Just do it!

[Bill Verburg]

Originally posted by mjshira
Bill

What is the best option for my 1990 C2?

Thanks,

James

As w/ a 993 g50/31, /21, /32 or /30.

If you don't want to swap in a 6 spd the stock 964 gearing isn't half bad, your speeds in gear are roughly 40, 68, 99, 129, 161 w/ reasonable drops. You can close it up a little but there just isn't the bang for the buck as w/ 993 6spds

[Tom W]

IMO, a stock gearbox works fine on a 964 until you want that last little bit in a race. I've done well with my stock gears in my 964. Only 2nd, 3rd and 4th get used and I find the fact that I don't need to shift as often allows me to concentrate more on the line I'm driving and other factors. Now, after a year of racing, I'm much more comfortable out there and am considering a gear swap to switch to a 6-speed with appropriate gearing and steel synchros.

[mjshira]

point taken. I imagine the 6spd swap is a big job.

[Bill Verburg]

Originally posted by mjshira
point taken. I imagine the 6spd swap is a big job.

from what I have heard it's not that bad. But like Tom and i both said stock 964 isn't half bad.

An asymmetric lsd would be a worthwhile improvement though

[mjshira]

mine has an LSD from the factory. what is the diff?

[Bill Verburg]

Originally posted by mjshira
mine has an LSD from the factory. what is the diff?

Your s is probably worn out, the motorsports diff help the handling w/ poer on like your 40% stocker did when new, on braking the 60 - 70% lockup will help there better than your stocker

[kurquhart]

Originally posted by Bill Verburg
from what I have heard it's not that bad. But like Tom and i both said stock 964 isn't half bad.

What about changing the final drive ratio (R/P)? It seems like you could just bring down all the gears; is this worthwhile?

[Tom W]

I'm with Bill. The stock 964 box isn't bad. I spent a bunch of money on my 993 box (and a LWF) shortly after I bought the car and the transmission failed (the spider gear failed at 22k miles). While it makes the car a bit more fun, I would not do it again for a street car (a lot of money for limited extra fun). It made the car noteably quicker for autox. It was fun on the track, but I was a novice then and had nothing to compare it to. It has significant implications in your car's class for racing or autox in PCA.

I see no benefit to changing the ring & pinion for track use with a 964. That would make 2nd to low to be worth much and not really improve anything else.

The stock 964 box is actually pretty nice for us lazy types. At Laguna Seca, Infineon, Thunderhill or Buttonwillow I spend most time in 3rd with occasional need for 2nd or 4th. Hell, at Reno-Fernley, I spent half the track in 2nd (which is why I think of it as a long, fixed autocross course - not a fun track to actually race at). It's nice not having to waste time shifting and almost like driving an automatic.

If keeping a 5 speed on the track, I'd make 2nd taller and shorten 3rd, 4th and 5th. With a 6-speed, I'd follow Bill's route. I'd only do this to a dedicated track car where I was in reach of a podium finish but just a tad out of the running where the re-gearing might make a difference. (Otherwise I think it's simply best to improve the driver or suspension - in that order). I'll find out next year how much my suspension improvements/changes have helped (and how much more I need to work on the driver).

My LSD also appears to be dying so it will be replaced/repaired this winter. I'm interested to see what is in there (stock or otherwise). I will be putting in a Guard unit (if that's not what it is). Driving without an LSD sucks (unless you like to slip-sliding around trying to get the power down out of turns). My Turtle is slow enough due to the driver, it doesn't need the extra handicap.

[Bill Verburg]

The topic of changing the cwp keeps coming up as a cheap alternative to regearing, it does help but is not an optimal solution
G50/20 stock


g50/20 w/ 8:32


G50/30 Cup


G50/31 RS

[Leland Pate]

As a strange twist of fate, I may be selling my custom gearbox as I attempt to bring the car back closer to stock this year. I am getting out of the military and may be facing up to a year of unemployment while I apply for other federal jobs.

[Bill Verburg]

Your box is a gem, you should hav no trouble selling it.

