any gearing gurus here? - Page 2

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by jmz

looking at an old post of Bills and one of his charts I have to ask.

Is the gearbox that 356RS recommends just the same as a stock 915/61 63 just with my 7:31 Rp in in it? I assume that a 25/25 is the same as a 26/26 correct or not? the s/n is a 28/23 right?

Getting more confused by the second!!!

Yes, /61 and /63 are are very close to the one 356RS posted, very minor differences except of course the 7:31cwp.

Yes, 26/26 is a 1:1 gear set like 25/25 just a little stronger

KTL · 02-08-2008, 01:08 PM

It's easy to get confused when you're just looking at numbers and not being familiar with what's interchangeable with other model years. You're getting the hang of it though. Seems like you're grasping the ratios and how things are affected by them. To make an informed decision as to what gears suit your desires, you MUST chart them as Bill has done for you. It's the only way to see what's going on from gear-to-gear. Believe me, I know how you feel!

Like others said, it's really a guessing game because there's no perfect ratio selection for every situation. Each engine-trans combination and intended use (street, street/track, track only, track specific.......) is different. At some point you must compromise, regardless of your intended use. If you just drive it on the street? It's easier to decide because you don't have to consider what the effects of changing ratios will do to your geographic location on the track. In my opinion, where you're at on the track is equally important as how the gears place the engine in the torque curve.

I've been diddling with 915 boxes the past 4-5 years, only changing out dog teeth, synchros and sliders. So i'm familiar with the parts and what they do. But changing gears has not been my experience and doing so gets your mind going about what rpms, rpm drops, mph will happen and where. Not to mention the can of worms you open when you have to reassemble and set everything up to work together properly.

I'm currently playing with my G50 trying to make it street/track friendly so I can retain some driveability on the street and still make it worthwhile (cost-wise) for improvement on the track. It really hard to make a decision. Especially if you DIY it because the transaxle is a precise mechanism that you can't just slap back together. There's tight tolerances that must be maintained with the operation of the trans that isn't measured with common tools- some specialty fixtures and instruments are needed.

jmz · jmz's Garage

Thanks Kevin. My head is swimming right now. I think I've got it figured out then all of a sudden I think about the occasional road trip(seldom) and the amount of gas I'll be sucking down let alone the noise etc buzzing along at 4000RPM at 80mph and I think no way - then I think ahh why not?

I am starting to lean to the QQ SN option above...

antaircooled · 02-10-2008, 01:59 PM

I have a very good SN that has just been checked by Porsche NZ. I went with an RO 27/24 in my 915/61. It's a track car and I needed more go in 5th on two of the tracks I race on. Similar gearbox to what you have but I have the 8:31 Ring and Pinion.