Setting Your C4 Up for Auto-X
11-28-2013, 10:37 PM
#1
Safety Car
Thread Starter
Setting Your C4 Up for Auto-X
This took a good bit of time to write and get together. I hope CF appreciates this short paper and if a moderator deems it worthy I'd LOVE to see this as a sticky for future C4 owners to use as a resouce.
That being said, please look it over for accuracy, typos, and other stuff. Be prepared to say WHY somethign is wrong, factually. I've made it clear much of this is based on converstations I've had with other forum members and my own expierence. If you've made some other setup work for you, that is great. However, a newbie may find this to be a GREAT place to start and go from.
See the attachment for a copy of the document. Feel free to distribute or print as you wish. I did this with NO intent of any payment or compensation in any form. These cars are so old now that the secrets are no longer secrets, so no sense in NOT putting this on paper.
Updated: Word Document, to reflect jacking issue. I feel it could confusing, so I eliminated it. The important thing is that you be aware that having too much rear roll stiffness compared to the front can/will cause funny handling. Shocks play into this some as well, but I want to keep this fairly simple. Beginners need a starting point.
That being said, please look it over for accuracy, typos, and other stuff. Be prepared to say WHY somethign is wrong, factually. I've made it clear much of this is based on converstations I've had with other forum members and my own expierence. If you've made some other setup work for you, that is great. However, a newbie may find this to be a GREAT place to start and go from.
See the attachment for a copy of the document. Feel free to distribute or print as you wish. I did this with NO intent of any payment or compensation in any form. These cars are so old now that the secrets are no longer secrets, so no sense in NOT putting this on paper.
Updated: Word Document, to reflect jacking issue. I feel it could confusing, so I eliminated it. The important thing is that you be aware that having too much rear roll stiffness compared to the front can/will cause funny handling. Shocks play into this some as well, but I want to keep this fairly simple. Beginners need a starting point.
Last edited by 93Rubie; 01-19-2014 at 08:05 PM.
The following users liked this post:
Tyler Graney (03-21-2023)
11-28-2013, 10:38 PM
#2
Safety Car
Thread Starter
Text in its entirety: Of the OLD Word Document Version for History Sake. See the above doc. for updated version.
Setting up your C4 Corvette for Auto-X
So you’re a 4th Generation Corvette (84-96) owner and you’ve got addicted to auto-x. Now what to do about the car? How do I make it faster and handle better? This article will seek to summarize and detail just how to do that but first some back ground.
In case you’re not aware the C4 has been a highly successfully auto-x platform since its introduction in 1984. Most notability Roger Johnson won many of his national titles at the wheel of a C4. The car was a winner in SS, AS. As well as BSP at the national level. The car was competitive and winning national championships well into the mid-2000’s. Jeff Cashmore won in 2006 in A Stock. Don’t ask me how I found this, but I hope it remains active as it is a great model for a stock class C4 this WAS Jeff’s 89 C4. Jeff Cashmore's 89 C4 Z51 The last coming in 2009 in AS when Scott McHugh’s 89 Z51 car took A Stock that year. The car has finally fallen out of favor with most national competitors moving on to newer machinery, many of which are now driving C6 GS’s and C5Z’s. While the car may no longer be as fast or favorable for national level competition they are still quite quick and fast enough to be competitive at the regional and divisional levels. Overall, for the car to have such a strong run and still be fast nearly 30 years since its introduction is a testament to the Dave McLellan team and the technologically advanced car they produced.
Basics-Selecting the Right Car
Although any C4 will work, finding the right combination of options and basic packages will save a lot of headache down the road depending on what class you seek to run in whether it be SCCA or NCCC organizations. Generally, the 89 Z51 cars are considered the best C4’s for auto-x. The answer why is because they had more favorable gearing with the ZF6 and 3.54 gears available from the factory for those in a stock class. They have the stiffest front spring and the standard 85-95 Z51/Z07 rear spring along with the big sway bars of 30mm solid front and 24mm solid rear. The L98 affords great off the line torque and power everywhere in the torque band. Finally, the 89’s had less weight than later cars due to not having airbag(s) and other weight increasing features. The 90 Z51 cars are a very close 2nd choice to the 89’s. The 85-88 Z51’s are very good handling cars some argue to a degree better than the later cars due to the scrub radius of the suspension being positive rather than zero. Which changed thanks to the 1988 year front and rear suspension revision. Indeed Roger’s yellow 86 is a testament to how fast the older Z51’s are. However, the latest winners have mostly been 89’s. Personally, if I could not find an 89 or 90 Z51 the 92-95 Z07 cars are a good choice. They have the later suspension, big brakes, ZF6, 17 inch wheels and LT1 engine. You’ll notice I left out the 84 Z51, the 91 Z07 and the 96 Z51 cars. This is on purpose. The 84 Z51 was the best handling car in the world in 84, however, the suspension was so stiff it tended to jack under hard cornering especially with sticky tires the SCCA Stock class was using (2014 rule is for street tires in street (old stock) classes, many of which are as sticky or more so than R-comps of 10-20 years ago and wear much better!). This is also true for the 91 Z07 cars which are identical to the 89 and 90 Z51 cars except they have FX3 shocks and a 26mm solid rear sway bar. The rear bar was just too stiff and with the suspension geometry being what it is, a jacking effect occurred during cornering. Jacking is where the rear roll stiffness is too much compared to the front and the rear of the car rises up and camber goes more positive and thus loss of rear grip and typically a spin. (I’ve PERSONALLY experienced this with stock bushing, max. front camber and my front bar setup to loosen the car (shims, more on this later), the rear jacked with the STOCK 24mm solid rear bar. Minor front bar adjustment corrected the snap oversteer behavior.) In 1988 GM changed the front and rear suspension to do two things. First the front suspension was made to have zero scrub radius instead of the positive scrub radius it had in 84-87. This was to offer better split co-efficient of friction function of the ABS such as pavement on one side and gravel on the other side of the car. The rear strut rod mounting brackets where revised to move the mounting location lower and thus lower the roll center. This was to help combat the jacking effect defined earlier. The 96 Z51 was a hollow shell of a “performance handling package” as it had some seriously soft springs, small bar in front, and softer Z51 shocks. This was all in the name of better ride not handling.
