7.31 gear swap to 8.31 costs

rching · 09-28-2008, 08:35 PM

Anyone know what the costs might be to change from a 7.31 to an 8.31 for better freeway rpm on a 73.5 911? I was thinking of doing that as I was fixing all the oil leaks on my 2.4 and the engine was out. Thanks for your input.

GrantG · GrantG's Garage

You might find someone willing to trade for no or minimal cost. If you do it, be prepared for a loss in acceleration. Have you also considered larger tires (assuming you're using ones with diameter less than 25" currently)?

rching · 09-29-2008, 08:24 AM

I just use my 911 as a driver, no track time. I'm running tall 15" tires on 7's and I'm around 4K RPM when running down the freeway at 85-90MPH. UH...that's how fast everyone is driving here in Southern California! : )

yel911 · yel911's Garage

I did this swap 2 years ago. I had a 74 911 with 7:31 R&P and a friend wanted to trade for his 86 Carrera track car. The main thing to keep in mind is, the mechanical speedometer. Most 8:31 are missing the section of shaft for the gear drive. My friend wanted the 7:31 badly, I guess, because he bought me a one year specific 8:31 R&P with gear drive. BRAND NEW!!!!! We never discussed the cost and he gave me a factory short shift to boot!!! The kicker on this whole story is, we did the job ourselves!!!!! Preload, backlash and all..... I was taken to school.

rching · 09-29-2008, 02:35 PM

Wow! I'm impressed. I don't think I'm going to tackle doing it myself though. That's why I wonder what it costs to have someone do it!

Bobboloo · Bobboloo's Garage

Why not just buy a used 8:31 transmission?

It's probably about the same cost as replacing the R&P and you have an extra tranny to boot. When the time comes to sell your car you can put the original 7:31 back in and then sale the 8:31 to get your money back.

Selling your 73.5' down the road with a 8:31 tranny conversion will be a hard sell. You'll take a hit on the resale value of the car.

Porsche84 · Porsche84's Garage

If you have a 1974 5 speed box, you may have a 5th gear that is a very high ratio (29:21). If that is true and you change to an 8:31 you'll lose the torque and need to shift on hills. Tell me what rpm you ran at 70 mph (or anyother speed). If you think the tranny is stock and can read the numbers on the bottom (like 915/08). The easiest solution may be to change 5th gear, although if you have a 29:21 that is about as high as you can get.

If you have a 4 spd, top gear is probably 28:23. In this case the 8:31 would make a good ratio combination. You can buy an aftermarket 29:21 4th gear and that may be the best bet if you want to keep the good acceleration in gears 1 - 3.

I do have a spare 8:31 r/p set. I'd swap my 8:31 for your 7:31. I'll rebuild your tranny if you want to discuss that.

Jerry

yel911 · yel911's Garage

FYI, my car was a 4 speed. I ended up witha 3.2 motor and the 8:31 R&P. Another thing, I tried looking for a 8:31 5-speed tranny with no luck. Even if you do find one, you'll have to go through it to make sure it's good. I would talk to Porsche84 and see what he can do. 7:31's are desireable.

rching · 11-08-2008, 07:43 AM

Porsche 84,' Still on this idea of less RPM at Freeway speed plus considering a 3.2L install. Rpm at freeway speed: 28K at 60mph, 32K at 70mph, and 36K at 80 mph. Is there a 5th gear only change to make it short for the first 4 gears and then a 5th like overdrive? I haven't done my research yet so these may be crazy questions laready addressed elsewhere.

rching · 11-08-2008, 07:46 AM

My 915 is a 73.5' so I don't think it has the taller 5th gear that you say the 74' box has.

moneymanager · 11-08-2008, 07:49 AM

Going from 15" wheels to 16's (an adventure in itself) will cut revs at a given speed about 7%...

Porsche84 · Porsche84's Garage

I'd suggest that you go to Carquip website: http://www.carquip.com/parts/gchart/index.asp. They are a good resource. Once you really figure out what you'd like to do, e-mail me (I don't use this site as much as I'd like) at austin@fairpoint.net. I'll be glad to help you decide and walk you through the 5th gear change. That is actually quite easy.

Jerry

yel911 · yel911's Garage

From my experience, a 4 speed vs a 5 speed is, 1st gear is the same for both, 2nd and 3 rd gear are taller than the 5 speed and 4th is taller than 5th. The 5 speed tranny is more usefull throughout the the gear range than a 4 speed, because of the added gear. 5th is not an overdrive, like most people would think.

Matt Monson · 11-09-2008, 08:21 PM

rching,
You likely have a TM or TL 5th gear in your car, so there's not much you can do about a taller 5th unless you buy an aftermarket gear like a .68 (which I have). That would be your easiest solution to get your revs down several hundred rpm and not compromise anything in your 0-60 funtime. You can change 5th without pulling the whole gearbox down since it's in the tailcone. Or even in you pay to do it, it will be way less labor than an R/P set up. Though, I should also mention that any of the aftermarket gears would make more noise than stock, and especially since you are putting it in 5th where you will be cruising at lower RPM's you will notice the sound of it.

As has been mentioned, finding the one year only 8:31 R/P with mechanical speedo drive is half the battle. Even those of us who specialize in Porsche transaxles don't see many of them. So factoring in an electronic speedo conversion into the budget may be one thing to consider.

Beyond that, expect to pay close to $500 to get the R/P set up by a shop. And they'll probably find a handful of "while you are in there" parts that would be worthwhile doing at the same time since you are paying a good chunk of labor to go all the way into. Budgeting $1500 total is probably a good idea just so you don't get caught off guard.

And to that point, are you going to pull the tranny yourself? I ask because there is that labor to consider as well in deciding if this is worth it. The flipside is that with what has happened to the value of 7:31 R/Ps in recent years you can likely barter yours against some of the labor because most shops will want that piece to sell to a race or hot rod customer down the road.

I haven't been checking in as much the last few weeks, so if you want any more input from me, just drop me a PM. I'll get the notification on that in my email and pop in and reply.

Regards,

Matt