915 trans gearsets or ring / pinion change

[najell]

i am looking to get more acceleration out of my 915 that needs a rebuild. it is just a street car. should i change the gears or go to a 7:31 pinion gear. what is cheaper and what will i enjoy more? thanks

[Steve@Rennsport]

Regear the transmission, hands down,...

Lowering the R&P does not address the main deficiency that the gear ratios are simply too far apart and the RPM drop allows the engine to fall out of its powerband at each shift.

Lastly, the 7:31 is nowhere near as strong as the 8:31 and thats important for engines over 3.0 litres.

[Cory M]

regear = better

7:31 = cheaper

[RWebb]

OK, Steve I have a related question. I hadn't planned to ask it for 2-3 years but since this came up...

I have a 1973 transmission (so it has the 7:31 R&P already). The 1975 version is still a Mg case, mechanical speedometer drive tranny, but is the first year with the 8:31 ring and pinion.

When I try to improve the transmission with different gears, etc. should I stick with the '73?

Or should I buy and build up a '75 transmission, then swap (and try to sell the '73 later)?

There is also the issue of whether the 1 piece bearing retainer helps strengthen the 7:31 R&P (adequately).

As ever, a big thanks!

[Grady Clay]

najell,
Welcome to the Forum
You will find a lot of help here.

The basic quandary is: the 7:31 R&P will lower ALL the gears. With your 8:31, the 11:35 1st gear is already too low IMHO. The 7:31 only makes it lower. The 7:31 gear pair are relatively available used. To properly set it up is time consuming (read expensive). There is always risk that a used gear pair is noisy.

Non-standard original gear pairs are very, very expensive. This choice depends on the depths of your pockets.

Another ‘go fast’ choice might be to install a stock 3.2 Carrera engine or buy a core and build yourself a 3.X hot rod. This lets you mothball your original 3.0SC.

Lots of choices. Perhaps a starting point might be to simply repair the shifting of your 915 and do a through tune-up on your 3.0. You never know.

Best,
Grady

[KTL]

Neither are cheap by any means! Steve is of course right that the 7:31 really doesn't address the problem at hand- the spacing/rpm drops of the gears. Keeping the car in the powerband with the right gear ratios more important than quickening the overall ratio, since the 8:31 is a pretty good ratio to start with.

In terms of "Detroit Iron" numbers that a lot of people understand:

7:31 = 4.428:1 axle ratio
8:31 = 3.875:1 axle ratio

both of the above are good ratios for accelerative purposes. The old domestic muscle cars run axle ratios like 2.73, 3.23, 3.42, 3.55, 3.73, 3.92, 4.10 The slow ratios like the 2.73 and 3.23 are typically changed to 3.73 or 4.10 for faster acceleration while still being streetable. You can see that the 8:31 Porsche ratio falls in between the so-called quick 3.73 and 4.10 gears.

[Bill Verburg]

It all depends on what you want to achieve. Too short is worse than too long. You can gear for fantastic acceleration but w/ a top speed of 30mph it's useless.

Just personal here but w/ just 5 speeds to use, a first that's good for ~40mph( a little less w/ a less torqey engine) and a top end of ~140mph and sequentially closer ratios is the way to go.

Here's one I like a lot


you can certainly close up the spacing but the cost is more frequent shifting and a more frenetic cruise.

[Matt Monson]

Originally Posted by RWebb

OK, Steve I have a related question. I hadn't planned to ask it for 2-3 years but since this came up...

I have a 1973 transmission (so it has the 7:31 R&P already). The 1975 version is still a Mg case, mechanical speedometer drive tranny, but is the first year with the 8:31 ring and pinion.

When I try to improve the transmission with different gears, etc. should I stick with the '73?

Or should I buy and build up a '75 transmission, then swap (and try to sell the '73 later)?

There is also the issue of whether the 1 piece bearing retainer helps strengthen the 7:31 R&P (adequately).

As ever, a big thanks!