For the most part unless your planning on running your C4 in a class that allows for changing of springs, sway bars, shocks, etc….stay away from the FE1 (Base), Z52 (87-88), FX3, F45, and ZR-1 suspensions. Yes, I said ZR-1. They might have been King of the Hill when it came to straight line speed and acceleration, but with an extra 200lbs on the nose of the car and a soft suspension setup, they are a great grand touring car not an auto-x’er. Also of note is the cruel reality of the 89 and 90 Z51 package when ordered with the FX3 package as well. If you did this you got base springs and sway bars. Avoid the 89 and 90 Z51/FX3 optioned cars. They are not what you want.
Also, if you happen to run into one of the 83 R9G cars, buy it! Not only will you own a piece of history but probably the best handling C4 the factory ever produced. These 83 cars where destined for the Corvette Challenge series and most of them raced in it and all where made in 89 or 90MY’s.
Finally, no matter what suspension package you want or need or what class you intend to run. Keeping as close to possible to the “base” car is best. Avoid heavy options like Delco-Bose Gold audio systems, the heavy power Sport seats, automatic climate control and other frivolous options that add weight. (Ironically my C4 is option HEAVY, but when I bought it, I had no intentions of auto-x the car, go figure.) You’d be amazed how 100lbs can quickly be on the car once the option sheet start filling up. In most motorsports weight is bad so keep this in mind when doing your search. It maybe more difficult to find “base” cars with proper suspension but they are out there and worth the search. Automatic transmission C4’s are more common and Rodger’s 86 was an automatic, however, most folks prefer the ZF6 manual box to the automatic transmission C4’s for auto-x. Buy what you like in this regard. The 4+3 works but it’s a 4+3, not the best idea ever by GM but they work.
Stock Class Modifications/Setup
Now that you found a suitable car to run in a stock class you want the advantage of taking advantage of the rule book. Simply having a properly optioned car is not enough. I’ll save wheels and tires for another section as this gets complicated in that regard. We’ll examine everything else and this is all based on the SCCA Stock Class Solo rules so your NCCC rules may differ but are similar.
Dampeners aka Shocks
This is a free mod as long as you don’t have more than 2 external adjustments. Budget and how serious you are will determine what kind of shocks you buy. At the low end is stock replacement Z51 Bilstiens. Bilstien can revalve their shocks to your liking. They do this for non-adjustable shocks and the FX3 adjustable. So called Morocca and Sears Point valving is available. A call to them and telling them what you want and need will determine how you revalve your shocks. You simply send them in and they do the work and ship them back to you. Note: If you have the FX3 shocks and revalve them this is ok, however, you CANNOT use the modified FX3 shock controllers, stock only. The next step is Koni dampeners. Koni makes a great shock in the Sport Shock line for the C4. They are well developed and are perfect for the 89 and 90 Z51 cars. They work well on the 92-95 Z07 cars as well, heck; they work well on the whole C4 line for that matter. Affordable, quality, and race ready. While the sport shock line is single adjustable, if you want to spend the money, they are available as double adjustable custom made by Koni, but you have to buy Sports first and then send them in. Part numbers for the Koni’s that fit 89-96 cars are: 8241 1097 Sport (Front) and 8241 1098 Sport (Rear). As of this writing Koni does not list sport shocks for older C4’s in their catalog. At the top of the line is $3,000+ Penske shocks and so forth. If you have the coin this is the way to go. The options here are endless and I won’t even try to cover them all. The Koni’s will serve almost everyone best.
Alignment Settings
This will primarily be based on the 88-96 cars but will be similar for the 84-87 cars. Starting with the back end, as you always align a car with the back first, set rear camber to be 1 to 1.5 degrees negative and give the toe an adjustment to the limit of the factory specs for toe-in. On to the front, take what ever shims you have in your upper control arms and take them ALL out. This will give you maximum camber that your car can get. Camber is VERY important and to be maximized even at the expense of caster, especially in a stock class setup. The reason for this is that the C4’s are camber limited in the front a maximum of around negative 1.5 degrees is possible if the bushings are wore some. A car with good bushings will typically read less than 1 degree negative. The right side is almost always more negative than the left. It is just how these cars where designed and built. Combined with the fact a C4 is left side heavy means these are Indy cars, which is to say, they turn better left than right. More on this is in a bit. Finally, set your toe to zero or around a total toe-out of approximately 3/16”. I run zero on my C4 as I drive it a lot on the street. The camber is not too bad, but toe will kill tires very quickly at auto-x settings. One trick is to set the car up at both your street and race toe settings and mark them with paint lines on the tie rod ends. This way you can quickly adjust them at the event and change them back to street just a quickly.
