alloy 915 vs mag 915?

[em78911]

I have a alloy 915 from my 78 SC
I have the chance to swap for a rebuilt mag case 915 that has a stacked ratio that will bring g the car to life a bit more that the taller
Box I have... I know the mag box is a 915 from earlier car would it even mate up to the 3,0 or is the clutch spline different?
What would be the disadvantage for this if any at all
The car will not be tracked but spirited drives around town

Thanks guys

[Matt Monson]

depends on what year mag case box and what was changed inside of it, both with respect to replacement parts and "stacked ratios".

Clutch is a non-issue. CV axle joint size and splines is.

Early mag case boxes aren't great. Middle ones are awesome. Get more details, and a build sheet if possible.

[em78911]

Originally Posted by Matt Monson

depends on what year mag case box and what was changed inside of it, both with respect to replacement parts and "stacked ratios".

Clutch is a non-issue. CV axle joint size and splines is.

Early mag case boxes aren't great. Middle ones are awesome. Get more details, and a build sheet if possible.

Thanks Matt I will do just that and get back
Thanks again!
Eric

[em78911]

Hey Matt
It's a 72/73 year tranny.
Work??

[Matt Monson]

Those had a shorter 7:31 ring and pinion but many of the other internals are weaker. The cases can also leak at the main seal. I'm not sure I would trade for it unless I got a build sheet that shows a lot of improvements made.

[lightbulb8817]

sorry to hi jack...whats the consensus on a 1975 915? i feel like mine needs rebuilding but not sure if its worth rebuilding this one.

[oldie44]

Keep in mind that the 72/73 915 has a cable drive speedo while the SC is electronic.

[jamesjedi]

Em78911, it would be better to reply with the numbers on the 915.

Matt is from GT Gears - it would be difficult to find someone with more knowledge on the Porsche transmission.

[em78911]

I'll grab numbers today James
Thanks just trying to see if the hurdles of doing this swap are worth it...the close ratio attracts me haha

[em78911]

Originally Posted by Matt Monson

Those had a shorter 7:31 ring and pinion but many of the other internals are weaker. The cases can also leak at the main seal. I'm not sure I would trade for it unless I got a build sheet that shows a lot of improvements made.

Thanks Matt for all input!!

[Crown]

The intermediate plate of a magnesium case is it's achilles heel: most are cracked or worn out causing leaks and bearing faillure. Replacement parts will cost you around $400 - $500 and the fun of replacing it.

[rkeele]

uh you mean side cover my 76 mag. case has a cast iron int. plate stronger than the later alloy 915, the earlier 915's aren't as strong, I think 74-76or 77 are the best ones not sure Matt would know

[Crown]

Nope I wrote the right part down: If your magnesium case still has a magnesium cast intermediate plate you'd better replace it if it's showing wear CMS Billet Intermediate Plate for 901 Transmissions with Magnesium Case CMS901BIP - CMS-901-BIP | Pelican Parts

[Bill Verburg]

Originally Posted by Crown

Nope I wrote the right part down: If your magnesium case still has a magnesium cast intermediate plate you'd better replace it if it's showing wear CMS Billet Intermediate Plate for 901 Transmissions with Magnesium Case CMS901BIP - CMS-901-BIP | Pelican Parts

That part is only used on 901 & 911 trans, a similar one is found in Sportmatic as well.

mg 915 have an iron plate between the diff housing and gear housing that strengthens the bearing area.

al 915 don't have this plate but the case can be machined to install one.
this is Wevo's version that is applicable to all 915 from '72 thru '86


The Bearing Retainer Plate is #65 here

[javadog]

Originally Posted by Crown

Nope I wrote the right part down...

As Bill pointed out, you're thinking of the wrong transmission and the intermediate plate on a Mag-cased 915 is usually better than the one on an aluminum-cased 915, once they have some miles on them.

Back to the original question, Matt's points are well taken but I'd still suggest you get the entire transmission number, so you know what type it is, what year it is for sure (there were improvements pretty much annually) and whether it might be anything special, like one fitted with an LSD.

JR

[Crown]

I apologize! My tiny brain overlooked the entire 915 part of this thread. Mag case is a 901 in my thoughts for some reason, maybe it's because I've got one on the workbench right now or maybe I just need another cup of coffee. Sorry guys.