89BP

Starting a C5 SCCA Solo Autocross STU build with my 2002 6 speed Convertible. I thought we could start a thread of resources/information to help those building cars for the class. As of right now, I don't know of any fully built STU C5's. I know of at least one about to be built. This means we are starting from scratch, no formula to follow, like an STC Civic. I have built 4 ST cars over the years, first was a STC Civic Si, then an STR MX5, next an STR S2000, followed by another STR MX5. This is my 6th Corvette, 1st C5.

I went with a convertible for one important reason, I like convertibles. The weight difference between a pop top coupe and a convertible is only a couple of pounds, 35lbs difference when you pull the top out of the coupe. FRC will be about 75lbs lighter. I went with the 2002 for the increased power and torque.

Let's help each other out, by giving what parts we will be running, and where you found them. Post up pics of custom built items, like exhausts, seat mounting, etc... Post up results against other ST classes, to track progress, STU needs to outrun all other ST classes.

Right now my car is completely BStreet legal, but I have my first STU part coming, wheels. I found a set of 18x11" Forgestar wheels, should see them late next week. I will be running 285/35/18 Hankook RS3 V2's.

Parts to research/order next, shocks, springs, sway bars. What rate springs? Where to source them? Shocks, do I need shorter bodies? Will Koni singles be a good start? Sway bars, which adjustable front bar? Rear bar, go smaller, or bigger? Who makes a good adjustable rear bar?

Future mods. Headers, high flow cats, custom built exhaust (I want to build a single 3.5" system, more torque, lighter weight, cheaper, unique sound) Seats, what fits? What mounting hardware is needed? Cold air intakes, which one makes the best power? Is it legal for STU (meaning, doesn't involve cutting/removing panels). Pulleys, do they make more power? Are they safe for our motors? Diff., is an aftermarket diff needed? Or will the stock diff perform well enough? Bushings, which bushings for which component? Will a light weight battery cause electrical problems?

I am sure more can be added and talked about. This is a good start for the discussion. I had great resources for the Civic, MX5's and S2000, let's continue that trend.

I am always up front about my car mods, I believe in sharing, I want everyone to be at their best at all times.

Jeff Cawthorne

hklvette

I'm slowly building mine towards STU as the toy budget allows. Currently, all I have is:

1130lb VB&P front spring + '00+ FE3 rear spring
FE1 front bar + '00+ FE3 rear bar
Rear wagon wheels on both ends
AD08Rs: 265/35 front and 265/40 rear.

Currently, my car is well balanced and puts power down well, but I would prefer to have more bar to keep it flatter during cornering and increase rear lateral grip by keeping the outside wheel below zero camber. To that end, I have a C6 Z51 front bar waiting in the garage that I took off when the bigger front leaf went in. It'll go back in once I get a rear bar similar to the LG G1 piece next year. I'll adjust from there as needed to get the over/understeer balance right again.

As far as I know, the vararam is one of the few STU-legal aftermarket intakes that pulls air from outside of the engine bay and doesn't require any cutting. That is assuming that my memory is correct that you can remove the block-off pieces in the nose like the Z06 is missing from the factory.

You forgot what is probably the most important item: SEATS. As I'm sure you're aware, SCCA requires a functional roll-bar to use a harness' shoulder belts (if applicable). A more supportive seat may be all you need, but its worth mentioning.

A big front spring will help a lot with curing the nose lift/ dive seen with stock C5s under throttle and brake application, at least it did on mine. I'm sure opinions will differ on how much rear spring is needed to go with it. Also keep in mind that there is some question/ doubt about VB&P's spring rate measurements. I expected the front leaf I put in to be three times the rate of the FE3 leaf that came out, but it certainly doesn't feel or act like it. It seems to be much lower than listed, but still higher than what came out.

As for results, the car is still faster than me. That said, a nationally-competing/ fully prepared STU WRX was at our event this weekend, and my first runs with the new spring and bar setup were one second off of his pace on a 35 second 1st-gear-only course. I'm sure with more seat time I can close about half of the gap.

my $.05

I'm curious what Lane has to say...

89BP

No update/backdate allowed, and Z06's aren't classed in STU, so we won't be allowed to remove any panels. I had to deal with this, twice, with my STR MX5's, the AEM intake required removing a splash shield, so I had to cut up the intake to make it fit legally, above the splash shield.

A better seat is high on my list, most likely after doing the chunky go fast parts, springs, sway bars, and shocks. As you know, the stock seats blow. I would like to go with the seats I had in my STC Civic, Sparco Corsa's, I really like their high thigh support, great for holding you in. I won't be running a harness bar.

Springs is my big grey area, I have no idea where to start. I run on bumpy, slick, asphalt, mostly, so I will need to be a bit on the softer side. I prefer a softer set-up, I don't like nervous cars (heavy spring rates). My car is a Z51, and I am shocked how soft this beast is, and is the number 1 reason I am already wanting to move from BStreet to STU.