Chassis Setup Tips and Tricks
Let us cover brakes really quick for auto-x. Stock brakes are fine J55’s or not, stock pads, rotors, etc…all fine. No real advantage in changing things for auto-x. The only things to do are making sure they work 100%, are safe, and run DOT4 brake fluid. DOT3 won’t cut it. I boiled FRESH DOT3 at an auto-x driver’s school in my 93 with the 12” base brakes. Went to Motul RBF 600 and had ZERO problems since. Any DOT4 fluid will work just fine for our purposes. If your car is a 1992 or older you can run braided stainless, if you have a newer C4 put on new rubber hoses, they are old.
In stock class not a whole lot you can do with the suspension adjustments other than the rear leaf spring and sway bars. First we will start with the rear leaf spring and its adjustments that will allow for better cross weighting. The C4’s are all left side heavy and also have less maximum camber on the left side. It should be no wonder then that these cars tend to turn better to the right than they do left. Unfortunately, auto-x cars tend to need to both well. This modification is pretty simple. Take the right side castle nut and run it all the way down against the cotter pin and your done with the right side. The left side needs to be adjusted upwards to jack the weight off the left front and onto the right front. The correct way to do this is to have the car at trim height with your approximate body weight in the driver’s seat and on a set of scales. Then adjust till you get the cross weight as close as you can to 50/50 side to side. This will tend to rise up the left rear of the car but it is all in the name of better handling. The other way to do it and the method I used is to just play with the adjustment on the left side until it feels right. Use your local events and so forth to do this. Keep the wrenches near your grid spot and you can adjust between runs and take notes on how the car feels turning one way or the other. A bit of trial and error yes, but it works.
The other HUGE advantage these cars have is in the sway bar mounting setup. You can make big changes in how the car rotates in turns by changing how the sway bar bushings clamp down on the bar. For the most part leave the rear bar alone just make sure it is mounted properly, the bushings are in good shape, and everything is torqued down properly at ride height. The adjustments are made with the front bar. The front sway bar mounts have two halves to them. An upper and lower halve. The idea is to adjust the preload the bushing has on the bar by shimming the halves. If you take four alignment shims, drill a hole in each end of them, and then attach a zip tie it makes for fast changes in your grid spot. If you place 4 1/16th inch thick shims between the halves you effectively loosen up the front end and create a more balanced car. This is the way my car is currently setup. However, if your car is still understeering you may try adding 1/8th inch thick shims and see what happens, if it is better, good, but more than likely your now into snap oversteer due to not enough roll stiffness in the front compared to the rear. If the car is too free you can remove the shims all together. If the car is still free, you can take large fender washers approximately 1 to 2” in diameter and place them under the bushing and with the mounts tight this will effectively crush the bar and clamp it very tightly increasing understeer for your loose car. Combined with the adjustable front shocks you can use the front bar for all day to day adjustments and leave the rear end alone to follow the front end.
Wheels and Tires
If you have a early 84-87 C4 your going to have a lot of trouble finding decent street tire rubber for auto-x if not impossible time finding it. The 88-96 17 inch wheels are much easier to find suitable rubber for. The two best (as of this writing) street tires for auto-x are the BFG Rival and Dunlap Dizzera ZII. I’ve personally ran my C4 on 255/40/17’s at all four with ZII’s on the four stock 17X9.5 “Sawblade” wheels. P255/40/17 is the widest 17 inch tire BFG or Dunlap makes. Others are available in the stock P275/40/17 but they are NOT the best compound for auto-x. The only one that might make you a good set of street/auto-x RAIN tires is the Continental Extreme Contact DW these are great auto-x tires for the RAIN. I feel the wider 9.5 inch wheels are better than the 8.5 inch wheels as they stretch the tires some and create a stiffer sideway. Turn in is sharp and immediate, the ZII’s do squeal but not very loudly and ONLY at the very limit of adhesion. Wear is excellent, I made over 200+ auto-x runs and 3K street miles in one set and have enough tread left for half a (road course) track day. Decent in the wet as long as the tread is deep enough, once it gets worn, slow down on the highways! The ZII’s seem to have better longitudinal grip and the Rival better lateral grip, but little difference between the two. However, that being said, with the torque C4’s make I feel the ZII’s are the better choice for putting down the power on corner exit. One thing is for sure the tire wars are alive and well, Kumho is rumored to be perfecting a true competitor to these two called the V720, lets hope it comes in 275/40/17 and is a better tire than the other two. In the mean time, the street class (stock class) rules for 2014, state that changing wheel diameter is allowed. Thus we can bolt on 18X9.5 wheels and run a ZII or Rival sized in 275/35/18 or even maybe a 295/35/18 in the BFG Rival!! This gives us back our wider and longer contact patch we lost by going to the 255/40/17. Make sure your offset is within +/- 6.35mm (1/4”) so it is stock legal either buy wheels with the correct 56mm offset or get some spacers to correct it. This is VERY wheel dependant so do your research!
Miscellaneous
A few housekeeping things in this section. A lightweight cat-back is a good idea, may or may not pick up any additional power depending on which engine you have. L98 can pick up more than LT1 as the LT1 system is very good. Stock catalysts must be retained. I’ve removed my factory mufflers and resonator. I replaced the factory setup with straight pipe and an X-pipe in their place. It’s not too loud or too quiet. Very deep tone but I did not pick up any power as my car dyno’ed at stock HP and TQ levels. The key is to loose weight and let that V8 make some noise. A harness bar and belts are a nice addition to the car as well. You might add a hollow 32mm front sway bar to loose some weight, but this has the same stiffness as a 30mm solid factory bar. Remember you can change only ONE bar not both.
