Best Year for a 915 Transmission?

[randywebb]

I've searched & never seen a thread that compares the various models of the typ 915 tranny. So I thought I'd start this thread.

HISTORY: As far as I know...
The 915 was introduced in 1972 with a Magnesium (Mg) case and a 7:31 Ring and Pinion. It followed the typ 901 (used from the Origin thru 1969) and later typ 911 transmissions (used in 1970 & '71).

The Aluminum (Al) case models were introduced in late 1977 -- not sure when the R&P changed to 8:31. At a guess that might have accompanied the bigger 2.7L motors, or maybe the bigger 3L ones...

Later, the side plate was strengthened with additional ribs (Bill V. posted pics of them some years ago).

There are also some with their own pumps - used in the ROW cars at some point (Carreras?), but rare in the US.

ISSUES:
Beyond the side plate and oil pumps (which are easy to swap onto a different trans.)...

I've also heard that the Mg boxes, tho weaker in stock form, are easier to strengthen than the Al ones.

Also, Wevo makes a stronger side plate.


But which is the best base model 915 to have? And why?

[IROC]

8:31 started with the 1975 model year. Electronic speedo drive started in the 1976 model year but not all 1976 cars have electronic speedos (mine doesn't).

Didn't the mag-cased 915s have some sort of bushings installed at certain wear points that actually make "stronger" than the aluminum-cased units?

FWIW, the 915 in my car is the original and has never been opened. 150k+ miles of hard use and it's still hanging in there.

[drl911]

The year it was replaced by the G50

[Deschodt]

I don't know the answer to that, but my 83's 915 is on it second "2nd gear syncro" by now, while my ex-73.5 915 was the smoothest shifting porsche gearbox I had ever driven ! Go figure !

[jkarolyi]

I imagine the '84-86 Euro 915s...they all came with a stock tranny cooler.

[Bill Verburg]

Best for what?

If I had a mech. speedo I would want ot stick w/ a matching trans. beyond that there are decisions that would need to be made

[Wil Ferch]

Correction....Euro 84-86 915 boxes came standard with an oil cooler..only if they weren't ordered with the optional cat ( like US cars)...which were sometimes ordered as a Euro tax break. The Euro cat cars were w/o cooler...just like USA cars.

- Wil

[dd74]

+1. Best depends on driver and what other modifications have been done to the car. For instance, I've heard one should not expect a 7.31 r/p to last long with a 3.6 swap. Supposedly the al 915s are stronger than the mag, but at a weight penalty. That's just two examples.

I've also been told one can make a fairly nice close-ratio hybrid 915 using a mag case with a 7.31 r/p and 4th and 5th gear from a later 915 box. To strengthen the mag 915, one need only get a side cover from a later al. 915 case.

The best thing about this combination is it stays within OEM parameters, and one doesn't need to spend $650 per gear.

But again, gearing is subjective as is the actual "feel" of the gearbox.

[randywebb]

Originally posted by Bill Verburg
Best for what?

- Deliberately open-ended -- I would say all the typical performance criteria in a crash-box:
- strength & reliability {power-handling}
- subjective "Feel"; ergonomic criteria
- weight

Originally posted by Bill Verburg
If I had a mech. speedo I would want ot stick w/ a matching trans. beyond that there are decisions that would need to be made

- This essentially says that the effort & cost to convert to an electronic speedo (or vice-versa) -- which cannot be much -- overrides considerations of wt. and strength

[Bill Verburg]

Randy, don't try to put words in my mouth, each has it's strength and weakness stock, each can be be improved to do one thing or another better.

For a street car I like the taller gearing of the late trans, for a 3.6 the 8:31 is prefered, for a mech speedo chassis a mech speedo trans is preferred, a lt wt is likely to want a mg. case, etc, etc...

[GrantG]

When I encountered this question, Hayden at WEVO (who rebuilt my box with custom Guard gearset, Guard LSD, Gateshift, Wevo Bearing retainer, etc.) recommended the last of the Mag cases (915/61) for its lightness, the stronger 8:31 R&P, electronic speedo, and because these less frequently require machining due to excess bearing play which the Alu boxes have more commonly (don't remember the precise description of the problem). Box is from late 76/early 77, AFAIK.

