jefnvk

I recently purchased a 1992 C4 with the intentions of making it an AutoX car. It also needs to be kept "sunny Friday to work" streetable, and may eventually move into some HPDE type stuff. I am not new to AutoX, but this will be the first time I've driven anything but a stock vehicle. I'm thinking that in SCCA, ST is where I would ideally like to end up (although I would also accept CAM). While I have looked over the rules on NCCC, I am not familiar with their competiton setup, and am not really sure how 2/3 plays out in comparison to SCCA competitiveness. I know I'm not fast, but I want to remain where massive drivetrain mods don't mean the difference between competitive and last.

Normally, I am a "use what you have until you know what is holding you back" type person, but a cheap car purchase means I am looking at all four corners of brakes and suspension needing fixed anyhow. While this is somewhat of a budget build, I also don't plan doing the "buy cheap, buy twice" thing. I'd like to get decent options under the car that will hold up for the next few seasons.

Brakes: if I am getting all new rotors and pads, is it worth just going to the C5 front brake upgrade now? Short of getting into a shift kit, it seems like the option that Could push me up the most classes, although I am not sure if the conversion fits ST's requirement of mounting in stock holes (via an adapter), and NCCC definitely takes me to 3 (the only thing to do so). Poking around eBay, I can get the adapters, calipers, and rebuild kits to freshen them up for around $300, which seems to push me in that direction. The car itself is already on C6 wheels (18 front/19 rear), so wheel clearance is not a consideration. Rears will remain stock in either case, but I am planning a DRM bias spring. Regardless of that decision, is there a particular rotor/pad combo people like? I see a lot of love for the Hawk HPS/HP+ pads, I don't see a lot of talk about particular rotors. Wheels and the want to do bushings while I have it up and ripped apart keeps me out of stock classes, so no need for stock consideration for brakes.

Suspension: the car has 170k, and is sitting on its original Bilsteins, which have about zero life left in them. I was originally considering just buying the off-the-shelf Bilsteins, but its only about $80 more to send them off to Bilstein for a rebuild and custom revalve. Has anyone gone this method, and is it a worthwhile option? I see KYB frequently mentioned as a preferred replacement option, but am I correct in understanding they are more for street comfort than any performance use? Any other options to consider in that $500 or less price range?

I think I've got the rest of my mods figured out myself, just not too sure on these. Thanks in advance for any insight!

69autoXr

While I don't own a C4, I've given thought to a C4 build for SCCA and NCCC crossover. With your car, my first priorities would be to get Z07 springs and bars, probably at least Bilstein shocks, and a set of 18x10.5 wheels for all four corners. Then with tires, you'd have to choose for SCCA whether you'd want to go to STU or CAM-S; STU has a tire width limit of 285 for Corvettes, while CAM-S is open for tire sizes. Myself I'd choose 315/30-18 Rival S's and run CAM-S. Assuming you don't have any engine mods like headers, cam or head work done then you'd have a competitive setup for 1SF/G in NCCC that could crossover to SCCA CAM-S. You can also do some limited aero stuff in Grp 1S that will crossover to CAM-S.

Big brakes aren't necessarily needed for autocross, many think that the base brakes are fine and going bigger just adds weight. If you want to do track days as well, then the bigger brakes may be more important. Otherwise good pads should be fine for autocross. I recently started using the Hawk Street/Race compound on my Z06 (same as DTC30), moving from HP+.

If you decided you want to go crazier in CAM-S, just realize that you'll be bumping up in NCCC to Grp 2 or 3, and those classes allow (and to a certain extent require, if you want to compete for FTD) DOT R-comps like Hoosier A7's.

jefnvk

Guess I should have tossed that qualifier on the classing too, I'd prefer to stay out of the A/R tire classes at this point. Even with a hefty Hoosier corporate discount, that isn't a cost I want to absorb now. CAM was pretty much only if I decided a shift kit was necessary.

I am a little unclear on the wheel sizing rules in NCCC though, as I read it I thought I'd have to bump to 2 with the C6 wheels. The redlined markup for group 1 next season mentions: "1984-NEWER - Any type wheel available according to the Stock Wheel and Tire Chart; adapters allowed", changed from last year's "1984-NEWER - Any type wheel available according to the Stock Wheel and Tire Chart for class & axle, +/- 1inch width, +/- 1inch diameter, 16 inch minimum; adapters allowed". Does that mean that any wheel size that was available as a stock size and is included on that list is now allowed?

If C5 brakes aren't a significant enough improvement, that simplifies this. Unfortunately right now, I have a new driver and a worn passenger front rotors/pads and rusted up rears coupled with fluid that looks like it came from 1992, they didn't feel awful but certainly aren't representative of how a new system would perform.

69autoXr

Ignore the redline rulebook for now, it didn't pass voting and will be resubmitted for 2020-21.

The wheel rules allow +/-1" deviation from stock diameter and width for the class. So for a 1SF/G car, which came with 17x9.5 x4 or 17x9.5/17x11 (GS) you can run on the front anything from 16-18x8.5-10.5, and on the rear 16-18x8.5-12.

