DREGSZ

I did a search and couldn't find what I was looking for so I'll ask away.
I've forgotten most of this stuff so refresh my memory.

I'm looking at an 86' Coupe 4+3 as a commuter car and the odd track day maybe a half dozen a year.

What are the shortcomings of the early C4s and how can they be overcome if necessary, obviously an 230 HP (when new) iron head TPI motor may need some heads and a carb and roller cam to wake up,

Correct me if I'm wrong, but the front brakes can update to 13" rotors with a simple swap and a new bracket for C5 calipers?

If the 4+3 live, fine, if not a 6 speed is a straight swap. THis car has 17" saw blades so no issue there.

No Zero Scrub but not the end of the world. Has ABS,

What else have I left out?

Thanks
Evan

UstaB-GS549

I think the brakes changed in 1988. From that year on, the C4 12" to 13" front brake conversion is pretty straightforward. Dunno about C5 conversion. Other than cheaper rotors, I don't see a big advantage.

Aardwolf

There have been some C5 brake adapters in the C4 parts for sale section. Both for the pre-'88 brakes and later.

0Paul Ruggeri

I started doing track days last year with my 86 Z51 4+3. I decided to use it instead of my 99 FRC, even though its faster, because I was less concerned about hurting the 86. If I was going to go out and buy a car I think I'd look for a later one, 89 6 speeds seem popular. The amount you're going to spend for brakes and horsepower to get the 86 up to a later model are probably less than the difference in initial cost. The later aluminum head roller cam engines have more potential too. Just my two cents.


Have fun, Paul

96LT1

I've got an 86 that I've turned into a track car... both the later C4 J55/GS/CE brakes and the C5/Z06 calipers need a bracket. The C5 upgrade is cheaper and affords better braking in the long run.

The ZF 6spd isn't quite a straight swap... the tail shaft on the ZF6 is bigger so the drive shaft needs a different yoke. The clutch, P/P, B/H and clutch slave cylinder all need to be replaced as well.

I'm in the midst of swapping mine over to a LSx and a T56 now.

Bill Dearborn

The 4+3 gear box is a Doug Nash box with an overdrive unit bolted to the rear. The O/D units sometimes fail due to poor maintenance by the owners. Used just as a 4 speed box it works but you are gear challenged compared to a 6 speed car. No matter whether you have an 85 or a 91 the TPI engine is a big limitation. The aluminum heads in the late 86 cars and newer help reduce weight but the big problem is the induction system. It is great for street and autocross work but just runs out of steam on a road race track. Effective engine speed is limited to about 4500 rpm. After that the HP produced just falls off a cliff. On a long straight like the back straight at Watkins Glen which is uphill an 86 will peak out at 124 on the speedo which is ~ 4900 rpm. It takes a while to get there since as the speed increases the HP is decreasing. Haven't looked at the HP curve on an 230 HP L98 for a while but I bet HP at 4900 is close to a 60 less than the peak at 4350 rpm. Throw another gear in there and the rpms drop below the peak HP point and the car will acclerate faster since HP is increasing as car speed increases. Where this is most noticed is coming out of some of the turns at the Glen. I could be on the bumper of a car coming out of T11 onto the front straight in 3rd gear and if I didn't shift to 4th at the exit of the turn I couldn't pass them. You could literally feel the HP going away as the speed increased. On the shift to 4th the car would take off and a pass could easily be accomplished until the revs got to high.

The nice thing about it is you could run the pee out of the engine without really stressing it since you couldn't rev high enough to stress the rods, bearings, etc.

When compared to my 97 C5 the 86 handled OK but the engine just couldn't get the job done. With the 97 going up the back straight the car was still accelerating when I got to the braking point and I saw speeds of 137 to 140 depending on how I entered the straight. With the Z06 I am seeing speeds of 145 at the same point.

The solution to the induction system is to go to a LT1 style injection system or a TPIS Miniram not a carb (shudder). Carbs are a pain to work with, provide poor mixture control at part throttle and subject to leaking (especially the Holley style) and I can't see why anybody would want to use them. At WOT they can equal the performance of fuel injection but anywhere else they fall short. I got rid of my last 4 barrel carb on a Vette in 1987 when I sold my 71 LS6 to get my 86. What a relief. I haven't had a fuel system issue since then.

