 octane tunes are mandatory it should close the low speed thrust disadvantage significantly. For low speed local lots we could even keep it in 1st for most of the course, and then it's just a case of getting the car pointed straight and shooting to the next corner, which is how most of the boost buggy STU cars seem to be driven anyways.

Ramo7769

Insightful post. I have some questions and comments.

What is the y-axis on your graph? Is that assumed power limited acceleration in terms of g?

In response to your question about springs, I have quite a bit of brake dive on street tires in my FRC and I think there would be some benefit to upgrading front and rear. Just to clarify, the Z51 and Z06 front spring is the same part # and the rear spring is where they differ.

For my setup, I'm going with Z06 rears and 285 ZIIs, so I'm with you there. In my parts bin I have some VB&P springs, 1130lb/in front, 830 rear and a mess of sway bars. I'm thinking it may actually be too much for street tires and the car could benefit from some more compliance if the site is bumpy.

The only thing you mentioned that seems off is static camber. That's more than I'm used to seeing for C5s. But I do not yet have personal experience. Perhaps the C5 has more camber gain than the Solstice?

Matt_27

Correct, the Y axis is supposed to be longitudinal acceleration but I think it might be more useful to just think of it as a relative number for comparison. I do see similar numbers to that though when I am logging longitudinal G's in my car with Solostorm.

Is there a resource somewhere that lists the various spring rates that were available in C5s? Then we could add some aftermarket options and look at the various ride frequencies one could achieve and the different possible "magic numbers" as Optimum G puts it. As far as the springs being too stiff, we should be able to use STR cars as a baseline, I know some Miatas and S2000s are running something like 700# fronts, 400# rears, so assuming equivalent motion ratios that would be 1400 and 800 lb Vette springs. I'm sure they aren't the same motion ratios but that gives an idea at least of what works with other RWD street tire cars.

For camber, yeah I run a lot but it's true it depends on the car's camber curve and that's less of a fundamental setup parameter, it can be tuned later.

Ramo7769

Now this is where leaf springs are a PITA. There doesn't seem to be a standard method of reporting rates. I dug through the internet looking at rates. For reference, the common spring rates you see for OEM C5 Z06 are 525 lb/in front, 714 rear. The T1 springs are a similar balance, but with higher rates. Mind you these are rates I've found on the forum and I have never seen them from an official source.

I asked VB&P some questions and here was the response: Now, motion ratios aside, I would argue that this method mentioned above may not be accurate to how the spring acts in the car since the spring is not mounted in the middle. It has two mountings offset from the center. So, I would expect a higher rate than what VB&P is reporting. What may be useful is to determine an equivalence between the reported spring rates and the associated ride rates or wheel rates. Ride rates might be easier to come by. But, I only have means of very rudimentary methods of determining this.

hklvette

Bumping this topic to see if any new data has come to light.