I also retrofitted a NOS later/stronger side cover with the additional annular reinforcing ribs...

Good luck!

[randywebb]

don't try to put words in my mouth

- it is an obvious implication of what you said.

I am still getting the impression that there is no single best trans. that can be made from a mix & match parts scheme.

However, the info Grant posted makes me think a '76 or so might be best for "most" applications.

It would help if we had a complete list of what changes were made when (e.g. if somebody wanted the lightest, strongest box but had a mech. speedo, maybe they could buy a '75 box...?)

[the]

Originally posted by GrantG
When I encountered this question, Hayden at WEVO (who rebuilt my box with custom Guard gearset, Guard LSD, Gateshift, Wevo Bearing retainer, etc.) recommended the last of the Mag cases (915/61) for its lightness, the stronger 8:31 R&P, electronic speedo, and because these less frequently require machining due to excess bearing play which the Alu boxes have more commonly (don't remember the precise description of the problem). Box is from late 76/early 77, AFAIK.

I also retrofitted a NOS later/stronger side cover with the additional annular reinforcing ribs...

Good luck!

The maincase bearing retainer area is much stronger on the mag case, I think it comes from the factory with a steel insert or some other strengthing of the area. They don't just bore out the magnesium and insert the bearings on on the mag cases.

With the al case, the bearing bores are just drilled out of the aluminum, and running high hp/torque through the trans can cause the bearing retainer area to get out of shape, which allows the bearings to get loose. Once they get loose, the trans tends to lunch itself.

Here is Wevo's description of the problem:

www.wevo.com/Products/TransmissionProducts/WevoTransmissionProducts_RaceCaseMainCase.htm

All things considered, it seems like the later mg cases ('76/'77) are, *overall,* all things considered, one of the better choices.

[dd74]

I've seen WEVO boxes on their website - gorgeous stuff.

Is there an advantage to an electronic over mechanical speedo?

[GrantG]

Originally posted by dd74
I've seen WEVO boxes on their website - gorgeous stuff.

Is there an advantage to an electronic over mechanical speedo?

In my case, the advantage was that I already had an electronic speedo, so replacing my gearbox (from an 86 Carrera) with a Mg-case 915 that also had the electronic speed sender required less hassle. I've had mechanical speedos too and there's not too much difference in accuracy or usability...

[84toy]

I rebuilt an aluminum 915 and the races were loose. I am currently working on 2 mag boxes right now and they both have tight races due to the factory installed steel inserts.

Clearly the later "box" made out of aluminum is stronger but it sure seems like the bore for the races is more of a problem for the average guy then the strength of the casting. You don't see nearly as many castings breaking compared to the quantity of bad bores in the aluminum. It seems like just about every aluminum box has bad bores. If they came with steel inserts that would be great but they don't....Just my$.02 though.

Here is a picture of the area discussed....

[javadog]

Randy,

A couple of other changes occurred in 1977. Porsche added the omega spring on the clutch linkage and changed some parts for 1st and 2nd gear, to improve shifting (less effort.)

JR

[Jack Olsen]

Like Grant, I've got an early '77 Mag-case transaxle. 8:31, stock 1st, tall 2nd, stock 3rd, short 4th, short 5th. Later side plate, Wevo gateshift, electronic speedo with a 1972 faceplate on it.

There are different horses for different courses -- but once you take one of these apart and put your wish list together, it's not a stock anything anymore.

[rusnak]

Which is the best transmission I think depends on your gearing and power curve. It's interesting driving the 911SC and the 911 3.2 back to back, because you really notice the difference in gearing, especially 3rd and 5th. The light ones would be the magnesium ones. It would take a lot to break one.

Some random thoughts, not trying to hijack the thread here:

I thought that Ferdinand Piech's first full charge project was the design of the 915 transmission. It cured one of the big glaring problems with the 901, which was the input shaft cantilevered out from the intermediate plate and often broke off, or broke the intermediate plate.

I'm not sure who I spoke to about the 901 at Andial but they say the 901 is good for up to 300HP.

A succinct history of the 915 here:

[rusnak]

(Continued










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