BSP and Beyond…
Things get messy and pricey at this level. I recommend buying a built BSP or BP or XP C4 before building one. It will cost you less and save you time. I’ll highlight the basics for a BSP build. As far as BP and XP go there are SO many options you’re on your own. Read the rule book, be creative, and bring money.
Basic BSP mods:
• LT4 Engine-Must be complete assy.
• Headers with lightweight exhaust
• 3.54 gears (Stock option in 89)
• Typically 17X11 wheels with 315/35/17’s at all four. However 315/335 combo’s are used. Some cases of 295’s at all four working well.
• Lightweight racing seats/harness bar/harness
• Polyurethane bushing everywhere except trailing links aka dog bones. These move in such a way that poly would bind them up.
• Offset lower control arm bushings or custom adjustable upper control arms. Other trick is to use later 88-96 lower arms and early 84-87 upper arms to get more camber in all cases.
• Aluminum Flywheel with any clutch you happen to like.
• 30mm solid front sway bar/22mm or similar solid rear bar, or adjustable.
• Stiff springs as possible. VBP makes some Extreme Springs that are stupid stiff.
• Fast earlier Z51 power steering rack with 2.0 turns lock to lock.
• Relocating battery to behind passenger seat and using a lightweight one.
• Car lowered front and rear. Keeping a negative rake to the car. Rear slightly higher than front.
• Braided stainless steel brake lines if not already on a older stock class car.
• A/C removal
• Tuning of ECM with changes in corresponding system, ie. Exhaust/intake/fuel injection/ignition timing.
• Alternate steering wheel
• Any intake manifold
• Splitter/Spoilers are permitted
• Roll Cages/Roll Bars
• Radio Removal/Fog Lamp Removal
• Any brake rotor/pad/lines
• Air cooling ducts
• Lightweight starter motor/alternator in stock locations
• Cold Air Intakes
• Etc….There is a TON of other small things that can be done. BSP allows for a LOT of modifications. Read the rule book carefully and have fun!
Concluding Remarks
Much of this is based on my own experiences and research on Corvette Forum, other racers, and sources found on the internet. So take that for what it is worth. Much of this is also based on discussions that I had with Solofast on Corvette Forum. He is well qualified to speak on such matters and much of the information is directly from him. Hopefully, the C4 Corvette community finds this information useful and may your auto-x competitors fear the stiff suspension, big tires, and loads of torque these cars have. Remember slow in, fast out and LOOK AHEAD!!!
By 93Rubie on Corvette Forum
93 FX3 FE1 car, turned into a Z07 package car with correct Z07 package components. These were NOT easy to find, but its doable. Contact me for exact car setup details if you wish.
Setting up your C4 Corvette for Auto-X
So you’re a 4th Generation Corvette (84-96) owner and you’ve got addicted to auto-x. Now what to do about the car? How do I make it faster and handle better? This article will seek to summarize and detail just how to do that but first some back ground.
In case you’re not aware the C4 has been a highly successfully auto-x platform since its introduction in 1984. Most notability Roger Johnson won many of his national titles at the wheel of a C4. The car was a winner in SS, AS. As well as BSP at the national level. The car was competitive and winning national championships well into the mid-2000’s. Jeff Cashmore won in 2006 in A Stock. Don’t ask me how I found this, but I hope it remains active as it is a great model for a stock class C4 this WAS Jeff’s 89 C4. Jeff Cashmore's 89 C4 Z51 The last coming in 2009 in AS when Scott McHugh’s 89 Z51 car took A Stock that year. The car has finally fallen out of favor with most national competitors moving on to newer machinery, many of which are now driving C6 GS’s and C5Z’s. While the car may no longer be as fast or favorable for national level competition they are still quite quick and fast enough to be competitive at the regional and divisional levels. Overall, for the car to have such a strong run and still be fast nearly 30 years since its introduction is a testament to the Dave McLellan team and the technologically advanced car they produced.
Basics-Selecting the Right Car
Although any C4 will work, finding the right combination of options and basic packages will save a lot of headache down the road depending on what class you seek to run in whether it be SCCA or NCCC organizations. Generally, the 89 Z51 cars are considered the best C4’s for auto-x. The answer why is because they had more favorable gearing with the ZF6 and 3.54 gears available from the factory for those in a stock class. They have the stiffest front spring and the standard 85-95 Z51/Z07 rear spring along with the big sway bars of 30mm solid front and 24mm solid rear. The L98 affords great off the line torque and power everywhere in the torque band. Finally, the 89’s had less weight than later cars due to not having airbag(s) and other weight increasing features. The 90 Z51 cars are a very close 2nd choice to the 89’s. The 85-88 Z51’s are very good handling cars some argue to a degree better than the later cars due to the scrub radius of the suspension being positive rather than zero. Which changed thanks to the 1988 year front and rear suspension revision. Indeed Roger’s yellow 86 is a testament to how fast the older Z51’s are. However, the latest winners have mostly been 89’s. Personally, if I could not find an 89 or 90 Z51 the 92-95 Z07 cars are a good choice. They have the later suspension, big brakes, ZF6, 17 inch wheels and LT1 engine. You’ll notice I left out the 84 Z51, the 91 Z07 and the 96 Z51 cars. This is on purpose. The 84 Z51 was the best handling car in the world in 84, however, the suspension was so stiff it tended to jack under hard cornering especially with sticky tires the SCCA Stock class was using (2014 rule is for street tires in street (old stock) classes, many of which are as sticky or more so than R-comps of 10-20 years ago and wear much better!). This is also true for the 91 Z07 cars which are identical to the 89 and 90 Z51 cars except they have FX3 shocks and a 26mm solid rear sway bar. The rear bar was just too stiff and with the suspension geometry being what it is, a jacking effect occurred during cornering. Jacking is where the rear roll stiffness is too much compared to the front and the rear of the car rises up and camber goes more positive and thus loss of rear grip and typically a spin. (I’ve PERSONALLY experienced this with stock bushing, max. front camber and my front bar setup to loosen the car (shims, more on this later), the rear jacked with the STOCK 24mm solid rear bar. Minor front bar adjustment corrected the snap oversteer behavior.) In 1988 GM changed the front and rear suspension to do two things. First the front suspension was made to have zero scrub radius instead of the positive scrub radius it had in 84-87. This was to offer better split co-efficient of friction function of the ABS such as pavement on one side and gravel on the other side of the car. The rear strut rod mounting brackets where revised to move the mounting location lower and thus lower the roll center. This was to help combat the jacking effect defined earlier. The 96 Z51 was a hollow shell of a “performance handling package” as it had some seriously soft springs, small bar in front, and softer Z51 shocks. This was all in the name of better ride not handling.