Kubs

For brakes contact Ken at KNSbrakes.com He can give you good adivce on pads and rotors. For autocross you can get away with the stock stuff and just upgrade to a good pad compound (as mentioned). C5 brakes are not that much better, and not worth the cost or effort either. I had them at one point.

I also agree that factory Z07/Z51 springs and sway bars are a good place to start and some off the shelf Bilstein Sports are a great budget friendly shock that provides good results. VanSteel has Bilsteins as well as lots of good suspension rebuild components if you need to address some other things while you're in there.

jefnvk

Drats, guess I'll be forced to stick with group 2 then unless I go buy another set of wheels. Although if I did that, I could just drop back to SCCA stock, might not be the worst decisions either.

Also, as far as the NCCC, I am starting to look for club options. Am I correct in my belief, having looked through the brief "About Us" sections on the local clubs, that each chapter may have varying interests, i.e. some are more competition oriented while others are more social or show oriented?

jefnvk

Is there any worry in SCCA ST about keeping stock springs? My reading of the rule seems to indicate that spring changes are allowed so long as they remain original style/mounting location, but my understanding of that is buried in a paragraph seemingly designated for ride height. If so, what all would go into a full Z07 "package conversion" beyond shocks and springs?

Solofast

In SCCA stock you'd have to have a 6 speed coupe to use the Z07 parts legally, and you'd have to add the big front brakes that were part of that package. Shocks are free and the adjustable shocks are part of the package, but since you wouldn't be using them anyway, you don't need them. In reality the Z07 springs and bars are mandatory. The base springs are so soft that the car will just wallow around. Just put the Z07 springs and bars in, nobody is going to complain or protest. Don't go crazy, the Z07 bars and springs are as good as you will get, going with more bar in either end will upset the car. Use as square setup. Don't go with more tire in the back, it doesn't work. You'd need more bar or spring and if you do that it will jack in the back and be all over the place.

You've mentioned a shift kit and that isn't legal in stock either although I rode in RJ's automatic car back in the day and it shifted like it had one... We had one in our BSP car and you had to be really careful not to be cornering with your foot in the power if you ran out of rev's in first, it would make the back end snap loose in about a half a heartbeat when it shifted. We used to leave the line in first, bump the gear lever to 2nd and then just drive the car in 2nd almost all the time. Only if you were doing a pin turn would you go back to first. In the 6 speed cars we were all over the place, shifting up and down as required. I'd skip the shift kit, it's more important to get the car to stick and hook up than anything else.

The advantage to the LT motor is that it likes to rev with the 6 speed you're in first or second and can keep the rpm's up. With the automatic the first gear is the same as the with the stick, but second gear is really tall. With the L98 you would be fine down low in 2nd in most autocrosses, but with the 92 LT motor you're really in nowhere land down low in 2nd. The 93 motor got a better cam and had the same low end as the L98, but the 92 motor is pretty gutless down low. I know you've got a 92 and it is what it is, but a 93 would be a lot better, just say'n…

C4's don't have adjustable spring perches like C5.s but the rear spring bolts have a little bit of adjustment. Run the LR spring bolt as tight as you can get it. Take the RR spring bolt and make it as loose as you can get it and still get the cotter key in. The reason is that the car won't have as much negative camber in the LF wheel as the opposite side and there is more weight in that corner (All of these cars were pretty much the same, they all came out a bit "twisted", I've set up about a dozen of them and they were all the same). You want to get the car as balanced as possible and that will get it to be as good as it can be. Take out all the shims in the front and set the front to zero or a bit of toe out, and set the rear to 1.5 deg of negative camber and 1/8 of a inch of total toe in in the back.

Sticky tires and a proper setup is more important than anything else (assuming that you've got properly maintained running gear and brakes). You don't have the best car for the class, but if you set it up properly you can have a lot of fun with it.

jefnvk

Yeah, knew it wasn't perfectly ideal when I got it, it was more of a right place, right time, cheap enough to impulse buy it before I sit around for another year trying to find the perfect thing situation. I'm considering it a "starter" car to play with for a year or two, truth be told I'm not much of an automatic transmission fan in general.

Is there a particular spring code/spring rate (beyond Z07, an actual part designation) I would want to be looking for? It seems from a few random charts I have found, FSK was the front and NYU on the rear, does that sound correct? Are there other codes similar enough to them?

blackozvet

https://www.corvetteforum.com/forums...84-96-z51.html

Kubs

If you are trying to stay stock then no you can't change springs as solofast indicated. However I think you are allowed in ST, but I haven't read the rule book in some time for SCCA. If you go CAM you can run whatever you want.

NCCC group 1 and 1S you can do the Z07 conversion as well, but sometimes it is easier to get off the shelf springs in a better rate than try to source OE.

jefnvk

After looking over the springs, I'm not really trying to stay stock at this point, as it is going to cost me far more money to package-add suspension and to get legal wheels than to just play in ST with a pseudo Z07 setup.

