vettehardt

I've kind of been in the same boat as you, but I am trying to stay in NCCC 1S/1 group with my 91. It is an automatic base model coupe that had the FX3 suspension and 193k miles on it. The adjustable shocks were long gone when i bough the car a year ago and a set of the Z51 replacement bilstines were installed. The car had the stock springs and sway bars on it. I bought a set of Falken RT615K+ for the stock 17x9.5 wheels. I have no idea at this point where the alignment is set (probably have it checked next spring). I installed hawk HPS pads. I am not happy with the brakes, but not because of the pads. I am not sure but i might have a bad booster or master. My brakes work better when i let up off of the accelerator for a second before i stab the brakes. I think it is loosing vacuum in the booster.

When I first started, the car was very, very loose. I would have to catch it in every corner to keep it from spinning. I did some research and found the 88-90 Z51 suspension is supposed to be the best for these cars. So, i started to try and acquire parts. I found a front sway bar (30mm), and rear spring (NYU). My car being a 91 already has the larger 26mm rear bar. The Z51 front spring (FHB) is nowhere to be found, used stock or aftermarket. Nobody is making aftermarket springs for the C4 at the moment since VBP went out of business. I have since installed the front sway bar and it made a big difference. However, it has now swung to the slightly tight/push side of things. I plan on doing the rear spring before starting to run next year. I am hoping it will balance it out a bit. If it still pushes I may lower the rear bar down to the 24mm.

Unfortunately many of my plans are put on hold since i am currently out of work. Hopefully I can find a new job and save some money for next year. My future plans include the rear spring, and 18x10.5 with 315 Rival S all the way around. I do want to upgrade to the J55 brakes with the 12" rotors also. If i can ever find a FHB spring, i may even try that also. Even with the setup i have now, i am still one of the faster cars on street tires at events. I have even thrown on some hoosier A7s and claimed some overall FTDs at some of the smaller, shorter tracks we use.

As far as brakes go for autocrossing, just remember this. If you can lockup the wheel and tire and get into ABS, there isn't any need for bigger brakes. They just get you into the ABS quicker. Proper brake modulation will serve you better than bigger brakes.

BrianCunningham

Lots a great advice here
I'm running CAM currently
but finding something in NCCC would be tough

jefnvk

I know the L98s put out a bit more torque, but doesn't sound like the auto hurts you much. I really need to look into what sort of Hoosier discounts I get.

Just out of curiosity, what do you do for your numbers? Are they permanent or removable? Didn't cross my mind til yesterday that the magnets I used on my Mustang don't really work anymore...

Heck yeah!

I suppose I should ask if anyone has stuck a "race" seat in a C4 as well. The original sport seat is pretty ripped up on the bolster and Corbeaus and Sparcos street/performance are far cheaper than OEM replacements.

Solofast

You need to get rid of the 26mm rear bar. If you look at Hib Halverson's suspension chart you'll see that the 26mm rear bar was only offered in one year with the stiff rear spring. That was, actually a mistake and they got rid of it the next year. The reason is that these cars have too high a roll center and because of that a combination of too much rear spring or too much rear bar causes the rear suspension to jack during hard cornering. ZR 1 cars use that bar, but they use it with softer springs and can get away with it. The 91 Z07 cars were purely evil handing when sticky tires are used. I had one and ran it in SS in those days. I ended up putting shims under the rear bar mount to reduce the preload on the rubber and soften the bar. Hey, you gotta do what you gotta do.... What happens is that once the car starts to corner, if you're off the gas or under any braking the back lifts up and starts to jack and you'll be looking out the side window or the rear view mirror to see where you are going. If you can squeeze on some gas it will plant the rear end, but you can't do that on every corner, and under trail braking the car was dangerous. Even with stiffer springs up front you'll still have the problem. We lowered the heck out of our BSP car where we tried the 26mm rear bar to see if the reduced ride height and more wheel offset would keep it from jacking. That didn't work either. We softened up the rear bar bushings and so long as you powered on it was ok. After half a season of effing with band aids we got rid of the 26mm bar and the car ended up sweet. If you don't have or can't get the 24mm bar you can put some shims under the rear bar brackets to reduce the rubber preload and that will help some, but it isn't as good as getting a 24mm bar.