[mjshira]

Originally posted by Bill Verburg
Your s is probably worn out, the motorsports diff help the handling w/ poer on like your 40% stocker did when new, on braking the 60 - 70% lockup will help there better than your stocker

I will have the trans out of the car shortly. I was wonder if it would be wise to open the trans and pull the LSD and have it rebuilt? Are there any threads on doing this or pictures? I have the skills to remove the part and have if rebuilt and reinstall it but I would need some guidance on the specifics of how to remove and reinstall it and who to send it to for the rebuild, etc.

The trans has a factory ZF LSD.

[Bill Verburg]

removing the lsd is no problem, it slides right out and back as long as the same unit is R&R'd

It's probably an easy job to replace the discks but I have no experiance w/ that.

Paul Guard rebuilt one of mine w/ plasma coated disks for a very reasonable sum.

[petevb]

To add to this very informative discussion, below is another option for cost effective re-gearing. In my box we kept 1st, moved 3rd to 4th, moved 5th to 6th and filled in the ratios in between with three new custom gears. Since it's a multi purpose car designed for autocross, street and track we didn't want to sacrifice either the low or high end. The resulting ratios:

1st- 11:42
2nd- 16:38
3rd- 19:34
4th- 27:38
5th- 23:26
6th- 38:35



RPM drops:
1-2 2457
2-3 1602
3-4 1388
4-5 1279
5-6 1204

This gearing is quite short on the low end with a stock rev limit, but works well with my modified 7300 rpm motor:



The results of both shorter gearing and a higher redline are shown below, comparing my motor with a 6500 rpm redline and stock gearing vs the same motor with a 7300 rpm redline and the shorter gearing. 25-65 is especially critical for autocross.

[Bill Verburg]

Comparison of 993 RS/Cup gearboxes including rpm drops and cruise rpms

[Bill Verburg]

I had nothing to do today so fiddled w/ a thrust chart comparing 2 different trans in the same chassis. I added the rev regimes of interest for track use in contrasting color

[Tom W]

Bill: Do you have the /32 gears correct above? It looks like you might have made a cut/paste error (or I have). My chart shows the /32 as have 23:35 (1.522) for 3rd, 29:36 (1.241) for 4th, 32:33 (1.031) for 5th and 35:29 (0.829) for 6th. You show the /30 and /32 as being identical.

It's interesting the difference a year makes in reviewing this thread. I still think the 964's 5-speed is a reasonable box, but I will be changing to a 6-speed this winter (likely something similar to a /32). Gearing now makes a difference in my driving and lap times (after 5 years I guess I'm getting to be a better driver) and I want to be more competitive next year.

[Bill Verburg]

Originally Posted by Tom W

Bill: Do you have the /32 gears correct above? It looks like you might have made a cut/paste error (or I have). My chart shows the /32 as have 23:35 (1.522) for 3rd, 29:36 (1.241) for 4th, 32:33 (1.031) for 5th and 35:29 (0.829) for 6th. You show the /30 and /32 as being identical.

It's interesting the difference a year makes in reviewing this thread. I still think the 964's 5-speed is a reasonable box, but I will be changing to a 6-speed this winter (likely something similar to a /32). Gearing now makes a difference in my driving and lap times (after 5 years I guess I'm getting to be a better driver) and I want to be more competitive next year.

oops, yes, I pasted the wrong data, it's now corrected and updated to 6800rpm shift and 24.6" loaded height for a 265/35x18


Thanks for keeping me on my toes, I need full time keeper these days

[Bill Verburg]

Here we have the HP curves overlayed w/ the thrust curves


It can be seen that the engine hp curves are always the same height but are stretched horizontally because more time is spent in each gear

As always the thrust curve also has the same shape though at a lower level in each gear. Thrust varies w/ the overall gear ratio.

The question always comes up, "What's the best shift point to maximize acceleration"
answer shift to maximize thrust in the next gear consistent w/ preserving engine life.

Here we have the HP & thrust curves for 1st and 2nd, note that shifting at 7K puts you in the fat part of the thrust curve for 2nd.

[Tom W]

Bill: You've probably noted it somewhere (but I can't seem to find it). How are you calculating the thrust? You've piqued my curiosity and I want to make the equivalent calculations for my car and potential gearing.