I'm curious what Lane has to say...[/QUOTE]

Me too. And what he decides to run.

ltborg

Is it? The rule techinically reads as follows:

The air intake system up to, but not including, the engine inlet may be
modified or replaced. The engine inlet is the throttle body, carburetor,
compressor inlet, or intake manifold, whichever comes first. The
existing structure of the car may not be modified for the passage of
ducting from the air cleaner to the engine inlet. Holes may be drilled
for mounting. Emissions or engine management components in the
air intake system, such as a PCV valve or mass airflow sensor, may
not be removed, modified, or replaced, and must retain their original
function along the flow path.

The bold section is the most interesting. The Vararam does not require structure modification between the air cleaner (filter) and the engine inlet (throttle body). All modifications are before the air cleaner. You also aren't adding additional holes so per rule 14.2.B:

Factory rub strips, emblems, mud flaps, bolt-on front valance lips/
spoilers, and fog lights (except those integral to a headlight or turn
signal) may be removed. Rear wings may be removed so long as the
vehicle retains any federally-mandated third brake light.

Technically, the covers are fog light mounts/vent covers. Does removing the fog light mean we can remove the whole assembly or just the actual light? That will be the determining factor on whether the Vararam is legal or not.

89BP

We will need to get a rules clarification. I would like to see the installation instructions, to see what else needs to be done for the VaraRam to fit. If even 1 panel needs to be cut or removed, it's out.

ltborg

Then it's out because you have to remove the fog light mounts.

ltborg

The other intake I like is the Calloway Honker, but that is also not ST legal. I think the only thing that would work is something like a K&N or a Halltech of some sort.

As for my build, I'll post a separate thread on it because I will be going a very different route from what I would recommend for most people. Here is the generic build that is cheap(ish) and will get you most of the way there:

CAI
Off the shelf header kit
Z06 mufflers
Tune
SEATS
Z06 Rear Wheels
B-Street suspension with a stiffer front spring
Poly bushings

A more moderate build would include better versions of everything above. For example, maybe double adjustable MCSs or Penskes instead of Koni singles, 18x11 wheels instead of 18x10.5s, and/or lighter mufflers that still make sound.

Either of those should get you a very competitive car locally that is easy to drive and easy on the wallet (relatively speaking). It should also be competitive at most National level events.

I plan on starting from the ground up for my build and am in the process of researching/calculating suspension geometry changes, changes to track widths, etc. and will likely end up with a car much stiffer than most people will want and with much more in the way of modifications than most. I'm still running through all the modeling which is why I haven't posted anything yet. The goal will be to figure out which options give a gain outside of measurement variability so when I attempt to get the car dyno classed in NASA, I stand a better change of not ending up in an aero class due to excessive modifications.

89BP

Are you planning on running the stock Z51 rear spring? Thoughts on the Z06 rear?

I have been looking at headers. Big question, is which ones will meet our rules about cat location? We can remove the front cats, and can replace and move the rear cats (replace with a high flow 100 cell), but only 6". Some of the headers I have seen extend rearward pretty far, far enough to question their legality for us in STU. Anyone have any details on the different headers and where their flanges end up?

ltborg

For a simple build, either would be fine. While we'll be making power similar to a C5Z, the gearing isn't as good so under that logic, I don't know that it would be worth the effort to put in a spring that's just a tiny bit stiffer. I plan on being way stiffer, but that's just an initial guess right now for my weird setup.

Most headers should work. The front of the new catalyst (actual material in the cat) needs to be six inches away from the end of the old catalyst. I'm not 100% sure, but I bet most header setups will do this, you just have to put the cats right after the collectors and have an X-Pipe after the cats as opposed to the other way around.

shrddr

FYI: Vararam VR-B2 installation instructions... http://www.vararam.com/VR-B2_instructions.pdf

hklvette

We will need clarification from the STAC on the legality of the Vararam I think since the rule Lane highlighted looks a little open to interpretation. The way I interpret it, if I can remove the fog lights I should be able to remove the brackets since they serve no other purpose. Others may not see it that way.

The change from the late Z51 spring to the Z06 spring is tiny. If you find yourself needing more rear spring, it would be best to step to at least a T1 leaf.

I think you're going to see two schools of thought on preparing a STU C5 and it isn't clear at this stage which one is better: One is to fix the deficiencies in the C5 chassis one problem at a time (add front spring, bars to match, etc). The other is to take a fully decked-out SSP C5 and undo all of the mods that are illegal in ST.

89BP

I agree, we will need clarification. Doing an intake is way down my list of mods, so if someone is wanting to do one now, I suggest writing a letter to get the process started.

Does anyone have a spring rate chart? I have no idea where to start from. All I know is, the Z51 springs in my car now are way too soft, I would think at least triple the rate of my stock springs would be a good start, but don't know what that is. This is one area where I will miss having coil-overs, springs were easy.

