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View Full Version : Distinguish a 915 from other Porsche g/boxes


Billman
March 25th 2003, 14:22
How can i distinguish a 915 from a 901 or other posche gearboxes????

lightning bug
March 25th 2003, 22:22
Not to be a smart ass, but you can tell by the # stamped on the case.

Billman
March 26th 2003, 05:21
How does these numbers look???? #915xxxxxxx ?????

lightning bug
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
March 27th 2003, 03:26
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
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
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
March 27th 2003, 12:26
http://www.911t.org/WklyReprint/type_915_transmission.htm

Tim

Wally
March 28th 2003, 04:07
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
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
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
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
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. :D ;) :D

chigger
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
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

chigger
April 11th 2003, 00:14
Dang! I have to get the old stuff and hack it up to make it work. Meanwhile in europe they already have it made up right. Please post some pictures. The latest and greatest for me anyway is always a modivator. Thanks.

lightning bug
April 12th 2003, 23:59
I really don't mean to argue but something doesn't make sense. 10 lbs may make a difference..... fine. But if we're so concerned about weight, then why the Type 4? We all know you can get more power out of the same displaced T1 for less money and weight. You said the weight balances on the bellhousing anyway. That's the center of the car L to R. You'll obviously have more weight toward the rear...... 10 lbs. Move your battery to the front, and you've more than compensated. I would think you'd want the strongest parts for a race car, never mind a street car. I would think it would be much easier to save weight in body panels and such rather than sacrificing drivetrain longevity and/or reliability. BTW.... I have the KEP 915 to T1 flywheel.... not cheap!

Alex
April 13th 2003, 06:53
That is a very strong argument lightning bug.

Alex

Wally
April 13th 2003, 15:29
Not really, because I have done all the other weigth saving issues Julian mentioned already and for what the weigth of the type 4 itself is concerned: a type1 can not reach the same displacement of a type 4 period. This was my mean reason to go type 4 anyway: 2,7ltr is possible in a type 4 with 'street' reliability. Especially with a set of NOS Oettinger Nikasils :-) .

One further argument: an older mag cased 915 is way cheaper (at least over here, because the 911 2,7ltr cars are the least popular 911's) than the later alu cased gearboxes. The price is almost half! The money item is the least important in this case, for the same reason Julian swears to his alu cased 915: you want to make just the decision on such a project and for me that was the mag cased 915.

In GB, a turbo'd type 4 with 911 heads and 300+hp drag races his stock magnesium cased 915 gearbox and hasn't broken anything after several seasons, so I don't believe I need even more strength. Tip: a G50 box is said to be even stronger...
Well, each his own I guess.
Walter

lightning bug
April 13th 2003, 16:07
2.7L. or whatever. We're talking about a 10 lb. difference in the trans. making a difference. Whether you have a T4 or a T1 its still 10 lbs. My point with the T4 thing is that you really don't have a weight concern anyway. You said you'd use the mag case for the weight savings and sacrifice the strength. So why not use a T1 case and save some weight and sacrifice strength. A trans. is just as important as the engine. I'm no expert, but I know a 2.7 T4 is not a good choice for a "reliable street" engine. We also know that a T1 can't be built to the largest T4. Build a 1.9L T1 and a 1.9L T4. The T1 will get more power. My friend built a 1915 T1 turbo engine for less $$$ than an N/A, carburetted 2.7L, and will definitely out-perform it. AND....its streetable, not to mention it will last just as long if not longer. I have a 2276 T1 turbo engine that was around the same price as a T4 2.7 and will DEFINITELY outperform a 2.7L T1. Both engines have EFI, EMS, and dry sump systems included and are still cheaper. BTW.. I'm not against T4's either, I have one. So if you built a 1915 turbo T1, you'd be lighter, more powerful and would last just as long. AND you can still use the alu case. Again... I'm not saying mag cases are crap, I'm saying that the alum. is stronger. Its a fact. And 10 lbs. is really no big deal. There have been drag cars that use T1 boxes and they last for multiple seasons. The 915 overall is a strong box, mag or alum.

Wally
April 13th 2003, 16:51
I did want to halt this 'debate'earlier, but was tempted again. I certainly will not go into a debate over which engine is better with respect to reliability of turbo's, turbo lag etc so I will stop here.
Like I said earlier, everybody has its own preferences, lets help each other with their respective projects and learn from other peoples experiences, rather than discuss what/which is better.
Walter

lightning bug
April 13th 2003, 17:46
That' fine with me. If you read my post completely, I mentioned the engines to compare the weight to their strength as to prove my point about the transmissions. I had no intention on going into a debate over engine reliability.

Quote: I certainly will not go into a debate over which engine is better with respect to reliability of turbo's, turbo lag etc so I will stop here.

Not sure what you mean here either.

Shad Laws
April 14th 2003, 15:04
Hello-

Just one quick point: the later boxes do _not_ necessarily have "better" gearing. It all depends what you want.

The 3rd/4th/5th gear selections changed for the various 915 boxes, but the 1st stayed the same. As a result, the overall 5th ratio for the late and early boxes are very similar. The difference is that the spacing of the later boxes is closer, and the 1st gear is significantly taller. While the first point is definately good, the last one might not be.

Let's look at some comparison numbers for 1st:
Early VW box with 4.12 R&P: 33/8 x 38/10 = 15.68
Late VW box with 3.88 R&P: 31/8 x 34/9 = 14.65
Early 915 box with 4.43 R&P: 31/7 x 35/11 = 14.08
Late 915 box with 3.88 R&P: 31/8 x 35/11 = 12.32

More isn't necessarily better...

Case in point: in my Fastback, my redline is going to be 9000rpm. The last thing I want is a really tall 1st gear! I'll accelerate better with an early box. Add that to the fact that my early box is a 923 (easy fitment with T4 components since it was on a 912E) and I have a winner.

Take care,

lightning bug
April 14th 2003, 16:37
Thanks for the info Shad. I never said that the later had "better" gearing, just "stronger". Saying that one or the other has better gearing would be too vague. Strength on the other hand is a fact. If you look at my second post in this thread, it is all facts. And I gave my opinion based on facts. I don't want to start a war or get flamed or whatever, but I didn't start the debate. As said, I gave my opinion and it was contested. So I replied back. No hard feelings toward Wally or anyone else and I apologize if I came off too strong. Everyone is entitled to an opinion, and I really felt, and still feel strongly about mine. That is all.

chigger
April 15th 2003, 01:22
We all build our motors and trans for what we wish them to do. I have yet to see two engine combinations exactly the same. I am a weight freak. Yes the aluminum case is stronger, but I won't be running the power you are. I am looking for a fast reliable street car at a decent price. One problem is I bought the trans thinking it was a 901! It actually turned out for the good as it will bolt up to a T4 motor easier than a T1. The gear ratios are fine for me.
I was originally going to race this car; however my daughter wants a bug and she is going to end up with my daily driver leaving me to try to make my race car more docile. Next generation is an all out race bug.

typ4boy
April 26th 2003, 13:28
good god you lot are so picky i have 2 cars 1 with mag case 915 1 with ally case 915 i coud not give a rats arse which one is better they are porsche boxes in vw bugs thats good enough for me . Lee L.A.P