911 3.2/915 gear ratios

[gliding_serpent]

Just wondering what people are running for gear ratios in their 3.2's with 915's? I have all stock now, but the gearbox is open and needs work, so may digest a closer/shorter ratio 2nd-5th.

My goal is 130mph in 5th at 6200rpm. A 4.43 ring and pinion final ratio (or stock 3.88 with closer/shorted gear ratios for a similar effect) with stock gears would give me the ideal balance of road and track compromise and get me around this goal.

A few things to ponder:
- As is stands I got to about 111mph on our back straight on 205/225 RE-11a's and will be running 225/245 toyo RE1's this year.
- The above speeds were after my first time with the car at our track. I was running 8/10 at best. Maybe pushing 6k occasionally. I figure I can do 120mph easy, thus a 130mph 5th gear target.
- we use second gear for one corner. I have the stock 1.83 (18/33) gear for second. I think it will be fine and I will not be shifting mid corner so long as I use my full RPM's in second.
- mostly 3rd gear track.
- I know it would be ideal to feel out the track some more with the more aggressive tires to get a better idea of gearing needs, but the transmission is open for other reasons.
- Matt of Guard sent me the gearing spreadsheet which I have been paying with.

Thanks for any words of wisdom/help

[stownsen914]

A couple things to consider:
- Using commonly available gearsets from one of the 915 variants that Porsche made over the years. They'll be much cheaper than aftermarket gearsets or rare, non-standard OEM gearsets. Be sure to consider 4 speed gearboxes, and they had very different ratios than the 5 speeds.
- Unless you really want a shorter 2nd gear, leave the stock one and just replace 3rd-5th. The stock 2nd is plenty short for most uses. In fact, in my race car I put in a taller 2nd.

You may find that you can get pretty close to your ideal set of ratios doing the above, or at least minimize how many of the expensive, custom ratios you wind up buying.

Scott

[gliding_serpent]

My gears ended up being is decient shape, fo there was no reason to change them. Bigger fish to fry as i ended up needing a top end rebuild.

I too consitered a longer 2nd as opposed to shorter 3-5th.

[Quinlan]

The norm in PCA club racing is to use 15 inch wheels with 275/35x15 Hoosiers. The overall outer diameter of these tires is about 23 inches, vs. about 25 inches for the Bridgestones. This gives an effectively higher final drive ratio.

[gliding_serpent]

Turns out I have a 76 7:31 (4.43)!!!

This explains a lot.

1984 911 3.2 - The beginning of the end

[Matt Monson]

Yes that does.

[DG624]

How many use 5th? If I change to closer 2,3,4th (1.08) then does stock (.76) 5th drop off too much for highway? Has anyone done this?

[gliding_serpent]

I ended talking to matt and getting his 1.933 second, 1.381 3rd, 1.08 4th, and 0.88 5th. 4th chosen to max out on our back straight with a little room to breath/grow as this is not a race car.

Lots of people keep stock 5th for highway use. It is a bit more work for the synchros, but a common choice. I personally wanted all four gears to match up, even though i would not be using 5th on track. Call it ocd.