93Rubie

In because this is interesting and your my kinda of people!!!

ltborg

Not even VB&P are triple the rate. We are limited in our choices. Basically there is VB&P and Hypercoil (plus other stock GM springs). Here's the list from a thread on here:

Springs:
* C5 Z06: 526# Front, 714# Rear
* C6 Z06: 531# Front, 782# Rear
* C6 Z51: 526# Front, 645# Rear
* T1 (C6): 582# Front, 850# Rear
* C6 Base: 420# Front, 657# Rear (VBP: 400-420 F / 600-620 R)
* T1 (C5): 582# Front, 793# Rear
* Pfadt Coil-overs: 425# Front, 575# Rear (Remember: Wheel rates!)
* Hyperco HPS (Street): 565# Front (12405HPS), #765 Rear (12406HPS)
* Hyperco HPT (Track/Solo): 625# Front (12407HPT), #850 Rear (12408HPT)
* VBP Sport 990# Front, 650# Rear
* VBP Extreme 1050# Front, 855# Rear

ltborg

I should add, not all of the softness feel is from the springs. The chassis to being with is on the softer side of things, and you will feel that. Obviously the tires are fairly soft too (remember, these cars were designed with OE run-flats in mind so they relied a lot on the stiffness of the tire for total roll/ride stiffness).

Bushings actually have a surprisingly big influence on the overall feel. You'd be surprised how much tuning you can do to a car by changing the bushings, including putting different materials in different positions within the suspension. Here is an example of trick bushing design from the McLaren F1, an example of a dual rate subframe bushing on a Honda, and an aftermarket dual rate bushing. Remember that the dimensions of the bearing have an effect on it's spring rate just as the material properties of the bushing itself.

Aside from bushings, the overall car design gives it some inherent "softness" feel. Some of what you are likely feeling is actually the poor inertia of the car. While it's not as bad as something like a 911 with the engine hanging behind the rear axle, the C5 isn't optimized for rotational inertia of the platform. The designers put the transmission at the back of the car to achieve the undoubtedly marketing specified 50/50 weight distribution. I won't argue that a good weight distribution is bad, because it's not, but there is a significant difference in inertia of the platform if the major mass centers are organized like a dumbbell rather than a more centralized mass as in a mid-engine platform. This is one of the reasons why manufacturers continue to push the wheels further and further to the corners of the car; all things being equal, it improves the rotational inertia of the platform. A longer wheelbase also improves the ride dynamics and high speed handling/stability. That said, we can't do anything about this in STU other than design around it.

The damping will have an effect on the stiffness feel as well. Getting higher pressure dampers on the car can help a little bit, even if you keep the damping curves constant.

I'll be the first to say that I think stiffer springs are necessary, but I think most people would be best suited to adding a stiffer front spring only and running a stock (or close to stock) rear spring. This will result in very progressive power application on corner exit. It should also allow for 60 foot times in Pros in the 2.2s range, maybe even 2.1s if 285s on an 11 are as good as I hope they will be.

troyguitar

Are you talking about NASA road course time trials?

If so, C5's run in TT3 or above so your mods have zero impact on classing. Just stay at 8.6 lbs/hp (weight with driver, hp at wheels on dynojet) or above and you're good. You can do anything else you want to the car.

93Rubie

Avoiding Thread Hijack-Addressed via PM

hklvette

They won't on classing, but will on the allowable lbs/hp IIRC.

ltborg

I need to look into the rules again to be sure, but last I checked, they allowed dyno reclassing based on the dyno curves and modifications list. I specifically am NOT trying to end up in a power to weight only class. If that's the case, I'd need 335 slicks on 13" rims and full aero. That's not fun. Primarily this is an autocross car so if it ends up being autocross and track days instead of autocross and time trials, so be it.

ltborg

No. A shorter wheelbase WILL change direction faster. At higher speeds where smaller inputs result in bigger changes in lateral g, shorter wheelbase cars get harder to drive because they rotate so much faster than a longer wheelbase. OEMs don't go to autocross to prove their cars and set records, they go to tracks, where a longer wheelbase creates more stability. There's a big difference between building a track car and an autocross car. This is why the BM tend to be older DSR cars (shorter wheelbase) but current DSR cars are much faster on a track than the old ones.

All things being equal, I'd rather have a C4 sized car than a C5 sized car to autocross because a shorter wheelbase is helpful on small, tight autocross courses and the C4's denser packaging of the large masses makes it more progressive in rotation than the C5. That said, the C5 has superior rear suspension design, a much stiffer chassis, a better engine, etc. so all things are NOT equal. With the C5's high inertia, it is harder to get the car to rotate as well as harder to stop the rotation. Once you get it rotating, you have a lot more mass to stop than in a more traditional layout. Think of all the oversteer issues Porsche has had to design around because of the huge amount of mass at the back of the car. This is why the early 911's developed their reputation for sudden and stronger oversteer. It's the same thing in the modern Vettes, just on a less dramatic scale. Hopefully that clears things up.

Also remember, as mentioned above, all things aren't equal in the C4 vs C5 case. The inertia issues come into play when you can design a car from scratch. The Cayman was faster than the 911 for this reason (until Porsche retuned it to keep the 911 on top). Given we can't get into moving things around in STU, I'll just take the benefit of a nearly 50/50 weight distribution and deal with so,e extra inertia in driving style.