For the most part unless your planning on running your C4 in a class that allows for changing of springs, sway bars, shocks, etc….stay away from the FE1 (Base), Z52 (87-88), FX3, F45, and ZR-1 suspensions. Yes, I said ZR-1. They might have been King of the Hill when it came to straight line speed and acceleration, but with an extra 200lbs on the nose of the car and a soft suspension setup, they are a great grand touring car not an auto-x’er. Also of note is the cruel reality of the 89 and 90 Z51 package when ordered with the FX3 package as well. If you did this you got base springs and sway bars. Avoid the 89 and 90 Z51/FX3 optioned cars. They are not what you want.
Also, if you happen to run into one of the 83 R9G cars, buy it! Not only will you own a piece of history but probably the best handling C4 the factory ever produced. These 83 cars where destined for the Corvette Challenge series and most of them raced in it and all where made in 89 or 90MY’s.
Finally, no matter what suspension package you want or need or what class you intend to run. Keeping as close to possible to the “base” car is best. Avoid heavy options like Delco-Bose Gold audio systems, the heavy power Sport seats, automatic climate control and other frivolous options that add weight. (Ironically my C4 is option HEAVY, but when I bought it, I had no intentions of auto-x the car, go figure.) You’d be amazed how 100lbs can quickly be on the car once the option sheet start filling up. In most motorsports weight is bad so keep this in mind when doing your search. It maybe more difficult to find “base” cars with proper suspension but they are out there and worth the search. Automatic transmission C4’s are more common and Rodger’s 86 was an automatic, however, most folks prefer the ZF6 manual box to the automatic transmission C4’s for auto-x. Buy what you like in this regard. The 4+3 works but it’s a 4+3, not the best idea ever by GM but they work.
Stock Class Modifications/Setup
Now that you found a suitable car to run in a stock class you want the advantage of taking advantage of the rule book. Simply having a properly optioned car is not enough. I’ll save wheels and tires for another section as this gets complicated in that regard. We’ll examine everything else and this is all based on the SCCA Stock Class Solo rules so your NCCC rules may differ but are similar.
Dampeners aka Shocks
This is a free mod as long as you don’t have more than 2 external adjustments. Budget and how serious you are will determine what kind of shocks you buy. At the low end is stock replacement Z51 Bilstiens. Bilstien can revalve their shocks to your liking. They do this for non-adjustable shocks and the FX3 adjustable. So called Morocca and Sears Point valving is available. A call to them and telling them what you want and need will determine how you revalve your shocks. You simply send them in and they do the work and ship them back to you. Note: If you have the FX3 shocks and revalve them this is ok, however, you CANNOT use the modified FX3 shock controllers, stock only. The next step is Koni dampeners. Koni makes a great shock in the Sport Shock line for the C4. They are well developed and are perfect for the 89 and 90 Z51 cars. They work well on the 92-95 Z07 cars as well, heck; they work well on the whole C4 line for that matter. Affordable, quality, and race ready. While the sport shock line is single adjustable, if you want to spend the money, they are available as double adjustable custom made by Koni, but you have to buy Sports first and then send them in. Part numbers for the Koni’s that fit 89-96 cars are: 8241 1097 Sport (Front) and 8241 1098 Sport (Rear). As of this writing Koni does not list sport shocks for older C4’s in their catalog. At the top of the line is $3,000+ Penske shocks and so forth. If you have the coin this is the way to go. The options here are endless and I won’t even try to cover them all. The Koni’s will serve almost everyone best.
Alignment Settings
This will primarily be based on the 88-96 cars but will be similar for the 84-87 cars. Starting with the back end, as you always align a car with the back first, set rear camber to be 1 to 1.5 degrees negative and give the toe an adjustment to the limit of the factory specs for toe-in. On to the front, take what ever shims you have in your upper control arms and take them ALL out. This will give you maximum camber that your car can get. Camber is VERY important and to be maximized even at the expense of caster, especially in a stock class setup. The reason for this is that the C4’s are camber limited in the front a maximum of around negative 1.5 degrees is possible if the bushings are wore some. A car with good bushings will typically read less than 1 degree negative. The right side is almost always more negative than the left. It is just how these cars where designed and built. Combined with the fact a C4 is left side heavy means these are Indy cars, which is to say, they turn better left than right. More on this is in a bit. Finally, set your toe to zero or around a total toe-out of approximately 3/16”. I run zero on my C4 as I drive it a lot on the street. The camber is not too bad, but toe will kill tires very quickly at auto-x settings. One trick is to set the car up at both your street and race toe settings and mark them with paint lines on the tie rod ends. This way you can quickly adjust them at the event and change them back to street just a quickly.
