Well, i've read Corvette Specs cover to cover twice, as well as Corvette from the inside twice. I'm in the market for a C4, mostly to autocross and road race. I will be keeping the car bone stock to race in stock classes.

SCCA puts all C4s in AS (A Stock) class. That means i might as well get the best handling C4 ever made to compete with the Honda S2000s and have a chance of being competitive.

I would like your opinions regarding the best C4 for autocrossing. A few of my thoughts are below.

Among base cars, a 90 Z51/FX3 or 91 Z07 seems like the way to go. From '85 to 90 there were continuous improvements, many of them brought about by the knowledge gained during the Corvette Challenge race series. So, the extra frame bracing, zero scrub radius suspension, and ZF 6 speed seem to be required, also the L98 was continuously improved so by 90 was pretty stout and reliable with lots of torque. I question the ability of the LT1 to come off the line as quickly as the TPI since the torque band is higher and the 92 and up cars are about 150 lbs heavier due to the LT1 weight penalty and the addition of traction control and some other options. Also by 90, stock tires were 275s all the way around. Would like your opinions, comments and hard data if you agree or dissagree.

The other two cars to consider, of course, are the ZR1 and potentially the Coupe Grand Sport with Z51.

I'm thinking the last year ZR1 would be the best handling/acceleration of all C4s, but am not sure...anyone remember any track testing done in any of the magazines between a Z51/Z07 TPI and a ZR1? In autocross, HP doesn't mean nearly as much as good handling and good torque. I don't really see how the ZR1 has a huge advantage over the Z51 90 TPI. While the TPI has 345 lb-ft at 3200 RPM, the LT5 makes 385 lb-ft but not until 5200 RPM. Also TPI was available with Z51 springs and shocks, while ZR1 received FX3. And ZR1 is over 200 lbs heavier than '90 TPI, and in the wrong places...anyone know the weight distribution of the ZR1/TPI? Obviously the big 315s of the ZR1 would be better in autocross or road race. The ZR1 is likely better for road racing where you can really get out and run the engine, but i'm not sure about autocross where top speeds don't always get over 60 and more often than not you are running 15-45 mph.

Then there is the GS coupe. Big tires of the ZR1, it was available with Z51 (276 GS coupes got Z51), makes 340 lb-ft at 4500 RPM, and weighs only about 50 lbs more than the '90 TPI.

thoughts?

thanks,
Paul

 

Hi Paul,
I know the flames will probably be out soon, but you should consider an 84 for your application. I come from a Porsche autocross/road race background, and consider this. 1984 = lighter weight, no ABS or ASR, yes you have to drive it yourself HA HA. Good botom end touque which is what you need to get from apex off the corner. Very high cornering G's in stock form (Z51). Quick ratio steering (2 turns) The car will be cheap because most people hate them. (Whats to hate)
Bob

 

'84 Z51 has the stiffest suspension. So stiff that many people complained and thats why it got softer over the years. The only concern I have is that if you have to keep the engine bone stock, will it be a healthy competitor? I'm kinda weighing the options, '84 has stiff suspension but only 205 hp, later LT1's have 300 hp but still handle quite well. You may want to compromise with an '85 as its got a lot of low end torque which is great for autocross. Its a shame you cant mod the '84 even a little.

 

The 85 doesn't have ABS, first year for L98 (as stated above) so you'll get some additional power and if you get the Z51 package, it has the fast steering ratio and the awesome super stiff suspension.

 

My votes on the ZR1 - if all C4s are in the same class, then you might as well go with the most output you can get. They handle nicely, and 405hp & 385 ft-lbs for 93-95 will dominate any L98 or LT1.

 

I do believe that to soften up the 85 Z51 over the 84, they softened the springs, but upped the swaybar diameter. You can still tell if a nickel is heads or tails if you roll over it. As far as the L98 getting better over the years, prolly not by much and the cars did get heavier.