I'm not Steve, but I will address your questions. Your 1973 transmission does not have a removeable mainshaft guide. This is a problematic with respect to inserting the seal, or more appropriately if the seal fails before the tranny needs a rebuild, you need to take out the gear stack just to do the seal. With a removeable guide, you can replace the seal when you do a clutch job, which you should be doing 2-3 times more often than rebuilding the gearbox. There are shops that machine the old cases and add studs for the removeable guide.

Your second question is a little more complicated because it involves two pieces. If you want to stay with a mag case tranny, you can build one around other years beyond 1975. And any of them would have the removeable guide. However the 8:31 R/P is where it gets tricky because you want to keep your mechanical speedo drive. There were only a small handful of mid 70's 915's that came with an 8:31 with the allowance for a mechanical speedo drive. It's that particular R/P that is the challenge to come up with. However, if you find one, you could actually build your tranny on any case you wanted, including a late aluminum case bell housing, as long as you use your tailcone with the mechanical speedo parts installed.

As for the one piece bearing plate, it doesn't really strengthen the R/P. It is more for strengthening the case itself and for maximizing bearing life. The one thing to really watch on all of these mag case 915's is the housing itself where the bearing race is supposed to be a press fit. Many of them are wearing out and getting lose, and a number of shops have taken to using the one piece CNC plates as a "solution" to the loose races. This is not the right fix and if yours is loose you should either find a tight one or have the case machined and go oversize with the races to make it tight and correct again...

[RWebb]

Thanks much. I guess I want to keep the mechanical speedo drive. I'm assuming the cost to convert to an electronic speedo (and run the wires) is high (I'd want to have the face with the chrom 'button' like the earlier gauges do).

I hadn't thought about keeping the tail cone part...

[KTL]

Hate to stray the topic from the question at hand (R/P vs. gear ratios) but since Matt brought it up and obviously has first hand experience, I gotta ask.

What are the results of a loose race besides bearing noise? I've got a friend who's constantly rebuilding third gear on his 915 race trans. and I have a feeling the loose races in his intermediate case section could be adding to the problem?

[Grady Clay]

Kevin,

The issue is with the two bearings on the pinion that press into the main (differential) casting. None of the other bearings have this issue.

Basically the casting support for the roller bearing and 4-point bearing isn’t adequate for the forces involved. Those two adjacent bearings take both the radial forces and axial forces of the pinion gear. The problem is much worse with the aluminum casting as the bearings ride directly in the aluminum. Porsche made the input shaft bearing smaller in about ‘86 in order to have more aluminum between the input shaft bearings and pinion bearings. This was intended to better support the pinion bearings. I’m not sure it did much good.

The magnesium castings have a steel bearing carrier cast into the differential section. While much stronger, it can suffer from the same problem. Possibly had there been the Wevo 1-piece bearing clamp from new, there wouldn’t be a problem at all.


Back on subject.

My choice of gears would be a taller 1st gear and a shorter 5th gear, say 3000 rpm @ 60 mph. The 2-3-4 gears would be appropriately spaced to give the progressive rpm drops that Bill V showed. I don’t mind ‘buzzing’ along the freeway at 3-4000 rpm. Even with an outrageous cam, I can operate the clutch in first gear to not have excessive wear on start-off. I agree about the 8-tooth pinion being stronger.

Best,
Grady

[Geary]

Sorry for the OT Grady, but Mike has been trying to reach you about a high demand part you were seeking, and he can only hold onto it for a few more days. (888) 894-3277

[84toy]

Originally Posted by Grady Clay

Kevin,
The magnesium castings have a steel bearing carrier cast into the differential section. While much stronger, it can suffer from the same problem. Possibly had there been the Wevo 1-piece bearing clamp from new, there wouldn’t be a problem at all.
Grady

Grady hit the nail on the head. While most people try to off-load their mag cases, I try to find them. They are typically in better shape because:
1. The steel insert cast in
2. The mag units were used in lower HP applications than the aluminum ones

Every aluminum case I have seen is egg shaped at the bearing bores but most mag cases are still usable. I have not heard of any blown up mag trannies as a result of the magnesium not being strong enough.