Chassis Setup Tips and Tricks
Let us cover brakes really quick for auto-x. Stock brakes are fine J55’s or not, stock pads, rotors, etc…all fine. No real advantage in changing things for auto-x. The only things to do are making sure they work 100%, are safe, and run DOT4 brake fluid. DOT3 won’t cut it. I boiled FRESH DOT3 at an auto-x driver’s school in my 93 with the 12” base brakes. Went to Motul RBF 600 and had ZERO problems since. Any DOT4 fluid will work just fine for our purposes. If your car is a 1992 or older you can run braided stainless, if you have a newer C4 put on new rubber hoses, they are old.
In stock class not a whole lot you can do with the suspension adjustments other than the rear leaf spring and sway bars. First we will start with the rear leaf spring and its adjustments that will allow for better cross weighting. The C4’s are all left side heavy and also have less maximum camber on the left side. It should be no wonder then that these cars tend to turn better to the right than they do left. Unfortunately, auto-x cars tend to need to both well. This modification is pretty simple. Take the right side castle nut and run it all the way down against the cotter pin and your done with the right side. The left side needs to be adjusted upwards to jack the weight off the left front and onto the right front. The correct way to do this is to have the car at trim height with your approximate body weight in the driver’s seat and on a set of scales. Then adjust till you get the cross weight as close as you can to 50/50 side to side. This will tend to rise up the left rear of the car but it is all in the name of better handling. The other way to do it and the method I used is to just play with the adjustment on the left side until it feels right. Use your local events and so forth to do this. Keep the wrenches near your grid spot and you can adjust between runs and take notes on how the car feels turning one way or the other. A bit of trial and error yes, but it works.
The other HUGE advantage these cars have is in the sway bar mounting setup. You can make big changes in how the car rotates in turns by changing how the sway bar bushings clamp down on the bar. For the most part leave the rear bar alone just make sure it is mounted properly, the bushings are in good shape, and everything is torqued down properly at ride height. The adjustments are made with the front bar. The front sway bar mounts have two halves to them. An upper and lower halve. The idea is to adjust the preload the bushing has on the bar by shimming the halves. If you take four alignment shims, drill a hole in each end of them, and then attach a zip tie it makes for fast changes in your grid spot. If you place 4 1/16th inch thick shims between the halves you effectively loosen up the front end and create a more balanced car. This is the way my car is currently setup. However, if your car is still understeering you may try adding 1/8th inch thick shims and see what happens, if it is better, good, but more than likely your now into snap oversteer due to not enough roll stiffness in the front compared to the rear. If the car is too free you can remove the shims all together. If the car is still free, you can take large fender washers approximately 1 to 2” in diameter and place them under the bushing and with the mounts tight this will effectively crush the bar and clamp it very tightly increasing understeer for your loose car. Combined with the adjustable front shocks you can use the front bar for all day to day adjustments and leave the rear end alone to follow the front end.
Wheels and Tires
If you have a early 84-87 C4 your going to have a lot of trouble finding decent street tire rubber for auto-x if not impossible time finding it. The 88-96 17 inch wheels are much easier to find suitable rubber for. The two best (as of this writing) street tires for auto-x are the BFG Rival and Dunlap Dizzera ZII. I’ve personally ran my C4 on 255/40/17’s at all four with ZII’s on the four stock 17X9.5 “Sawblade” wheels. P255/40/17 is the widest 17 inch tire BFG or Dunlap makes. Others are available in the stock P275/40/17 but they are NOT the best compound for auto-x. The only one that might make you a good set of street/auto-x RAIN tires is the Continental Extreme Contact DW these are great auto-x tires for the RAIN. I feel the wider 9.5 inch wheels are better than the 8.5 inch wheels as they stretch the tires some and create a stiffer sideway. Turn in is sharp and immediate, the ZII’s do squeal but not very loudly and ONLY at the very limit of adhesion. Wear is excellent, I made over 200+ auto-x runs and 3K street miles in one set and have enough tread left for half a (road course) track day. Decent in the wet as long as the tread is deep enough, once it gets worn, slow down on the highways! The ZII’s seem to have better longitudinal grip and the Rival better lateral grip, but little difference between the two. However, that being said, with the torque C4’s make I feel the ZII’s are the better choice for putting down the power on corner exit. One thing is for sure the tire wars are alive and well, Kumho is rumored to be perfecting a true competitor to these two called the V720, lets hope it comes in 275/40/17 and is a better tire than the other two. In the mean time, the street class (stock class) rules for 2014, state that changing wheel diameter is allowed. Thus we can bolt on 18X9.5 wheels and run a ZII or Rival sized in 275/35/18 or even maybe a 295/35/18 in the BFG Rival!! This gives us back our wider and longer contact patch we lost by going to the 255/40/17. Make sure your offset is within +/- 6.35mm (1/4”) so it is stock legal either buy wheels with the correct 56mm offset or get some spacers to correct it. This is VERY wheel dependant so do your research!
