emptnest

I ran my first PDX - HPDE yesterday with my 1993 stock LT1 C4 Vette. Great event at Summit Point WV, organized by Washington DC SCCA.

The day began rainy, and wet track conditions kept speeds low. Dry afternoon conditions allowed greater speeds, leading to this question.

I accelerated for the full length of the straights, and at the end of the main straight, approaching the braking zone for turn 1, RPMs would max the tach at 6000. In hindsight, I probably should have shifted from 4 to 5, but being literally only 50 yards or less from the braking zone, I stayed in 4th. A couple of times in that condition, I noted a momentary loss of power (almost like a miss), and simultaneously hit the brakes and backed off the throttle. As I shifted to 3rd, hit the apex and added throttle, all was well.

Does the C4 have a rev limiter? Might that explain the momentary power loss? Or was something else going on? She ran perfectly afterward each time. My instructor was a Porsche guy, and couldn't explain it.

Diagnosis please.....

s346k

yes, that was your rev limiter. stay away from it if you want the engine to live.

emptnest

Thanks. I studied the track via Youtube, and used one particular video from a C5 or C6 ZR1. That driver stayed exclusively in 3rd and 4th, so I tried to do the same. It worked pretty well, except now I realize he has more RPMs available to work with.

I will watch my tach closer, and adjust accordingly.

DanielRicany

They have different gearing than you do.

Tom400CFI

How do you know? You don't know what gearing the OP has. In fact we don't even know what CAR the OP has. My C6 Z51 had virtually the same gearing including rear gear, that my '92 has. Had 750 more useable RPM though.



*'84-'91 have NO rev limiter other than the TPI intake itself, and the stock valve springs.
*'92 - '96 LT1/4 have a rev limiter at ~5850 RPM. What car does the OP have?

My '92 LT1/6spd/3.45 is good for about 120mph in 4th before I hit the limiter. I've never had to go into 5th at a track yet -our track here, Miller, has a 3800' front straight and I just barely get to 120 when I have to brake.

s346k

it says in the original post what car he has...

Tom400CFI

Ah yes...read right over that.

Stick or auto? Need to know that to make any claims about gearing.

Though we (I) know know it does have a limiter. ~5850.

MrWillys

The ECM shuts off the injectors at 5851 and turns them back on at 5817. Any good programmer can modify this if you like.

emptnest

Car is 6 spd. It has minor mods: MSD ignition, under drive pulley, shorty headers.

Stock black tag ZF6 transmission, stock rear end. I recently purchased it and have no idea if it has been custom tuned. Although a clue is that the engine temps seem to run cool; around 194 with ambient air temps in mid 70's. And I just noticed that at least one cooling fan on the radiator seems to run even at start up regardless of engine temp. So I assume that indicates a tune might have been done.

Tom400CFI

Ahhh, so that IS about the same gearing as a C5Z or a C6Z. They have 700-1200 more RPM than you do, so that is where the added speed is coming from. And maybe from the rear wheel diameter if it's a C6.

My car runs at the same temps, so your temps aren't indicative of anything out of the ordinary. Stock T-stat is a 180, so 194 on a 70* while moving is just about right.

EDIT: Fans will run at anytime/any temp, if your AC is on and compressor is engaged. Needs to, to pull air over the condenser. Defroster runs the AC, FYI, so that would do it too.

s346k

pretty sure a c6z has a longer 3rd than a c5z and stock for stock I think the tire diameters are the same.

emptnest

Good info. I didn't realize that the AC would require the fan full time. You just saved me unknown hours chasing an imaginary problem.

Tom400CFI

While there are slight variations in gear ratios through out the years and models the differences are...slight. However, in this thread, the OP was asking about running out of power at the track in 4th gear. 4th gear is the same in all stick shift Corvettes, C4 - C6, and that is 1:1. The OP's car has a 3.45 rear, and so does a C5 stick and a C6 ZR1.

Daniel Ricany said the difference is due to gear ratios and it's not since all the cars discussed in this thread have the same 4th gear ratio and the same rear gear. It is due to a different RPM limit, and in very small part, tire size.

The C4 has a 17" rim, and a ~25.6" tall tire. The C6ZR1 (car cited above in vid) has a 20" rim and the total diameter ends up being about 1" larger. This is a very small difference; ~6 mph diff at 140 mph, so about as meaningful as different gear ratios in the various T56's that were offered in the C5's and C6's (other than 4th gear).