 

I wouldnt recommend a ZR1 for autocross...Heavy, "touring" type springs, slower steering, etc. Was more of a high speed endurance runner rather than a quarterback to put it one way. Stick with the Z51 TPI cars.

 

Another good C4 to consider would be an 89 Z51 car. By 89, the suspension had been refined to where it had excellent handling, the ZF was a good tranny and autocrossers had sorted out the car to make it handle. The J55 13" brakes in front would be a plus. The 89 was a very popular car in SCCA back at that time and it was hard to beat.

Also, 275/40-17 tires on 9.5" wide wheels appeared in 88 as an option and were standard from 89 to '92. In 93, the front tires and wheels were smaller, but RPO Z07 included the previous 275 tire and wheel setup.

The ZR-1 would be a decent choice but IIRC, it never really was a strong competitor in SCCA when it first came out. The sheer cost of the car plus the extra weight up front were factors. And at low speeds and engine RPM, the LT5 motor really didn't have that much of an advantage. Tires would be an issue too with the 315's out back and the narrower 275 up front. With different size tires, you can't rotate them for longevity so expect to pay more for race tires more often.

You have to carefully check out FX3 cars; FX3 was an option by itself and got the base springs. Z07 cars had FX3 but came with stiffer springs, larger sway bars and poly bushing in the lower control arms. FX3 works, but it can be expensive to fix.

The trick in A/S is to use every advantange and loophole in the rules that you can. At local events, you could probably get away with mods that are hidden, but once you get to regional or national events, people start to take notice that your car is working better than it should.....

Don't rule out the LT1 cars if you can find a good deal. Z07 was still an option and the suspension was more "tuned" or refined. Traction control was standard equipment, but it shuts off with a push of a button. And don't forget the added 50 HP and extra torque the LT1 motor has. For faster courses, the LT1 would most likely offer a broader power band range along with a nearly 1000 RPM extra range. Brakes and tires will be the same size as the later L98 cars and 13: front brakes came with Z07 and were standard after 1994.

 

Wow...this thread brings a tear to my eye...

Finally a bit of love for the lowly '84.....

 

'89 Z51.

1) ZF6

2) 3.54 D44

3) Better cam, 113's

4) MAF

5) J55

 

Z07 is a better choice for autocross.

 

A stock has the s2k AND the Subaru STI. Those are some quick cars. When it rains the subies tear it up. Those honda's move on tight courses. Solstice GXP, Boxter, M3's NSX are classed there too to give you an idea.

I drive an '89 zf6 poorly and regularly get shown up by a 3rd gen camaro. He has a 305 auto, open diff and runs 205 50 15's because they are cheap. AX requires high skill specific to auto crossing. If your already good at it and you are starting from scratch I would go with a 84. Because lighter is better. An auto will probably be faster. There are a few c4's still ranking nationally and I believe (from memory) that they are mostly '89's.

 

Lower the front end with some lowering bolts. It's still stock then! Id do the rear if you want to. It will handle better and isn't a "mod". I reccomend BFG Pro force or Michelin Pilot tires.

 

89 to 91 Z51's have won more national championships than any other C4 in AS. Next are 96 Grand Sports (most are conversions from collector editions, but a few real ones). The nice thing about the Grand Sports (and converted CE's) is the rear wheels, no other non-ZR1 C4 got the 17x11's.

Whoever suggested ZR1 is completely wrong. Auto-x has nothing to do with power and everything to do with weight and handling. ZR1's all got soft springs and more weight up front, both of which lead to poor performance. Z07 LT1 cars do OK, but if you're looking at national title winners, 89 to 91 is where it's at.

There is an old thread on SCCA forums that covers this exact topic.

C4's are still very regionally competitive in AS; if I I'm not mistake one won nationals back somewhere around 2006.


C4's don't have bolts in the front end, and lowering the car in the rear on non-factory bolts is not SCCA Stock class legal. If you're not racing on Hoosier A6's (or some other R compound), you're doing it wrong for auto-x.