Miscellaneous
A few housekeeping things in this section. A lightweight cat-back is a good idea, may or may not pick up any additional power depending on which engine you have. L98 can pick up more than LT1 as the LT1 system is very good. Stock catalysts must be retained. I’ve removed my factory mufflers and resonator. I replaced the factory setup with straight pipe and an X-pipe in their place. It’s not too loud or too quiet. Very deep tone but I did not pick up any power as my car dyno’ed at stock HP and TQ levels. The key is to loose weight and let that V8 make some noise. A harness bar and belts are a nice addition to the car as well. You might add a hollow 32mm front sway bar to loose some weight, but this has the same stiffness as a 30mm solid factory bar. Remember you can change only ONE bar not both.
BSP and Beyond…
Things get messy and pricey at this level. I recommend buying a built BSP or BP or XP C4 before building one. It will cost you less and save you time. I’ll highlight the basics for a BSP build. As far as BP and XP go there are SO many options you’re on your own. Read the rule book, be creative, and bring money.
Basic BSP mods:
• LT4 Engine-Must be complete assy.
• Headers with lightweight exhaust
• 3.54 gears (Stock option in 89)
• Typically 17X11 wheels with 315/35/17’s at all four. However 315/335 combo’s are used. Some cases of 295’s at all four working well.
• Lightweight racing seats/harness bar/harness
• Polyurethane bushing everywhere except trailing links aka dog bones. These move in such a way that poly would bind them up.
• Offset lower control arm bushings or custom adjustable upper control arms. Other trick is to use later 88-96 lower arms and early 84-87 upper arms to get more camber in all cases.
• Aluminum Flywheel with any clutch you happen to like.
• 30mm solid front sway bar/22mm or similar solid rear bar, or adjustable.
• Stiff springs as possible. VBP makes some Extreme Springs that are stupid stiff.
• Fast earlier Z51 power steering rack with 2.0 turns lock to lock.
• Relocating battery to behind passenger seat and using a lightweight one.
• Car lowered front and rear. Keeping a negative rake to the car. Rear slightly higher than front.
• Braided stainless steel brake lines if not already on a older stock class car.
• A/C removal
• Tuning of ECM with changes in corresponding system, ie. Exhaust/intake/fuel injection/ignition timing.
• Alternate steering wheel
• Any intake manifold
• Splitter/Spoilers are permitted
• Roll Cages/Roll Bars
• Radio Removal/Fog Lamp Removal
• Any brake rotor/pad/lines
• Air cooling ducts
• Lightweight starter motor/alternator in stock locations
• Cold Air Intakes
• Etc….There is a TON of other small things that can be done. BSP allows for a LOT of modifications. Read the rule book carefully and have fun!
Concluding Remarks
Much of this is based on my own experiences and research on Corvette Forum, other racers, and sources found on the internet. So take that for what it is worth. Much of this is also based on discussions that I had with Solofast on Corvette Forum. He is well qualified to speak on such matters and much of the information is directly from him. Hopefully, the C4 Corvette community finds this information useful and may your auto-x competitors fear the stiff suspension, big tires, and loads of torque these cars have. Remember slow in, fast out and LOOK AHEAD!!!
By 93Rubie on Corvette Forum
93 FX3 FE1 car, turned into a Z07 package car with correct Z07 package components. These were NOT easy to find, but its doable. Contact me for exact car setup details if you wish.
Last edited by 93Rubie; 01-19-2014 at 08:06 PM.
The following 4 users liked this post by 93Rubie:
3JsVette (07-25-2016),
blackozvet (07-08-2016),
BrianCunningham (07-11-2016),
carguy604 (07-09-2016)
11-28-2013, 11:11 PM
#4
Race Director
The driver is far more important than any mods you can make to your car. Getting good instruction and LOTS of seat time are the keys to fast times.
In many cases the horsepower is NOT important. A good example of that is my 2013 Grand Sport. I am in the same NCCC class as the C6 ZO6 (1SM -- stock on street tires), even though I have 430 hp and they have 505 hp. NCCC doesn't consider the extra horsepower to be an advantage. At the most recent autocross I ran I beat both of the ZO6 cars in my class. That was for all three events of two laps each.
In many cases the horsepower is NOT important. A good example of that is my 2013 Grand Sport. I am in the same NCCC class as the C6 ZO6 (1SM -- stock on street tires), even though I have 430 hp and they have 505 hp. NCCC doesn't consider the extra horsepower to be an advantage. At the most recent autocross I ran I beat both of the ZO6 cars in my class. That was for all three events of two laps each.
100% with driver mod. However, selecting and properly setting up the right car is part of the game as well.
11-30-2013, 11:55 PM
#6
Burning Brakes
Autocrossing the C4
Thanks for your impressions of mods for the C4. If you could add some illustrations that would be helpful. Your notes on the front sway bar and the shims would benefit from pictures.
My Corvette club sponsors autocrosses, and the best advices seems to be, 'get a Miata!!'
As for me, I have GS wheels with Kumho 700s. that helps a bit.
My Corvette club sponsors autocrosses, and the best advices seems to be, 'get a Miata!!'
As for me, I have GS wheels with Kumho 700s. that helps a bit.
12-01-2013, 02:04 PM
#9
Le Mans Master
As Cliff said the driver is the deciding factor. I have done some mods to my car but still you would think I would not have a chance against a car with 100-200 more HP. Well I beat most of the C5 Z06 and C6 Z06's unless they have a seasoned driver.
