Gear ratio's for a 915 for the street

[carrera1975]

Any recommendations for a street 911 3.0 915? doing a rebuild now so if I'm going to change gears now the time. Facts:

It is a 3.0 engine in a 1975 carrera.
ZF limited slip at 60%
Org. magnesium case
CIS
suspension is fully dialed


The car is a street car with limited track driver ed. ( Laguna Seca, Sears point)

What I'm looking for is quicker acceleration, but flexibility in the power band for the trip over to the beach (mountainous terrain)

thank in advance

Craig

[Bill Verburg]

'75 US 911 & Carrera came w/ 915/40 5speeds. Stock gearing is very good for most purposes


You have nice progressive & reasonably sized drops and a reasonable speed in each gear.

If you don't have a lsd that would be a great upgrade, best for the described use would be a GT 40/60, but a ZF 40 would also be good

[carrera1975]

Thanks again Bill, I have a zf in place now, is the lock adjustable? I think the last time I had the Trans rebuilt it was at 60% It is great, hammer it and it will go straight!

How about any internals for durability?

Craig

[RoninLB]

I feel stock gearing dealing with speed limits is nice. I never have to race engine or lug it. A nice 2.8-3.2k rpm keeps engine clean and cool.

I also have wevo parts so it'll support a 250hp 3.4

[Bill Verburg]

Originally Posted by carrera1975

Thanks again Bill, I have a zf in place now, is the lock adjustable? I think the last time I had the Trans rebuilt it was at 60% It is great, hammer it and it will go straight!

How about any internals for durability?

Craig

on the ZFs lock % is only notional, the actual breakaway torque is adjusted by arranging preload and disc assembly order, notionally from 20% to 80%(street to track only setups)

the down side of higher lockups is increased understeer and potential for severe handling issues on ultra slippery surfaces, you really do not want to hit an oil slick w/ an 80% unit

later units used in 964 up also have an asymmetric ramp setup that alters lockup differently on acceleration or deceleration. these are very much improved units, performance wise over the older straight units because you can have the lower % for acceleration and higher on braking where you want the stability that lsd imparts to the chassis

Wevo has a very nice 1 piece intermediate plate and reinfoced side plates, alternately late 915 side plates can be used, g50s have available steel syncros and billet shiftforks bot are extremely expensive, not sure if they are available for 915s

[RWebb]

"flexibility in the power band"

- drive the road a few times and note the mph, gear, rpm and whether a curve or up or down hill - do a few times and in each direction

record notes on a voice recorder - reduce to writing -- analyze...

it's obvious you want something other than what you have now... but are you willing to do the work and pay the $$ to change?

[carrera1975]

I guess what I'm after is a way of "quick n ing" the 3.0. with the reliability of my rebuilt CIS system

in place now

(7 and 8's -16's)
alum pressure plate

[Bill Verburg]

What you need to understand is that it's all a compromise, sure you can gear lower
here's an an example of a really tight close ratio 915 race box


39mph@2000
69mph@3500
137mph@6500

and another for the SC/RS, also a race box



41@2000
72@3500
134@6500

and compare that to your stock 915/40
46@2000
80@3500
149@6500

what is gained w/ the lower geared boxes? acceleration in gear, the engine can be kept w/i a few hundred revs of max hp most of the time
what is the penalty? increased frequency of shifting(when you shift you are slow), increased rpm@ cruise(compare the 3500rpm #s then think what your car is like @ 3500), a narrower range of speeds in a gear(you are more likely to have to shift at an awkward place or time)

increments of 10 mph difference in top make a major difference in the feel of a trans.