Mag 915 trans. Can they be trusted?

markmaunz · 08-03-2005, 08:45 PM

I saw a trans from a 72 911 going for a cheap price. Would you guys trust these older lighter trannys? Are they as strong? Or would you wait for an aluminum unit???? Opinions are great, aren't they? Thanks, Mark
84 Coupe roller

CRH911S · 08-03-2005, 08:51 PM

I could be wrong but I don't think they had 915 for 72. Someone will come along with the infor you seek.

LakeCleElum · 08-03-2005, 09:05 PM

I have one in a '73 and it's great. 116,000 miles and never a problem. Still shifts well w/o any syncro problems.....They had 915 in a '72's, 901's in the earlier models.....

Jason Cullen · 08-03-2005, 09:40 PM

The 915 Transmission was introduced in 1972 on all 911 Models.

They are desired for there short gear ratio's

Wonderful transmissions!!!

Are you looking to use an early transmission on an 84 911?

Jason

markmaunz · 08-03-2005, 10:05 PM

Jason, Would you use one of these or use the later ones? Mark
84 Coupe

randywebb · 08-03-2005, 10:27 PM

Th '70 & '71 models used type 911 transmissions - they are a bit different from the 901.

The 915 was introduced in 1972 with a Magnesium case and short gears. Later ones used the Al cases and longer gear ratios -- the ratios are 8:31 & 7:31. I don't know if the Mg case was intrinsicly weak, but P AG would not change for no reason.

I posted some pics to help externally ID the 901, 911, 915, and G-50 trannys some time ago.

The real question is what sort of strength does your application desire?

IROC · IROC's Garage

My mag-cased 915 has lasted about 150k miles without anything other than fluid changes. About 140k miles of it was attached to a 2.7 and the last 10k or so has been behind a 245hp 3.2. No problems.

Mike

Wayne 962 · Wayne 962's Garage

Believe it or not, according to Jerry Woods, the mag cases are better than the harder aluminum with respect to the bearing races wearing and enlarging the bores. The aluminum ones are far more likely to have out of spec bores than the magnesium ones. Ironically, it may have to do with the fact that the aluminum is inherently stronger...

-Wayne

coldstart · 08-04-2005, 04:19 AM

The flange for the clutch cable on my 915 mag case broke off and had to be repaired. Also, the transmission shifts better when the car is cold!!!? Opposite of how every other tranny I have driven...

Todd Simpson · 08-04-2005, 04:30 AM

The '72 915 has a bad rep due to a wierd oil seal that can only be replaced by disassembling the trans. Sometime during the year Porsche changed that. May be a factor to consider.

I use a stock '73 mag 915 with a euro 3.2. I don't think durability is a concern, just find one in good shape.

Bill Verburg · Bill Verburg's Garage

The resident gurus on transmissions are Wevoid and Geary. A search will be well rewarded. Some snippets of wisdom from them.

Wevoid(Hayden) Windrush Development(WEVO)

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It is true that keeping the outer race of the pinion roller bearing anchored in the case is a reliability issue with the 915.

However, the mag cases are superior in this respect as there is an iron insert resident in the casting, meaning that the pinion roller bearing is fitted into an iron bore - with lower expansion than the surrounding mag, or the aluminum bore in the aluminum cases (that do not have the iron insert)

Both mag and alu versions suffer in this respect, the aluminum version significantly more so.

The bare cases have a 14 lb difference in weight, a mag case with the late model side cover in aluminum still enjoys a 12 lb advantage over a late model aluminum case.

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The 8:31 CWP with a mechanical speedo drive gear (on the end of the pinion shaft) will go directly into your 1972 transmission.

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I assume you would still recommend the aluminum reinforced side cover for my mag case as an enhancement to the steel bore.

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That is correct.
Not so much as an enhancement to the steel bore, but to eliminate the flexibility of the Mag side cover.

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Anyone who knows me knows my penchance for '72 911's.....

Also in conjunction with all the work Windrush has done on 915 transmissions, it seemed logical to develop a solution for the '72 input shaft seal - even if only for selfish reasons !

We have a machining operation and replacement guide tube that solves the problem and expense of input shaft seal leakage on the '72 and all non-bolt-on guide tube 915's. (early and late '73's)

We machine the final drive housing to accept a new guide tube. The guide tube has the seal correctly positioned for the '72 input shaft. The Guide tube is retained by a large circlip, that can be installed or removed with the side cover removed from the final drive housing and differential removed (a 10 minute job). The guide tube is sealed in the housing by the same O-ring used on later 915's.

When the job is complete, the final drive housing has a new steel guide tube that is easy to remove without disassembling the trasnmission and uses the later model input shaft seal that is common on 915's up to 1986.

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I would also recommend the later differential side cover with the additional annular rib.

The 7:31 is making a special effort to exit through the side cover and the later, stiffer version will maintain your backlash better than the version that came on the 915/61.

The flexing of the side cover leads to excess backlash and excess stress in the pinion gear teeth, the 7:31 components are not inherently weak (sorry to contradict you Tyson) but the O.E. installation was challenged by higher output motors.

Move along the development line to the factory solution for the 915 as they added more torque at lower rpm.

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The vector of the attraction / rejection forces is the factor flexing the side covers.
The 7:31 has a vector skewed more towards the side cover, the 8:31 delivers this force more along the axis of the pinion shaft. The 930 installation I suspect is even less taxing on the side cover and more aligned with the pinion shaft axis.
In support of this - apart seeing the helix angle on the 930 pinion gear - is the fact that the 930 transmission has the same pinion roller bearing (for radial loads) as the 915 transmission and a larger 4pt ball bearing for the thrust loads.
We prepare the cases of a 930 transmission for a customer who has over 600 ft/lbs of torque and he uses the stock 930 side cover. His failures (which are not insignificant) never seem to be related to the stiffness of the side cover.
I would not choose to use a billet side cover based on stiffness.

If you change the side cover, it is almost certain the backlash will change. This will be due to the change in distance from bearing to bearing. Even if the new cover was a precise replica of the cover you remove, you will need to establish the correct pre-load to restore the installation to what it was prior to the swap.

Unortunately, despite the logic at first take - you will have to make these adjustments on both bearings - not just the one fitted to the side cover end. The Final drive housing as an element, is flexible at both ends. The backlash is a product of specific deflections by each end of the final drive housing. A new billet cover will be changing that stiffness ratio between the two ends and I would expect to have to change the shim stack for both taper roller bearings to set backlash correctly.
With the torque of turbo motors and cost of 930 CWP assemblies - it will pay to do it correctly.

I hope this helps?

Geary(Paul Guard) Guard Transmissions

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Unless the cover incorporates an oversized bearing, a heavier side cover probably won't be "better" than a late model 915 side cover.

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A 1-piece bearing retainer is wise for any 915 rebuild, and an absolute must in any track car.

randywebb · 08-04-2005, 10:11 AM

also BV posted pics of the late model side cover along with the earlier one some time ago.

It would be a public service for someone to compile all changes to these over the years...