12-01-2013, 08:16 PM
#11
Safety Car
Thread Starter
Thanks for your impressions of mods for the C4. If you could add some illustrations that would be helpful. Your notes on the front sway bar and the shims would benefit from pictures.
My Corvette club sponsors autocrosses, and the best advices seems to be, 'get a Miata!!'
As for me, I have GS wheels with Kumho 700s. that helps a bit.
My Corvette club sponsors autocrosses, and the best advices seems to be, 'get a Miata!!'
As for me, I have GS wheels with Kumho 700s. that helps a bit.
12-26-2013, 09:22 AM
#13
Race Director
Brian, what class does your supercharger put you in?
Bsp or something different?
theres a lot of egos at some of the autox i go to (the road course events Are friendlier)]
Bsp or something different?
theres a lot of egos at some of the autox i go to (the road course events Are friendlier)]
12-28-2013, 10:36 PM
#15
Safety Car
Thread Starter
part of my C4 notebook
01-12-2014, 08:59 PM
#18
Safety Car
Thread Starter
Bumping this up, as some folks asked for pics of sway bar shim setup. Tore apart both front and rear suspension today. Man, those LCA bushings are shot. The rest not as bad as I thought but still old and worn.
01-13-2014, 10:32 PM
#19
Melting Slicks
Thanks for the write up, myself and many others appreciate your efforts. My car has been a project for many years and many years ago I sent my FX-3 shocks in and they came back with new dark yellow "Bilstein" paint and decals and I had them revalved to the Morocca. Since you hurt my cars (91 ZO7) feelings, can you tell us what the "Morocca" valving is? I can't remember but I didn't send in my controller.
01-14-2014, 12:07 AM
#20
Team Owner
Pro Mechanic
Awesome write up! Wish more members contributed as well as you have or do!
You asked for input corrections...I have one: your explanation of "jacking" is not quite right; Jacking occurs on an independent rear where there is a single lateral swing arm (the axle shaft). The cornering force applied to the bottom of the tire leverages the suspension arm and forces the rear tire into massive positive camber, while "jacking" the rear of the car vertically. The "Jacking" is detrimental (relatively speaking) as much as the camber change is. The camber change reduces the amount of tread on the road, and the effectiveness of the tire. Grip goes away, car goes into massive oversteer.
The stiff springs on an '84 do not cause jacking, and in fact, would mitigate it; it's a rate and it decreases as fast though rebound as it increases through compression. If the pressure drops as the wheel moves in rebound, how is the stiff spring contributing more to 'jacking', then a softer one? It can't.
Also, the '84 (and all 'Vette's since 1963) use a two lateral arm rear suspension which does two things;
1. It reduces or eliminates the forces that cause jacking
2. It allows the tire to stay perpendicular to the road during suspension travel in corners.
IMO, the only reason the '84 isn't a killer autox car is the engine. Not the stiff suspension.
Also a stiff sway bar doesn't contribute to jacking either, It can't, and in fact it would also mitigate any jacking effect. A "too stiff" rear spring, a "too stiff" rear sway, or a "too stiff" combination of the two items causes or contributes to oversteer because the weight transfer is more dramatic on the stiffer end of the car...forcing that end of the car to rely on one tire (more so) rather than two, for grip. Imagine if you had no sway up front, and a very compliant front set up. In the rear, you weld the suspension to the frame of the car so it's like a go-kart. When cornering, ANY body lean will transfer 100% of the load onto the outside tire, in the welded rear. In the front, the suspensions travel and compliance allows a near 50/50 weight distribution- both tires are splitting the load. So the car looses grip in the rear. Oversteer. Not jacking, just too stiff a rear, which is tuneable in the swaybar.
You asked for input corrections...I have one: your explanation of "jacking" is not quite right; Jacking occurs on an independent rear where there is a single lateral swing arm (the axle shaft). The cornering force applied to the bottom of the tire leverages the suspension arm and forces the rear tire into massive positive camber, while "jacking" the rear of the car vertically. The "Jacking" is detrimental (relatively speaking) as much as the camber change is. The camber change reduces the amount of tread on the road, and the effectiveness of the tire. Grip goes away, car goes into massive oversteer.
The stiff springs on an '84 do not cause jacking, and in fact, would mitigate it; it's a rate and it decreases as fast though rebound as it increases through compression. If the pressure drops as the wheel moves in rebound, how is the stiff spring contributing more to 'jacking', then a softer one? It can't.
Also, the '84 (and all 'Vette's since 1963) use a two lateral arm rear suspension which does two things;
1. It reduces or eliminates the forces that cause jacking
2. It allows the tire to stay perpendicular to the road during suspension travel in corners.
IMO, the only reason the '84 isn't a killer autox car is the engine. Not the stiff suspension.
Also a stiff sway bar doesn't contribute to jacking either, It can't, and in fact it would also mitigate any jacking effect. A "too stiff" rear spring, a "too stiff" rear sway, or a "too stiff" combination of the two items causes or contributes to oversteer because the weight transfer is more dramatic on the stiffer end of the car...forcing that end of the car to rely on one tire (more so) rather than two, for grip. Imagine if you had no sway up front, and a very compliant front set up. In the rear, you weld the suspension to the frame of the car so it's like a go-kart. When cornering, ANY body lean will transfer 100% of the load onto the outside tire, in the welded rear. In the front, the suspensions travel and compliance allows a near 50/50 weight distribution- both tires are splitting the load. So the car looses grip in the rear. Oversteer. Not jacking, just too stiff a rear, which is tuneable in the swaybar.