7:31 R & P

The engines out and tranny is being rebuilt on my 81 SC. Long story short some of the work i was anticipating needing to do won't be necessary.. so I've got some car money burning a hole in my pocket.

Has anyone out there switched to the 7:31 ring and pinion in a 915 . I know it will reduce my top speed, and I don't really care. I don't spend a lot of time up around 150mph. I would like better acceleration. I just wondered if it negatively impacts daily driving ability.

Thanks in advance

Jeff C

81 SC

It will also raise revs at interstate speeds. This can become really annoying. I feel that the car is already geared very low compared to more modern designs. Do you already have ssi and a good free flow muffler? Or, if you go to the 7:31, take the opportunity to upgrade tires to better use the increased mechanical advantage of the gearing.

I would prefer the 7:31 over my existing 8:31. And some day this will happen to my tranny. but I intend to do the SSI upgrade before that happens.

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'83 SC

IMHO 7:31 R&P is not the way to go. Heres why, to me first gear was low enough. Maybe a notch taller would be better that stock in a late 915 8:31 tranny. Fifth was just perfect for crusin' the freeway. I changed second from 18:32 to 17:34 down three gears. Third was changed from 23:28 to 21:29 down two gears. Fourth was changed from 26:26 to 25:26 down one gear and fifth is stock. In my 1971 911 with 3.0 motor and 915 late tranny and ZF LSD set a 80% I think that this is the best set up.

For just changing the R&P to 7:31 you make first gear worthless. It's like the old pick-ups that had a 4 speed, my first thought was, cool, not understanding that first gear was called compound low. Why build a box with one gear that is usless.

Randy Jones
1971 911

I have put the 7/31 in my 915 (see the gallery). The car just FLIES! At speeds between 25 and 70, you are always in the right powerband. Yes the noise on the highway can get tiring. I wouldn't have done it if it were my only Porsche. I've put that against the SSIs (I've removed the first muffler). I didn't take the exchangers out because they never were in 26 years, and I didn't want to break a stud, as the engine is really going perfect.
Beware that the mating sorface between the R&P is small! So go easy on power launches.
GeorgeK

Oops

[This message has been edited by GeorgeK (edited 02-17-2001).]

Again...

[This message has been edited by GeorgeK (edited 02-17-2001).]

Call Roger at Carquip. They make alot of custom boxes and custom ratio gearboxes and know alot about what mods will do what for you. They have ads in Excellence and Panorama. Phone#303-443-1343

Also, how 'bout a Torque biasing limited slip differential, that is, if you're ichin' to spend that cash.

Hope this helps.
Nick.

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Nick Shumaker
1982 911SC Coupe

I agree with Randy. The 7/31 change is effective but not ideal for all around use. What Randy did was target only the "less than ideal ratios" and left the others alone. 2nd gear is the worst and 3rd and 4th are a little tall as well. 1st is just fine for the torquey 3.0L. My 72 915 trans with 7/31 is too short geared in first and fifth for my taste, but when I change to E cams from T cams I may change my mind and enjoy the low gearing with the free revving cam profile. It's all about matching power band to gearing.

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Tyson Schmidt
72 911 Cabriolet

Gearing has just as much to do with driving style and preference as it does with performance. If the owner of the car does not like to drive at high revs all of the time the performance factor is moot.
The gears in my SC are presumed stock and I would not change a thing. Redline is 140mph, spacing is acceptable, all gears pull well. Interstate driving speeds are around 3200-3400RPM. I don't like to drive harder than that for long periods of time. If I were going to change the gearing I would personally prefer to tailor each gear rather than just lowering them all. Last time I checked gears ran around $100 or so each.

Gear pricing from Guard Transmision
First gear = $1200
Second thru fifth = $550

First is so expensive because it has the imput shaft. GT makes an imput shaft thatthe gear is not fixed like the factory.

Randy Jones
1971 911

Thanks gentlemen,

A useless first gear and revs up around 4000 plus to maintain highway speeds huh? Oh well it was worth tossing around..I'll put the cash toward next years supercharger project.

Jeff C
81 SC

Randy - I'd never buy that stuff new, unless I was building a racecar or high output street machine. Obviously you have to be careful when buying wearable moving parts used, but as you point out, the price of new is not worth the effort.