I know I can probably stick whatever under there and not have anyone call me out, but I prefer to do things the proper way. The way I interpret that is I am fine with any spring rate in ST, was just getting confused about it being in the middle of a paragraph that started with ride height. The actual SCCA wording is:

"Ride height may only be altered by suspension adjustments, the use of spacing blocks, leaf spring shackles, torsion bar levers, or change or modification of springs or coil spring perches. This does not allow the use of spacers that alter suspension geometry, such as those between the hub carrier and lower suspension arm. Springs must be of the same type as the original (e.g., coil, leaf, torsion bar, bellows) and except as noted herein, must use the original spring attachment points. This permits multiple springs, as long as they use the original mount locations. Coil spring perches may be changed or altered and their position may be adjustable. Spacers are allowed above or below the spring. Coil springs may incorporate spring rubbers. Suspension bump stops may be altered or removed" (emphasis mine)

I looked at new springs, but with eBay takeoffs going for ~$150 and new ones going about three times that, its eBay or stock. I'm not cheaping out if there is a very real cost-benefit rationale, but I'm not just spending wildly either.

Thanks all for the help thus far! Beyond the springs/bars, I've decided to just keep stock brakes, with some sort of high-performance street/autox pads (like the sounds of R4S right now), and probably will have Bilstein revalve/rebuild my OEM shocks when I figure out springs I think that'll get me to a place I can play for a season and decide if I really want to stick with this car, or start looking for something more specific.

Kevova

Google Danger Zone Corvette. It's series of posts related to Vorshlag 92 Corvette build to run NASA TT. There is a lot of good information.

Kubs

Reading that paragraph I would say the RATE is free but you are correct that it has to remain a stock style leaf. No converting to coil overs or modifying the front to be height adjustable.

Sounds like to have a good plan. Springs, bars, re-valved bilsteins, and good pads.

jefnvk

Well good news after crawling under the car, I apparently already have a 93.1 spring up front, so only need to source a rear and appropriate bars

FAUEE

Dont waste money on the c5 brakes. If you car has the j55 brakes there is virtually no improvement, and since you already have larger wheels I would move up to c6 z06 brakes immediately, skip the c5 brakes, not worthwhile.

As far as shocks go, I have koni yellows. I am satisfied with them. In your case I would rebuild the bilsteins

l98tpi

Sorry, but it drives me crazy with brakes. C4 needs C5 brakes, c5 needs c6 brakes, c6 needs C7 brakes. It's all in the pads. Use the calipers you have and just get good pads; especially autox. Now if you were going to do 20minute lapping sessions, yes larger calipers would be nice because they dissipate the heat better. For autox, I use 12" front and 12" rear on my C4 with Hawk HP Plus pads, with my C6z I use stock brake hardware with Hawk HP Plus pads. Both cars have similar braking with the C4 probably more consistent because I don't experience ice mode as I do with the C6z if I get excited and stab the pedal.

C4 suspension; in its day was the latest and greatest but the geometry cannot match that of the c5 or newer. Not saying they cant be competitive because they can, just takes a lot more. With that said, stiffer is not always better. C4 stock suspension has two short comings; first is the lack of lowering capability and second is inability to get much negative camber. Try the car with what you got already and get as much negative camber on front as you can, 1/8th" toe out and for the rear around 1.5 degree neg camber and 1/16th toe in. Keep around 1/8th" positive frame rake too (front lower than rear). Get car corner balanced too. Along with alignment get some good shocks like Koni Yellow adjustable shocks. Then get your tires; Bridgestone RE71's or BFG RIval S.

Last item I'd add to your combo would be a rear spoiler. These really help the C4. And with your stock fuel tank, need to run more than 1/2 tank of fuel to prevent fuel starvation when cornering hard. Lastly, tire pressures are very important; if you don't know how to mark and read your tires then research and learn. The tires are the most important suspension component. Good Luck and I guess since I didn't mention it; the best upgrade is the driver

jefnvk

Got a response back from Bilstein that they don't service these anymore, so rebuild/revalve is out. Gotta decide if adjustable Konis is worth it over off the shelf B8s, or if it's just something else for me to screw up!

Hence, the asking for advice on whether it was worth it!

Thats the point of this whole exercise! I bought this because I was pretty slow at performance drivers training in powerful RWD cars at work.

blackozvet

If you can use single adjustable shocks I would definitely go that route - you cant really stuff it up, you either go up or down until you find the sweet spot ! Adjustable shocks are mostly rebound anyway with very little compression being added as you go 'stiffer'.

I used the single piston calipers for a number of years hillclimbing (which would have similar demands as autocross) and have only recently gone to c5 brakes - as the guys above have already said its all in the pads, my advice would be DONT use a ceramic pad for autocross - they dont work until they get plenty of heat in them - there are companies that make pads that are specific to autocross / hillclimb, they have good braking from almost zero degrees.

If you are going to use OEM stuff in the front, go with the 30 mm Z51 bar and FHB spring if you can get one - the extra rate will help with roll stiffness.

jefnvk

For now I am going to stick with the FHA that's on it. I'll get a NYU to match it on the rear, and the appropriate 30/26 bars. If that is really too soft still, then when I make my decision on keeping the auto v finding a manual, I'll add specific year and packages to my MTX search or I'll go buy actual new springs if I keep the ATX