There are several versions of Hib Halverson's C4 suspension chart out there and some of them have a mistake in that they show 26mm rear bars on the 1990 Z51 cars. That is a mistake the 1990 Z51 cars actually had the 24mm rear bar. The 26mm rear bar was only fitted to 1991 Z07 other than ZR1 cars. Notably, the chart on Corvette Action Center has this mistake in it. Below is a link to a correct chart.

http://www.netmotive.net/articles/hib/c4/sustunch.pdf

vettehardt

I use permanent vinyl numbers. I have a vinyl plotter and cut my own numbers for my dad's, brother's, and my car. We've all been leaving our numbers on so I haven't had to make any new ones until i bought my 91 last fall. Honestly, cops don't really pay any more attention to the cars on the street as long as you aren't doing something stupid.

I've been trying to find some inexpensive removable vinyl for a while now. I know sites like izoomgraphics.com sell some, but i haven't found the vinyl needed for a "reasonable" price to try it out.

If you want to go cheap and easy to peel off, use contact shelf paper and cut the numbers with scissors. It doesn't have as sticky of an adhesive and comes off easily after a weekend. However, i have found if you leave it on for a couple of weeks, it does get hard to remove without leaving some stickiness. It will clean off with goo gone or goof off though. Permanent vinyl numbers will also come off easily after a weekend if there is a good coat of wax on the paint. I was cutting about 10-12 sets of numbers a year for my dad when he was removing them after each weekend. He now just leaves them on the car and doesn't worry about it.

vettehardt

I'm going to have to check which bar i actually have. For some reason i thought all 91s got the 26mm bar. Since mine is a FX3 car, it might only have the 24mm bar. Checking will have to wait until next spring when it's warm though as my car is sitting outside with a cover on it (my 76 is in the garage). I did already have that chart you posted. It's sitting right here next to my computer. Just haven't looked at it in a few months until now.

jefnvk

I ended up picking up a set of original 17x9.5 wheels cheap tonite. I'm keeping the C6 wheels and tires for street use, I do like the way they look, just easier to deal with a square setup and be able to rotate them for something I'll beat on.

Tirewise, is it worth moving up to 285, or just stick with 275? I've made the decision I'm not going to play in R or A comp land this year. Not much cost difference in the street comp sizes, just less choice.

Kevova

There are far more tire options running 275s opposed to 285s. IMO 275 would be the better choice.

69autoXr

Solofast

The biggest problem is getting sticky tires in a 17 inch wheel. Other than the Falken or Toyo Proxes, there aren't any 275/17's available anymore. I had 275/40/17's in the Hankook RS3's and I was really happy with the grip level and tire life, but these are not available anymore. Even the newer RS4's don't have anything wider than a 255 in a 17 inch rim. The Hankook's wear like iron, much better than any of the other 200 UTOG tires. If I were doing it I'd get a set of 18 inch wheels and get a set of 275/35/18's and go with the RS4's. The Hankook's also have a construction that doesn't wear excessively on the inside when you're running negative camber on the street. For that reason you can run them with a reasonable amount of negative camber and the tire will still last on the street and you aren't going to give up any performance. That and now you don't need a separate set of street and track tires makes this an ideal setup. I've been amazed at how well the RS3's lasted. You buy rims once, you buy tires as they wear out. After years of buying 3 sets of Hoosiers a year the performance and life of a good 200 utog tire is pretty refreshing...

jefnvk

Understood, but the biggest reason behind no A/R tires is it is a cost I simply don't want to pay right now, the 200TW top options aren't really that much better when it comes to price. Can't remember if I mentioned it earlier, but I get a sizable corporate discount from Conti/General in addition to Hoosier, and going to the A7s may well be cheaper for me than the other 200TW options anyhow. Until I'm ready to jump up to Hoosiers it'll likely be the ExtremeContact Sport or Gmax RS. I cant justify going from $300-400 for a set 275 Conti/General to the $1000 range it seems to cost to get into the 200TW for my first year.

Also, this can put Street class back in play for me if I leave the springs and stainless brake lines alone for the first year.

