Budget close ratio gears in a 915?

[G450X]

Hello All,

I am seeking advice on changing gears on a 915 box for my SC. I have read through numerous threads on 915 gear swaps and the 7:31 R & P swap. I am planning a modest H.P. increase on my '82 SC (Max Moritz short stroke P & C's, early SC big port heads and CIS, 964 camshafts, and SSI's with a M&K 2/1).

I recently picked up a hybrid mag (final drive case) and aluminum (the rest of the case) 915 from a '77. If I recall correctly, the '77 box received most of the updates that made the 915 more reliable (less the reinforced side cover). The previous owner said it had been rebuilt in the last 5k miles, and it was very clean, so I tend to believe the story. I would add a reinforced side cover, internal 1 piece bearing carrier plate, and possibly a Wevo gateshift (if I find the funds).

I was planning a 7:31 R & P swap with a taller 5th gear, but after reading of the negatives (less strength of the R & P and the cost of setting it up, unusable 1st gear), I am leaning toward a gear swap in the '77 915. Tire size would remain normal for 16" Fuchs (R 8" and F 7").

I'm thinking that the stock 1st is low enough, and I could keep 2nd from the '77 915 (915/61) because it has a shorter 2nd (1.83 versus the later 1.77) than my stock '82 915/63. Third would be 21/29 (1.38) and 4th would be 25/27 (1.08). Since this is only one set away from stock (26/26 1.00), could I keep a stock 4th to save money, or would it be too big of a drop from 3rd?

I would keep 5th stock for the freeway. The rpm's are reasonably low with the stock 5th, but not too much of a 4th to 5th rpm difference to worry about having the rpm's too high when shifting up (or down).

I never saw such a combo in my searches, it seems like a fair compromise for a primarily street car - although I might auto cross it a few times a year.

Thanks for any input.

[kenikh]

Wow, no responses

[DG624]

Did you see Bill Verburg's post on the 915 gear options? He has some good information. I was interested in revising 2,3, 4 and maybe 5th:

They used to be
915/71
2nd 15:30 915.302.902.05
2nd 16:35 915.302.202.40
3rd 21:29 915.302.903.10
4th 25:27 915.302.904.15
5th 27:24 915.302.905.10

Much better ratios and better acceleration.

See Bill's post here:
Dan, Here are your transmissions