Bill

DaveL82

A few of us in the C3 forum run the later aluminum diff and are using rea toe links like the C4 to cure the C3 toe steer problem. In working on the C3 a few folks found that in the early C4's the roll center was too high and was corrected in later C4 cars. Might be worth a search in the C4 section to see if this is the case and if so the cure might be as simple as relocating the rear toe bracket on the diff.

c4cruiser

Early 86 coupes had iron cylinder heads, but you could swap in a set of later aluminum heads or even aftermarket versions. The factory TPI intake keeps the engine from delivering high RPM power. I have a ZZ3 cam, some head work and port matching in my car and power still drops off at 5200-5400 RPM. You can go with a Super Ram or other aftermarket manifold and that will help but they are expensive.

My 87 is a 4+3 car and I use it for street driving, autocross an d occasional track days. As mentioned, the key is regular maintenance for the OD unit with fluid and filter changes every 12-15K miles. The 4-speed was made by Doug Nash and it is really a Borg-Warner Super T-10. I don't use the OD on track so my car with some mild mod is pretty much limited to 125-128 on the top end. Everything I have read here says not to engage the OD unit on the track.

If the tranny in the car is not in good shape, look at the units from Keisler or Richmond Gear. The Keisler 5-speed is supposed to be a bolt in and there is a Richmond Gear 6-speed made specificilly for C4's. The prices are high ($3K or more) but they are supposed to be decent trannies. SK Speed also rebuilds the 4+3 with heavy duty parts.

For me, the stock brake 11.5" size works 'OK' for track use but I buy generic "white box" rotors thru the shop where I work and toss them after they show wear. Race-level pads for the early brakes are hard to come by but you can get Hawk HP+ for the front and rear. I run HPS pads for street use and swap to HP+ and new rotors for track use. A brake cooling duct set-up would be a big help for the stock brakes.

Make sure the PS pump and rack is in good shape. The chances are that you can blow the seals in the rack with lots of high RPM use from track use (I did). I replaced the rack with a Z51 version. I use Red Line PS fluid in my car. I don't have a PS fluid cooler and the factory Z51 cooler is hard to find. Go with an aftermarket cooler. A small 2-pass cooler and custom lines will work fine.

Suspension upgrades like larger sway bars, poly busings and good shocks will help the car handle. Vette Brakes has a nice C4 suspension package but it's pricey. Install a set of Z51 springs and shocks and larger sway bars and that should make a base FE1 car handle much better. Vette Brakes also has some good alignment specs you can use as starting points. Add an R-D Racing Camber Brace to stiffen up the front end and a good 5 or 6-point harness bar.

My acrylic top has shown some signs of additional cracking over the last couple of years and that may be due to flexing from autocross and track use. Consider the R-D Racing Targa Truss or something similar to keep the body stiff.

AUTO_X_AL

DRM has a bracket to adapt early C4's to the J55 or GS brakes. Not a real big deal. The VBP or DRM bias spring will help flatten the car out under hard braking. With a 3.54 gear and 25.6'' tire your top end is about 127 MPH @ 5900 rpm so usable is about 120 without OD. I have heard of some guys using the od with the clutch on track. If you had an on demand od system it would work but the OD is a bit tempermental.

0Paul Ruggeri

I haen't had any problems with my 4+3, but I don't use the OD on the track. I would definitely get a Z51, it will save you a lot of time and money changing the springs, sway bars and steering. I added a R+D camber brace, rebuilt the front end with poly bushings and lowered the car an inch. The turn-in feels much quicker now. I run Hoosier 295/30-17 A6s on the track and I can keep up with most street tired C5s in the corners, but they run away from me on the straights. I also use Hawk HP+ pads, but the stock 86 brakes aren't up to it. They get hot and fade if you're to hard on them.
Part of the fun of this for me is gradually improving the car and my driving.


Paul