Kevova

Although not mentioned previously you need to verify or install 3.07 (G92) the base 2.59 axle ratio is sad and is only good for super speedways. It's ok in first but car won't go in second. 700r4 3.06 1st to 1.68 2nd kills it. LT likes rpm but beyond 3.07 puts car in SM or full on race car

Solofast

Even with a discount, getting about 40 runs out of a set of HoHo's makes them an expensive proposition. We found that Kumho's and Hoosiers were about the same in terms of life, after about 40 runs both of them fell off, the Kumho's fell off faster, but if you're trying to run with the big dogs, it doesn't matter, after 40 runs you're needing new tires. If you run more than a dozen events you're looking at 2 sets in a year and that's pretty expensive. Since it didn't seem like you were looking for the last increment in performance the Hankooks, with more than a season of life on a set makes them a lot less expensive in a long run. If you only have a single driver you'd get more than a season of serious autocross on them. While the 200 TW tires aren't that much less than race tires, if you don't have to have 2 sets (street and track tires) as well as the fact that they'll last more than a season and still have decent grip makes it much more affordable. It's the number of runs you get out of set of tires that determines the cost, not so much the initial price

jefnvk

Just a random thought too, anyone know when SCCA is releasing the 2019 rules? Doubt much will change, would just like to read them over.

It's the 2.59. I've been keeping an eye out for 3.07 replacement, unfortunately I haven't found a source besides watching eBay or changing out the entire rear axle. Anything besides 3.07 will take me at minimum to SM, which I certainly don't want to be.

A dozen events a year would likely be near an upper limit, I do have other hobbies and interests. As short as the racing season is in Michigan thats roughly every other weekend anyhow!

I am keeping the second set of wheels, honestly I like the looks better, and I can toss a nicer touring type tire on them for commuting and cruising.

jefnvk

Actually, forgot a bushing change takes me out of street, and remembering that I ended up ordering a new VBP rear spring for the rear to match the FHA up front. It is marked k=369, which puts it to about 64n-mm, if my math and understanding of VBP product markings is correct. Bit stiffer than the normal Z51, but for it being the price that used NYUs are going for on eBay (and cheaper than a couple still on there), I felt it to be the better decision.

For my engineering knowledge, I understand that increasing either sway bar will effectively increase turning on that end of the vehicle. What happens if the bars are balanced, but both are either undersized or oversized? Bars are one thing I am thinking of leaving alone until I have a few runs and then make a decision on.

Solofast

More rear bar increases oversteer, more front bar increases understeer. If both bars are a bit soft but your are balanced you just lose a bit of overall stiffness. Not a big deal.

The front bars on these cars are relatively stiff, and are mounted in relatively soft rubber bushings with some compression preload on the bar bushings. You can fine tune the bars on these cars by tuning the stiffness of these bushings to adjust the stiffness to be "between" bar sizes. In the front you can put alignment shims between the two halves of the front bar bracket to soften the bar. Don't use more than 1/8 of an inch of shims as this will let the bar be too loose. If you want to stiffen the front bar put a big fender washer between the bushing and the bracket. This will increase the rubber preload and stiffen the front bar. Put a tie wrap on the shims or the washer and you can quickly loosen the bar mount, pull out the shims and make a fine adjustment to dial in the car between runs. Same thing in the back, if you find the rear end is upset under trail braking or on corner entry it is likely that you have too much of a combination of bar and spring stiffness. To address this loosen the rear bar and slip a 1/16 shim between the clamp and the chassis.

You can also tune with poly bar bushings, but with those you only get a big increase in stiffness and no adjustability after that...

jefnvk

OK, yeah, that makes more sense than how I worded it. Poly bushings were something I was going to do, but most of the kits don't include sway bars due to sizing variances, is that something you would suggest leaving rubber (for the bars only)?

blackozvet

25 year old rubber is probably not the best if your going to track your car (might be ok for street use)
I know that Solofast has had success with shimming rear bushes to alter the bars effectiveness, but that tuning is after all other mods have been done and the car is sorted. You can still use that method with poly if you wanted.

Get the poly bushing in the size smaller (if you cant get the exact size) and hone it out (use a small brake cylinder hone in a drill) till it fits the bar just right.

jefnvk

Yeah, not using the old ones, I just see that new rubber bushings are available for sway bars, was wondering if it were a better material for that particular location.

Unfortunately the spring sale fell through, but since that freed up a chunk of money this month brakes are on their way! Went with plain basic rotors with Porterfield R4-S pads, and I'll keep watching for a decent NYU or another deal on a VBP.

blackozvet

rubber has too much compliance, the sway bar isnt as effective while it waits for the compliance of the bushing to be taken up (under cornering).
poly is a much better bushing for a sway bar, and delrin is even better. Ive got poly on the rear and delrin on the front of my car.