Help with tranny decision

[Paul Thomas]

This winter i am thinking about pulling my tranny to replace the 1st and 2nd syncros and while i am at it i want to change something about the gearing. I say something because i am not sure whether to change the R&P, as some have suggested, or to change all of the gears. It boils down to the fact that i want more torque.

The box is a euro 915(LSD) with the loop cooler, and all of the gears are pretty tall, so much so that, even on the highway, i rarely use 5th. I think 1st gear is the same as the US version but the rest are taller. This is speculation because when i see gear ratios in print, it might as well be greek.

Who is the tranny guru? Getting the work done nearby is no problem, i just want some feedback on what to do. I want it to be driveable around town and in the hills.

Thanks,
Paul

[Elombard]

Greek - I agree why are P car ratios so strange? My old Nissan Comp box had a 1:1 5th I think a .89:1 4th etc. it was relatively easy to understand. Hopefully some gurus will enlighten us.

[Bill Verburg]

There is a ton of gearing data in the archives, here is the results of a search on "gear* and ratio" that I just did, Search Results

The bigger the ratio the lower the gearing, tires, f/d and individual gears must be considered as a package.

If you have some more specific questions ....?

[masraum]

Porsche gears are usually listed by the number of teeth instead of the decimal equivalent, ie 25/22, if you divide 22 by 25 you get the effective ratio that most of us are used to seeing for 25/22 you'd get .88 [I made up that combo])

The difference between an 88 US car and an '88 Euro car 5th gear is I believe .89 (US) and .86 (Euro). THe 4th is a little higher too, but 1-3 are the same. Since your car is a '70 it could have a wide range of ratios since back then there were often several different sets that you could order as an option. If you give us your tire size, and then rpm at a certain speed we can tell you what the gear probably is. I think a '70 had the 7:31 ring and pinion which gives you a pretty low gearset already, 4.43. If that's the case then you will likely have to change your gears rather than your R&P.

That's the theory, some one more familiar with the early cars will likely chime in with more accurate numbers before too long.

[Paul Thomas]

I just realized that i wasn't clear on the fact that the tranny is 1986 915. If i still had the stock tranny it would be a 901.

Bill- I am just looking for input on gearing to give me more low end grunt in the 2-5 range. I believe the 2 options would be to change the R&P to a 7:31 and shorten everything OR get different gears for 2-5, i think 1st is fine. I guess the other option is get an earlier 915 or even a beefed up 901 with LSD. The 901 idea would even save a fair amount of weight.

Edit - Thanks for the link, i dont know how i missed a topic that i am considering right now. sounds like the change in gears is the way to go over the 7:31.

Paul

[Bill Verburg]

Edit - Thanks for the link, i dont know how i missed a topic that i am considering right now. sounds like the change in gears is the way to go over the 7:31.

That would be my rec.



The 915/67 is the euro trans. the data in the table is from my car, tires are a very important factor

Also consider if freeway type driving is important to you, one way to do that is to drive around and pretend that your 4^th is your 5^th

This is Jacks trans


The torquiness of your engine also makeas big difference, my gearing w/ the 275s is great for a 3.8 would lilely be un satisfactory for a 3.0. There are numerous compromises that need to be decided on.

I really do hope that you did a search, I have posted tons of stuff looked at from various perspectives.

[ChrisBennet]

I wouldn't just change syncros. If 1st and 2nd are giving you problems change 1st/2nd slider and dog teeth as well. Price gears before you get too emotionally invested in the idea.
-Chris