Distinguish a 915 from other Porsche g/boxes

Billman · #1 March 25th 2003, 14:22

How can i distinguish a 915 from a 901 or other posche gearboxes????

lightning bug · #2 March 25th 2003, 22:22

Not to be a smart ass, but you can tell by the # stamped on the case.

Billman · #3 March 26th 2003, 05:21

How does these numbers look???? #915xxxxxxx ?????

lightning bug · #4 March 26th 2003, 20:36

915 will look like 915/XX on the bottom of the case. Not sure how or where the 901 is stamped. It most likely will begin with 901 though. If you're interested in a 915, look for a later version as these are all aluminum cases (after '78), stronger R&P 8:31 (3.88) and the early boxes didn't shift as well as the later boxes. If you can find a factory LSD box even better. 915 was installed from '72-'84. 901 was '65-'71 (I believe on the 901). Also 915 has a conventional H pattern and the 901 has a dogleg 1st. A Bentley manual should tell you the "XX" designations as far as years.

Wally · #5 March 27th 2003, 03:26

Quote:

Originally posted by lightning bug
If you're interested in a 915, look for a later version as these are all aluminum cases (after '78),

Well, it depends what you find more important, but I went on purpose for the earlier magnesium 915 case because of weight advantages.

The 915 type number is on the bottom of the case, the lowest point of the case (and the car in fact) and therefore the number has been grinded away on most cases,making type reading very difficult. The other part numbers (nose cone e.a.) are clearly visible and all start with 915.xxxx so you can't miss it.

Walter

Billman · #6 March 27th 2003, 10:10

For the beggining of the project i will have the 915 gearbox with the original motor!!! The gear ration is tou big for the horsepower of the motor isnt it?? Someone knows the gear ratios???

lightning bug · #7 March 27th 2003, 11:42

True the early mag trans. is lighter, but the later all alum. are stronger as well as the stronger and higher geared 3.88 R&P as opposed to the pre-75 4.43. All alum. case is '78 and up BTW. Me personally, I would take strength over weight any day.

Tim · #8 March 27th 2003, 12:26

http://www.911t.org/WklyReprint/type...ansmission.htm

Tim

Wally · #9 March 28th 2003, 04:07

Quote:

Originally posted by lightning bug
True the early mag trans. is lighter, but the later all alum. are stronger as well as the stronger and higher geared 3.88 R&P as opposed to the pre-75 4.43. All alum. case is '78 and up BTW. Me personally, I would take strength over weight any day.

But Julian, The 915 can handle up to 300 hp! The (magnesium composite) case is probably not the first to break anyway (most likely a gear or diff will go first). Besides the final gear ratio of a mag case is already way longer than the bug's IRS with 3,875 R&P because the shorter R&P of the 915 mag case is more then compensated by its longer gears.
Sorry, but weight ditribution is very important and in this case you can influence it by choosing the right part and you really don't sacrifice strength. Unless you drive a 350 hp turbo on the street...
Walter

chigger · #10 April 5th 2003, 13:12

My mag cased trans must weigh at least 25 to 30 pounds more than a bug trans. I don't know how much more an aluminum one would weigh. That and the greater weight of a T4 motor all in the rear of the car may alter the handling a bit. The combination somewhat balances itself at the bellhousing mount.

lightning bug · #11 April 5th 2003, 21:13

An all aluminum case is only 10 lbs. heavier than the mag. Honestly, do you think 10 lbs. will affect handling??? You may be right about internals going south before the case, which is another reason to go with a later case......... as I said above...... Stronger diff, case and mainshaft. So for a mere 10 lbs. and a stonger overall package, this seems like the right part.

Wally · #12 April 6th 2003, 14:40

Julian,
I guess eventually it comes down to (personal) choices. Too bad we can't 'settle' this difference of opinion by a race of some sort :-) Its nice to see you have a strong opinion on this. I respect that.
Later,
Walter

lightning bug · #13 April 6th 2003, 20:42

I respect your opinion also. Personally, I'm going with an '86 case for the strength as I am building a 2.3L EFI Turbo T1. I just don't want to take a chance. Who knows, I could be over-doing it. Another point I should add.... I'm not saying the early case is crap, I just feel the later is better. Who knows... maybe it'll blow up in the first 100 miles. I guess that's when I go G50.

chigger · #14 April 9th 2003, 01:53

Ah but ten pounds is ten pounds when you are racing. Since I won't be running the HP you are, my engine will probably be happy with the mag case. I can probably also gain weight loss by dropping the weight of the hugh flywheel to near a stock bugs weight. For racing I would go even lighter.

Wally · #15 April 10th 2003, 02:08

Chigger,
I just received the special KEP flywheel and it is relatively ligth because there is no starter ring gear on it (just like the original 911 flywheel. Now I can use the original 911 pressureplate, which is also quite ligth cause it is lined with aluminium and has the starter ring gear on it.
Just some info.
Walter

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