Swapping to 7.31 R&P
 

I am considering changing the 8.31 R&P on my 915 tranny to either a 7.31 or an 8.35.
With regards to using a 7.31, is it a straightforward process of swapping one with an 8.31?
Can I use any good, used 7.31 R&P from earlier 915's, and put it into my later-915 tranny?
I know the 8.35 from Guard Trans is available, and is a more robust solution.
Just wanted to know the pros and cons of of both options...before I jump into it.

I'd call Matt at Guard, he will give you the correct advice.

To look at the finished installation of each, the 8:35 is a far more confidence inspiring solution.

Pete ... Whats are the pros/cons for a 8:35 RP vs the others. I dont think Ive ever seen/heard of one .. Im curious. Thanks!

Lot's of info here:

http://forums.pelicanparts.com/porsche-911-technical-forum/722760-has-anyone-used-8-35-ring-pinion.html?highlight=pinion+bearing

...and here:

http://forums.pelicanparts.com/porsche-911-technical-forum/720070-broken-915-pinion-bearing.html?highlight=pinion+bearing

Thanks Pete. Good info. Im guessing a modified 3.6 will tune up one of these in a hurry? Slower then a 7.31 but faster then a 8.31?

And an 8:35 is much stronger than a 7:31!

Thanks for chiming in Pete...and I've come across that link you provided, and read it multiple times to familiarize myself with the nuances of a 7.31 vs. 8.35 r&p's.
From a purely "$" perspective, it's a substantial diff... $500-600 for a good, used 7.31 vs. $2895 for the 8.35.
And there are many 915's happily running around, both cruisers and track cars, that have the 7.31 r&p...with no apparent negative consequence.
No question, the 8.35 is intrinsically stronger...but definitely a more expensive proposition.

I was hoping that I could be part of the "7.31 gang"...and enjoy the benefits of a 4.42 final drive ratio on the track...:D

What does this entail?

I would be interested to know why the 8:35 is stronger than the 7:31 as the cost difference is significant (at least for the Tandler parts).

Is a tooth loading or a scuffing issue?

I thought the rule of thumb was to have no fewer than 9 teeth on a gear to reduce the speed variation (maybe it was 11 to have a prime number). Isn't it also undercut?

More teeth = higher Lewis form factor. Not sure how much difference there is between 7 and 8 teeth, offhand. I recall Porsche once ran a gear with 6 teeth.

The issue with the number of teeth between the two gears is to make sure that you don't have the an individual tooth on one gear making contact with the same teeth on the other gear every revolution. As an example, an 8:32 pair would not be ideal.

JR

That is a good question.

Porsche used a stronger differential side plate on the Carreras. I wonder how a stronger side plate enters in the equation.

I think that when I wrote that an 8:35 was stronger than a 7:31, I was suggesting that based on how Porsche engineers parts. For instance, as soon as the 930 Turbo went into production, Porsche switched to a 9-tooth pinion. It's big and heavy, and if Porsche didn't think it was necessary to cope with the Turbo's increase in power, they would have continued to use the earlier part, or something similar.

Porsche used the same reinforcing strategy on the Sportomatic in 1975. When they switched to a 3-speed unit, they changed the R&P from a 7:27 to an 8:27. Not sure how long they would build Sportos, they probably had an eye to the future and made the change based on higher future hp & torque numbers.

When Porsche switched from a 7:31 to an 8:31, I'm sure they did that with an eye toward the future; bigger displacement engines producing more hp and torque. I feel that an 8:35, for those reasons, is a better choice. I know that they're new, and expensive, but under normal conditions should last the life of the car. A used 7:31 might not be able to deliver that.

The load limit for a gear set is proportional to 2N2/(N1 + N2) for the 7:31 the proportionality constant is .36897 for the 8:31 it is .41025 a strength increase of ~12%


The potential issue w/ swapping cwp besides strength is that it doesn't change the rpm drops which is what most really want to address.

Changing the final drive ratio will change the absolute rpm drop between gears but not the percentage change in rpm.

Nope...

The Z man knows the deal
 

The Z man has a very valid point. Look up the sheared gear teeth threads on overloaded 7:31 gear boxes here on Pelican. I went with a 915/72 for my Euro 3.0. The 7:31 would work nicely on a 2.7 high RPM motor (call Henry at Supertech for one) or a pleasure use 1980s US stock SC 3.0. However, javadog hit the nail on the head about gear design calculations. The 7:31 went away after 911 motors went up in displacement and started making lots more torque(not just peak HP @ RPM). Basic Torque/HP equation. The 7:31 behind a built 3.6 will have a very short life span and a high maintenance/replacement frequency. Torque needs more "teeth in contact" and/or "bigger gear teeth" to keep from killing a gear box too quickly. IMHO Save the 7:31 R&Ps for the long hood restorations and use a R&P that will consistently hold a 3.6.

Nope

Here's an example of a g50/30 that was configured w/ 9:31 for use in Cup cars and 8:32 in RSR's
http://forums.pelicanparts.com/uploa...1386132605.gif

Bill according to 'your' equation the 8:35 has a proportionality of 0.372 which is a strength improvement of 0.8% from a 7:31.

Hardly seems worthwhile bearing the cost involved?

I am also not sure the teeth in contact argument works either as a spiral bevel generally has two teeth in contact - sometimes 3 depending on the accuracy of the pitch - I am not sure changing from a 7 to 8 tooth pinion changes this factor.

7:31 r&p...
 

I was faced with the same decision a few years ago. If you're not in a hurry and have a good 8:31 R&P already, I'd suggest swapping the gear sets. Stock 1st is low enough, and based on feedback I thought a 7:31 would make 1st a bit too low.

I picked up a good used GT set and found another new in box GT set for a great price. I went with new Albins for the other two and negotiated a fair price on the set.

I went with:

2nd - 15/30 (2)
3rd - 21/31 (1.476)
4th - 24/27 (1.125)
5th - 28/24 (.857)

5th gear was tough for me because it was only a slight drop at a big expense, but I didn't want to spend all this coin on the box (I also have a Wavetrac LSD, one piece bearing retainer plate, billet side cover, Wevo internal gateshift and I plan on plumbing it for a cooler - $$$...) and have a "compromise" flat 5th shift.

My 3.0 build (Max Moritz P&C, early big port heads & CIS, 964 cams, SSI's, M&K, etc..) will hopefully compliment it. I plan on a conservative 6500 prm redline.

I guess my main point is that if you are patient and have some time to wait there are usually some nice gear sets that pop up every few months. You could also post a WTB, it seems that some folks on the forum are very helpful and reasonable in their pricing.

If it's a pure "budget" build, it's hard to beat the 7:31 R&P swap though...