It’s winter, and that means winter projects!
I’m looking to re-gear the 915 in my 3.2SS + 993SS cammed + SW chipped SC. Should have done this when we rebuilt the engine & box in May, but you don’t know what you don’t know & the 10k miles since then have given me a better take on what I want out of the ratios. I’ve read through many other threads to help guide me to where I’m at now.
All gears except 5th (a longer 29:22) are currently stock for a 915/61.
Car is primarily a backroads/mountain rallye car. Lots of weekends & events in WV (sweepers and some hairpins), NC (tighter, more technical) and VT (a mix). I mix in some track days & AX where I can.
Problem to address: stock 2nd (1.83) to 3rd (1.26) is a big drop. 6.8k RPM in 2nd drops you to ~4.7k in 3rd, which is just below where the cam is really working. You’re trying to wring it out without bouncing off the limiter.
Going to have Steve Wong set the limiter to 7.5k (have the rods & valvetrain for it) but that’s a bandaid, not a fix. No more power up there, and while it’s nice to hang onto 2nd for longer vs upshifting in some circumstances, it’d be nice to keep the car in the sweet spot of the 993SS cams. The pull from ~5k to 7k is
strong. They’re good from 4k+, but there’s a real step function from 5k+.
Current thinking: stock 2nd, shorter 3rd and 4th - “Option A” in the table below.
- 2nd: 18:33 (1.83) — stock
- 3rd: 21:29 (1.381)
- 4th: 24:27 (1.125)
- That shrinks the RPM drop from 2nd → 3rd from ~2,250 rpm down to ~1,770 rpm, and 3rd → 4th from ~1,490 rpm down to ~1,330 rpm.
- 2nd → 3rd is really what matters for me.
- I could go with a shorter 2nd: 15:29 (1.933), but then I'd need to find a matching 3rd to get back into that ~1,800 RPM drop window.
- My concern with the shorter 2nd is that it ends up too short, which means more shifting (2nd → 3rd → back down) than is actually useful.
Summary of options + links to charts:
Option A: Stock 2nd + short 3rd+4th
Option B: Short 2nd+3rd+4th
SCRS:
Put the SCRS ratios there for comparison, but they’re almost definitely too short for what I want.
All of these look to be available via Guards (
https://www.guardtransmissionllc.com/products/transmission-gear-sets/915-gears/) as well.
Thoughts? Feedback from folks with re-geared 915s appreciated.