901/911 Transmissions

[chris_seven]

I am trying to gain a better understanding of the differences between a 901 and a '911' transmission.

I am sure that there are many detailed differences apart from the Aluminium to magnesium material changes and some basic facts would be a good start.

I also think but am not 100% sure that a Coarse splined 915 LSD will fit the 911 transmission. can anyone confirm.

Thanks

[304065]

Chris, I'll start with the most basic difference, the clutch actuation.

In a 901 the cable housing is fixed and the cable actuates the clutch.

In a 911 the CABLE end is fixed and the HOUSING actuates the clutch. Strange but true.

[Steve@Rennsport]

Coarse-splined 915 LSD's fit the 911 transmission.

Bear in mind that coarse-spline LSD's require special drive flanges (NLA),......

[MarkS]

911 uses 225MM clutch and pressure plate, 901 uses 215MM C & PP as used from begining to 1969.

[Christien]

Chris, I'll start with the most basic difference, the clutch actuation.

In a 901 the cable housing is fixed and the cable actuates the clutch.

In a 911 the CABLE end is fixed and the HOUSING actuates the clutch. Strange but true.

John, could you please describe this a bit more? Not sure if it will help the OP but I'm curious. If I'm not mistaken (and I very well could be) 1972 was the first year of the 915, and it shared a lot in common with the 901, including the cable attachment to the clutch, which as I understand is adjusted in exactly the same way as the 901 (i.e. no helper spring, just tension on the cable held with 2 nuts locking each other, measuring about 1.5" of free play in the pedal).

So with that set up, it would still be the cable itself which actuates the clutch, even though it's a 915, correct?

[Grady Clay]

Christien,

Whoa haus. We are now talking about three transmissions; types 901, 911 and 915. Your ’72 has the type 915. The type 901 was used ’65-’69, the type 911 was used in ’70-’71 and the type 915 from ’72 to ’86.
Do we need to confine this discussion to only the difference between the type 901 and type 911 transmissions?
What are you looking to do?

Chris,

The big difference between the 901 and 911 is the clutch actuation method as noted above. There were magnesium 901s in ’69 so you can’t totally identify them by material.

All 901s and 914s have a 215 mm ‘push-type’ clutch. This is where the clutch release bearing pushes in (toward the engine) on the spring fingers of the pressure plate. The bearing is not attached to the pressure plate.

In this configuration, the cable inside the Bowden tube pulls on the clutch release arm as it protrudes out the bottom-side of the bell housing. The Bowden tube is held rigid by a cast ‘ring’ on the differential sidecover.

Starting with MY 1970, the clutch was enlarged to the 225 mm ‘pull-type’ clutch. This is where the clutch release bearing pulls out (away from the engine) on the spring fingers of the pressure plate. The bearing IS CAPTIVE on the pressure plate with a circlip.

Since the clutch release arm now has to operate the other direction, Porsche came up with a ‘Rube Goldberg’ solution where the inner clutch cable is rigidly mounted to the transmission casting. The Bowden tube has a bend in it and the Bowden tube free end pushes on the clutch release arm. It does this when there is tension in the cable and it tries to ‘straighten out’ the Bowden tube. The tube slides through a larger (clearance) ‘ring’ cast on the differential side cover and pushes on the arm through the dreaded 'plastic piece'.

The type 901 and type 911 (and 914) are all basically the same internally and share the same shift pattern. That is a modest oversimplification as there are a lot of subtle differences.

While the 915 has a 225 mm ‘pull-type’ clutch; the disc, release bearing and the method of actuation are totally different. The flywheel changes with the 3.0 911SC in ‘78. The 915 has larger gears that are both wider and the shafts spaced farther apart. The shifting is different than the 901/911.


The reason this type 911 transmission being more than a 2-year peculiarity is it can be used to fit the larger 225 mm clutch into earlier cars – SWB in particular. It is an easy swap into a ’69 but requires some creativity in SWB cars.

The usual reason for this is retrofitting a larger engine. The 215 mm clutch will work with larger engines but has a much shorter service life and is very intolerant of abuse. Most agree that a good 901 or 911 will survive just fine up to about 2.8l displacement. Some even say 3.0l is possible.

The type 911 will fit in an earlier chassis without modifying anything. To fit a 915 in a SWB requires modification (or replacement) of the sheet metal between the rear seats.

Search Pelican. There are a lot of images and diagrams about all this.

Best,
Grady