Transmission gear swaps?

[czwbkx]

For any gearbox experts on the board; I'd like to change my shift points a bit by making changes in the gear box. The goal is to reduce the rpm of the engine a bit (to account for some changes i'm making to the torque band). Ideally, I would like to use existing gear-sets, but before tearing too far into the transmission I'm hoping to learn from your experience.

Is it possible to use the 32/21 (1.52:1) fourth gear from a 930/33 gearbox as a direct replacement for the fifth gear position in a 915/61 gearbox? Also, can the 8/27 (3.375:1) ring & pinion gear from a 925/13 Sportomatic by used as a direct replacement for the 8/31 (3.875:1) in the 915/61?

Any other thoughts on building up a race transmission will be appreciated.

Thanks,

R

[TimT]

Im not sure about using the 930 gears in a 915 box, or using the r/p from a sportomatic

but I re geared my 901 box this year.. I determined how fast I go and at which track I see my fastest speeds (watkings glen, summit point) and I realized I was hardly ever using 5th at these tracks.. ( my car is 2.2 ltr 1900 lbs) so I chose a shorter 5th, and basically evenly divided the other gears btween 1st and 5th, making sure I have lessening RPM drops as I shift through the gears, this is because the faster the car is going the harder it is to push through the air.

You should do the same, chose a 5th gear that gives you a terminal velocity you want, then select your other gears making sure you have no drastic rpm drops between shifts.

you can take some gears and invert them and use them as the inverse of the ratio.. for example take 901 gears, if you invert a ZD 5th it becomes the same ratio as K 3rd.

I have some excel spreadsheets that make visualizing gearing alot easier, let me know if you want a copy




tgif!!!

[Bill Verburg]

Here are the available sets
Fourth gear

• 22:28
• 24:28
• 25:27
• 25:26
• 26:26
• 26:25
• 27:24
• 27:25
• 28:22
• 29:22
• 28:23

Fifth gear sets

• 26:26
• 27:25
• 27:24
• 28:24
• 28:23
• 28:22
• 25:26
• 29:22
• 29:21

The only 2 r/p that I know of are the 7/31 and 8/31, to put a 7/31 in a late 915 there is a new design which is not the same as the older 915 7/31

[czwbkx]

Bill, you're awesome. I'll plot the curves with these gear sets to see which will work best. Are these Porsche parts or aftermarket? Can you point me to a source or part numbers?

thanks,

R.

[Bill Verburg]

They are all factory parts. If you narrow it down I can give you the part #s

Steve Timmins has posted some nice gear calculators, he does it backward from the way I would do it but the results are the same.

Dr. Steve Timmins