915 Trans, Ring and Pinion Question

[constaf]

What performance difference would i notice by going from a 8:31 to a 7:31 ring and pinion on my 1980 SC?

thanks

[constaf]

thanks. Do you know where i can find their comments?

Fred

[Rich76_911s]

Here is a great article on it by Noah:

http://www.dorkiphus.net/porsche/showthread.php?t=8642

[constaf]

awesome article. awesome. I got the information that i was looking for.

[Cory M]

here's some reading material for you (I'm going to be doing the same r&p change soon too):

7:31 R&P torque capability or 8:31 gearing issue

7:31 vs 8:31 ring and pinion

915 R&P Whine - How do you get rid of (fix) this??

Best Year for a 915 Transmission?

All the Way Into 915 & Comin Out (Long)

[Bill Verburg]

Originally posted by constaf
What performance difference would i notice by going from a 8:31 to a 7:31 ring and pinion on my 1980 SC?

thanks

It depends on what you are looking for,
a drag racer? sure it would probably help

dual purpose?
ok but not great on track, ok but busy on the street

dedicated track car?
no way
here is what a stock 915/63 used n "80 -81 US SC looks like


And here is a 915/63 w/ 7:31


Compare top end, drops through the gears, speed in gears and speed at ~2k. Those are the main factors to consider

Besides the fact that you lose a little reliability, probably not an issue w/ a 3.0 though.

[chrisp]

another way to look at this is if you run 22" diameter tires (versus the stock 25") you'll get the same affect, feel like you have bigger brakes, have a much lighter wheel/tire combo, etc.

Something like a 225/40-15 and 245/35-15 would be perfect.

[chrisp]

Bill, how about a bigger tire. Figure it for 25" and also 25.5" if you could. Most commonly used DOT-R race tires run within these diameter limits.

Topping out at 130 mph is a definite problem on track.

[Bill Verburg]

I know it says 245/45x16 but the dynamic loaded rolling radius of the tire is what is important.

24.5" is the # to use in this app to aproximate a 265/35 x18 tire

a 245/45 x16 would need ~23.9"
a 275/40x17 ~24.9" etc.

[constaf]

Thanks. But then why is a 7:31 r&p ratio considered an upgrade by PCA club racing rules? Is it because you will get a quicker initial acceleration or because you can stay in 3rd gear more consistently instead of waisting time down shifting to 2nd?

Originally posted by Bill Verburg
It depends on what you are looking for,
a drag racer? sure it would probably help

dual purpose?
ok but not great on track, ok but busy on the street

dedicated track car?
no way
here is what a stock 915/63 used n "80 -81 US SC looks like


And here is a 915/63 w/ 7:31


Compare top end, drops through the gears, speed in gears and speed at ~2k. Those are the main factors to consider

Besides the fact that you lose a little reliability, probably not an issue w/ a 3.0 though.

[constaf]

I just sold my 1985 Carrera and bought a 1980 SC. If the SC has a 8:31 ratio, will it feel different than the Carrera? Some other posts suggest that it will. Thanks

[Bill Verburg]

Originally posted by constaf
Thanks. But then why is a 7:31 r&p ration considered an upgrade by PCA club racing rules? Is it because you will get a quicker initial acceleration?

PCA stock classes are stock, Yes, While in a given gear you get quicker acceleration but at a lower speed, the penalty is the shortness of the gears which on a track means more shifting. There is no improvement in the rpm drop between gears.

What you want is to decrease the rpm drop between gears, this does the same as lowering the cwp ratio and keeps the rpms in the fattest part of the power band more of the time.

As always the other penalty is higher revs at highway speeds.

Here is something that has the best of both worlds. stock 915/63 w/ short 3, 4, 5

[Bill Verburg]

Originally posted by constaf
I just sold my 1985 Carrera and bought a 1980 SC. If the SC has a 8:31 ratio, will it feel different than the Carrera? Some other posts suggest that it will. Thanks

An '85 US CArrera has a 915/68 which is geared identicaly to a 915/63 in an '80 except that the 915/68 has a taller 5^th

The feel through the gears then depends on the relative weights and torque characteristics of the engine. Shoud be pretty similar though except in 5, where the Carrera will feel slower at a given speed.

[philippeF]

Hello M. Verburg

Do you have the same charts for an euro carrera (1974), and the 73 RS ?
those are nice !
thanks


Philippe

[john walker's workshop]

fun around town, higher revs and poor mileage on the freeway. i've done it and gone back to the 8:31.

[Bill Verburg]

Originally Posted by philippeF

Hello M. Verburg

Do you have the same charts for an euro carrera (1974), and the 73 RS ?
those are nice !
thanks


Philippe

'73 2.7RS 915/08



'74 2.7RS 915/06

[philippeF]

thanks again
Philippe