 

FYI, a '90 (or '89) with FX3 and Z51 will have the base spring rates and anti-roll bars. It will have the other Z51 equipment, though.

You could not get the stiff springs and anti-roll bars with adjustable shocks until the '91 Z07 package.

For the 1992 MY, all suspension packages got softer, including the Z07.

I've no idea how all this pans out on the track. But 1984 Z51's have by far the highest spring rates. The brakes got a little bigger in 1988. Then I believe bigger still in 1990 with the J55 option. Also in 1989 a 6-speed manual transmission appeared.

Edit: Sounds like you got your answer in the post above mine. Well, everything except GS's being the only C4 with 17x11" wheels...

 

Not being partial because I have an 86, but I thought on one of these types of threads, another very popular year with many wins was the 86 Z51 auto - which was light, and had thicker sway bars or something....

 

Oops, forgot about ZR1's. Good catch, I fixed my post.

 

Thanks for all of the lively responses!

Looks like i had done my homework in narrowing it down. While the '89 is a bit lighter than the '90 (32 lbs according to Corvette Specs) i actually prefer to have ABS II and the Airbag as i'll be driving the 'vette to the events so prefer it to be as safe as possible. Also '90 upped the HP a bit with the same torque and had lighter wheels than '89s. Lastly, the '90s had the sloped radiator that supposedly has the lowest radar signature of any production car ever made in America.

Regarding the stiffness of the springs in the '84 Z51, while it is true they were the stiffest put on C4s, according to Dave McClellan in Corvette from the inside, the floppiness in the chassis that was put in at the last minute when someone in management demanded a targa when the car was designed as a T-top hampered the car, requiring the super-stiff springs. In '87 they added the bracing developed for the '86 convertible to the Z51 and Z52 coupes, which allowed them to soften the springs dramatically while improving handling. For this reason i would not consider a C4 earlier than '87 for racing. Then of course you get the ZF in '89... and in '90 you get 5 more HP and 10 more lb-ft (and ABSII and the airbag as i mentioned earlier).

The question remaining for me is...does anyone know how the '90 Z51 and '91 Z07 compare? Anyone recall any road tests from back in the day?

Thanks Again!
Paul

 

I agree on everything you said here...but I've been really confused over the years about everybody's FX3 problems.

I've owned my car since new. I ordered it with exactly what I wanted on it. I now have about 60k miles on it, and have had to rebuild my shocks once, replaced 1 actuator and replaced the controller with a Rippie unit with the modified chip in it (only because the shocks were valved to Rippie specs and needed the new controller to make them work right).

The car got flogged on the track for 5+ years, hitting every autocross in the Southwest I could afford to go to.

That said, your driving style will need to change based on the car you have. You'll find the limits to where the car can go based on your skills as a driver. The 1991 Z07 package is killer...I was spanking cars in Group 2 with nothing but a poly suspension upgrade (since the bushings wore out from too much aggressive driving) and everyone else had engine/suspension mods. The Z07 rewards a smooth driver with very fast times and loads of grip.

I can also agree with a few previous statements about the ZR1. The extra weight on the front end really hurts the ZR1. I've known a few guys to mod the car to Z07 suspensions, but they are still handicapped by the additional weight in the nose of the car. Understeer is a big problem with the ZR1, but if you do track events where it can have some legs to go it will be fun to drive.

True Z07 cars are hard to find. You'll need to search and confirm it with RPO codes to make the car a "legal" Z07 for SCCA. I have not been involved with SCCA for a long time now, but one area why the Z07 worked so well was the high durometer lower control arm rubber bushings. Unfortunately, they are not available from GM. Poly is the only other choice you have, and for the class you want to race is it legal?

Good luck in your search...

 

i know...and there is one on eBay right now:
http://cgi.ebay.com/ebaymotors/Chevr...fCarsQ5fTrucks
But i'm not a fan of the white... or a Pennsylvania car for that matter. But with only 700 made, you have to wait awhile to find the next one if you pass over one...