Cyclonite

I have a 91, L98/ZF6 with 3.43 rear end (unless it was changed by a PO), and seem to run out of gear awful fast without much top speed. What gearing are any of you other C-4 guys running? Do you feel you run out of go before getting up to a good speed?

For comparison, at Summit Point, either the main or Shenandoah circuit the max I hit is about 100-105.

AU N EGL

ina C4, C5 or C6 for most ppl the 3.42 rear gear is more then adequate.

if you need more then that use 5th gear

anything else, 3.90 or 4.10 you will use 5th

Cyclonite

I frequently go into 5th and occassionally red-line 5th and have to hit 6th. I was thinking higher (numerically lower) gearing, like 3.10 or 3.03 range. Or would this kill my low end too much?

The car is not a dedicated track car, and I drive it to the track (therefore have to drive it home) so I don't like over-revving it too much; I don't need to blow the engine. It just seems like I hit 5500-6000 and . 6th doesn't really accellerate too much once I hit that.

Kubs

It sounds like you have a 3.73:1 rear or higher if you are hitting 6th! Or the tracks you run have super long straights. To eliminate the guess work you can take the cover off the diff and count the teeth to see what ratio you have. What RPM do you run in 6th at 60 mph on the highway?

spazegun2213

sounds a lot like a 3.90 if you are going into 6th!

7k rpm in 4th with a 1:1 and a 3.42 is about 155. My vette is not super fast, but it takes me a LONG time to reach that.

AU N EGL

Could be a 4.10 or 4.11. IIRC many C4s came with 4.11s

Cyclonite

Thanks guys...sorry I haven't been around much lately.

Okay, I had forgotten what my revs were at speed, and had to wait for decent enough weather to go for a ride.

At 60 MPH, I'm at 2200 in 6th, 3200 in 5th, and 4200 in 4th.

I say it's a 3.43 because of the RPO code. As far as I know it's still stock, but I haven't verified it. Same with the engine, AFIK it's a stock TPI L98, so redline is pretty low. My tach only goes to 6K, redline is 5000 or 5500, and it starts losing it's power around 4600.

As far as the tracks I've run, it's only been a few; Limerock Park, Summit Point (Shenandoah and main), and the Lightning track at NJMP. The Shenandoah has a 2900 ft straight (according to their website), I don't know what the rest are. The highest speed I've hit is 117... Usually I'm more in the 100-105 range.

Eventually I plan on doing work on the engine to be able to safely get more revs, but for now...

Thanks again.

Carolina C4 Racer

If you are turning 2200 RPM in 6th gear @ 60MPH it ain't a 3.42!!

When my 92 had the stock 3.54 rear gear, I was running 80mph when I hit 2000 RPM in 6th gear.

Another thing to consider - When the rear gear was changed was the speedo corrected???

mrc24x

The stock TPI is holding you up more than any gearing. When I had a stock L98,TPI w/ headers I would max out at ~128 mph at Summit. I was limited by the 4sp w/ a 3.07 rear. (1988)

The '91 should have a 3.45 if it hasn't been changed. I think a few were equipped w/ a 3.33 but it's rare. You can always jack the car up, rotate the rear tires 1 revolution and count how many turns the drive shaft makes. (3 1/2...) to get an idea.

Cyclonite

My bad...yeah the RPO code is GM3 which is 3.45. It sounds like it may have been changed then. Speedo is pretty accurate, according to the various automatic speed-signs I've checked it against it's within 1-2MPH, depending on which wheels I have on it.

Some P.O. must have wanted a little more SOTP out of it...maybe running it on 1/8 or 1/4 mile straights.

Looks like I'll have to put it in the air and count the drive shaft spins. I need to change the rear fluids too, so maybe I'll count the gears while I'm in there, just to be sure.

Thanks for all the input.

T