[Matt Monson]

Originally Posted by 84toy

Grady hit the nail on the head. While most people try to off-load their mag cases, I try to find them. They are typically in better shape because:
1. The steel insert cast in
2. The mag units were used in lower HP applications than the aluminum ones

And 3. They are lighter.

I have a number of race customers that want to use nothing but mag cases.

Kevin,
In case Grady's answer wasn't totally clear, the consequences are that the pinion howls and wears out prematurely.

As for your buddy and his third gear issues, does he run a super short 3rd, like a 19:32 for example? The really short 3rd like 1.68-1.63 tend to have fairly short lifespans in racing applications.

I'm pretty new around here and am sorry if the OT comments are a problem. the other boards I post on don't care if you go OT with related technical discussions. It's only if you start in with OT banter about fitting a car seat in your 911 in a tranny discussion or talking about the price of seat covers that it's an issue on the boards I've been on prior to here. If I am breaking local protocol, please let me know and I'll just talk to people about such things privately via pm.

And as for the original question, I do concur with Grady and Bill and their approach. I favour the 8:31 and regearing the tranny. Something you rarely see discussed anywhere is pinion speed. Lower pinion speeds put less wear on the whole transaxle. Using a taller ring and pinion coupled with short gears is the best way to achieve the desired goal. I also agree that the stock first is the best approach for a car that still sees street time. Changing fifth to a shorter gear or not is a personal decision, but 2-3-4 are pretty much a given.

Or, if you've just got to have a short ring and pinion, Tuthill makes a very high quality piece that is an 8:33 which gives you virtually the same final drive as a 7:31 but is stronger. The tradeoff is that it will cost you almost as much as it would cost you to buy 5 brand new gears and a billet mainshaft to regear the whole gearbox!!!

[KTL]

Forgot to respond to the helpful input people gave me.

Thanks to Grady, Paul S and Matt for your insight to the bearing issue. Much appreciated.

[Matt Monson]

You are quite welcome. Drop me a line anytime something comes up...

[KTL]

Oh, and I don't think anybody's going to give you grief about OT information around here when it relates to the technical aspects of discussion. Especially when it comes to transmissions, which are somewhat beyond a lot of people's abilities to diagnose & assemble properly. I think I speak for many people around here when I say we're very thankful to those who are willing to share their experiences about things that can't be known without years & years of experience working on these cars. Most of us have dealt with only a handful of engines & transmissions and it's hard to learn the intricacies of them from such few experiences.

Thanks!

[Paulporsche]

Just a thought here.

I recently noticed (I think) greater acceleration out of my car by going to smaller dia tires--24.1 vs previous 25.2". I kept the same 715 w/ 8:31.

This would seem like an easy way to do this, and I like the look better.

Anybody have any comments?

[Bill Verburg]

Originally Posted by Paulporsche

Just a thought here.

I recently noticed (I think) greater acceleration out of my car by going to smaller dia tires--24.1 vs previous 25.2". I kept the same 715 w/ 8:31.

This would seem like an easy way to do this, and I like the look better.

Anybody have any comments?

the loaded rolling radius of the drive tires is one of the variables that affect gearing, acceleration and speed

[sbrown.nw]

Originally Posted by Grady Clay

najell,
Welcome to the Forum
You will find a lot of help here.

The basic quandary is: the 7:31 R&P will lower ALL the gears. With your 8:31, the 11:35 1st gear is already too low IMHO. The 7:31 only makes it lower. The 7:31 gear pair are relatively available used. To properly set it up is time consuming (read expensive). There is always risk that a used gear pair is noisy.

Non-standard original gear pairs are very, very expensive. This choice depends on the depths of your pockets.

Another ‘go fast’ choice might be to install a stock 3.2 Carrera engine or buy a core and build yourself a 3.X hot rod. This lets you mothball your original 3.0SC.

Lots of choices. Perhaps a starting point might be to simply repair the shifting of your 915 and do a through tune-up on your 3.0. You never know.

Best,
Grady

I have an 85 Euro 3.2 and 915 in my 72 T Targa. Would going to a 7:31 be close to the US gears in the 915? How many labor hours is it usually for a R&P swap?

Thanks,

Scott