8:31 to 7:31

[Jim Hunter]

Hello,

I want to change my R&P from an 84 915 euro (8:31) to a 7:31.
How many hours it may take in a Porsche only shop ???

Also what is a fair price for a 7:31 r&p ???
8:31s has so if any value also ???


Thanks!

[Bill Verburg]

Why?

[Bryan Beaumont]

Why??? Are you kidding? If so, I do not get it.
Perhaps you can simply think about your question of "why" for maybe
20 seconds and come up with some of the many possible answers.
Disappointing that you would lob out your comment, which is nothing
more than a meaningless negative jab.
Bill, thanks for your many contributions to this board, but WHY....did you see the need to respond in this way? You are generally a huge asset to this board, not to mention a moderator. I guess I just was surprised that that post came from you. Are you having a bad day? If so, I hope your weekend is better.

Bryan

[2.7RACER]

Bryan,
Sorry, but Bill's question is legitimate. Essentially Bill is asking Jim what are his expectations.
There is an up and down side to this type of change.
Jim,
I switched complete transaxles for the following reason:
A decent 7:31 ring and pinion will cost $500. Set up in your or my trans will run 5 to 10 hours labor, not counting removal and replacement.
These days that is $300 to $600 just for setup labor.
I bought a nice '72 trans with a 7:31 for $1500.
If I'm still happy, my '76 trans will be for sale $1200, no issues.
Net cost to me $300 vs $800 to $1,100.
What is your reason for the switch?
Guy's like Bill are very knowledgable about these things.

[Jim Hunter]

Well,

I think that with the 7:31 the engine will rev more easily and the decrease in top speed is not an issue for me...

Thanks!

[Bill Verburg]

Here is what a 915/67 looks like stock


For Autobahn use it is nicely set up.

for sporting use the issue, which is what I think you are concerned about, is the spread from 3^rd to 4^th and 5^th

here is what a change to 7:31 does


The max speed in each gear is decreased by ~15% but the primary issue of awkward spreads has not been addressed.

here is a better(IMO) solution short 4,5


this can be juggled a bit, say a little longer on 5, or a more expensive 3,4,5 change.

This has the advantage of closing up the top, where you need it, but not compomising the spread or strength of the bottom, or the strength of the cwp

[beamonk]

I have it in my 3.0 euro SC. I don't particulary care for it. It doesn't seem quicker than the old 8:31.

[jaydubya]

Great info here!

Bill, I imagine that changing tire diameter just scales the numbers?

The reason I ask is that race tire sizes are always forcing me to deal with an increase in diameter with an increase in width on my '82 SC. In Hoosiers I can either run a rear with size 245-45-16 (or 245-40-17) with a diameter of 24.4" (24.5"), or if I go wider in the rear I can run a 275-45-16 (or 275-40-17) with a diameter of 25.3" (25.2"). It would be useful to see exactly how the differences affect gearing.

Most people that I ask say that in terms of lap times it's better to run the smaller diameter tire even though it has a smaller contact patch (you can generally lower the car more, better gearing, somewhat less rotating mass). I guess it depends on the particular track too.

Are these data coming from an Excel spreadsheet? If so, perhaps I could get a copy to play with?

For PCA club racing, changing R&P or gears bumps you up at least one class.

Cheers,

Jeff

[Bill Verburg]

I imagine that changing tire diameter just scales the numbers?

Yes, It does exactly what changing the cwp does, only less so.

to calculate what a different diameter tire does you need to know the % difference in increase or decrease of the dynamic loaded radius from your base tire is as a decimal then just multiply.

[jaydubya]

^^^ Thanks!

Cheers,

Jeff

[1980 911 sc]

Bill,

way to bring the info despite the flame... I tip my hat...

[GrantG]

I've heard that the 8:31 is significantly more stout for those who track their cars more frequently (7:31 can crack for those who are unfortunate or stress the box alot). I think the best option (though not cheapest) is to keep the 8:31 and change the gear stack making the ratios closer together.

[Steve@Rennsport]

Originally posted by GrantG
I've heard that the 8:31 is significantly more stout for those who track their cars more frequently (7:31 can crack for those who are unfortunate or stress the box alot). I think the best option (though not cheapest) is to keep the 8:31 and change the gear stack making the ratios closer together.

Amen, to that advice,....

[Bill Verburg]

This question came up in a PM, paraphrased, Is a shorter lighter tire better on track than a taller wider tire?
It's related to Jeff's ?
If you look at the thrust curves for a US Carrera w/ 2 different tires you can see what happens.

The taller tire will provide slightly less thrust but the main thing is that the speed where thrust peaks is moved up(to the right), In a drag race 0-whatever it is most important to get the max thrust as soon as possible, but on track as long as you aren't shifting in a corner it is more important to carry a higher speed through the corner.

[Bill Verburg]

Also note that in a drag race it's the lower and middle gears being used, on track it's the middle and uppper gears where the thrust difference is less.