915 Tranny

[ppp]

HURRY...looks like great price.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33733&item=2435107953

[Panelfantastic]

Thanks for the link. I'm not quite ready yet, got to get this rear suspension sorted first. Plus, I think I want to hold out for the 930 trans (is it the G50??).

[jhelgesen]

Panel,

The 930 is the 4 speed version of the 915, made for big HP. The g50 is the later model of the 964/993 tranny, much taller gears. Any of them are excellent trannies.

I'd be real nervous about buying a used porsche tranny on ebay, you're looking at a 1.5k rebuild if not careful.

[Shad Laws]

Hello-

IMHO, the 930 isn't such a great choice for most people. It has a VERY tall 1st gear... an overall ratio of about 9.50. For most people, that's almost like starting out in 2nd gear. Strong? Yes. Usable on the street day to day? Not unless you have a HUGE crapload of torque... and even then it's kinda painful.

The 915 is very, very strong as-is. Unless you have a 400hp+ engine that you want to use in a high load, high duration activity (like 12-24 hr. road racing), I really think that the 930 is unnecessary.

The G50 is a great tranny... quite strong, and the ratios tend to suit a street car well. The problem is that they are much heavier (~50lb more than even a 915!) and much bigger. It will be a major chore to get them to fit.

IMHO, stick with 901 or 915.

Shad

[Panelfantastic]

Thanks guys for the advice.
I focused on the 930 unit because its for my bus, which is considerably heavier than a bug, and you know it will take some serious torque to get it moving. I can supply the power but the weight and power is gonna kill transaxles even without over zealous stop light antics.
Please feel free to share ANY thoughts...

[Shad Laws]

Hello-

Thanks guys for the advice.
I focused on the 930 unit because its for my bus, which is considerably heavier than a bug, and you know it will take some serious torque to get it moving. I can supply the power but the weight and power is gonna kill transaxles even without over zealous stop light antics.
Please feel free to share ANY thoughts...

If it's for a bus, then that makes the 930 even a WORSE idea.

The first gear is REALLY, REALLY tall. You'll have to slip the clutch like mad and lug the engine all the time to get the danged car moving.

The torque applied to a gearbox has to do with the engine, not the car. The torque applied to the rear wheels has to do with the engine output AND the tranny reduction ratio... you multiply them together! So, an early 915 with a 7:31 R&P has a 4.429x3.182=14.08 1st gear, so you get a 14.08:1 torque multiplication. A 930 has a 4.22x2.25=9.50 1st gear, so you only get a 9.50:1 torque multiplication.

In other words, a 930 over an early 915 kills your torque to the wheels in 1st by about 1/3!!! That's a HUGE decrease, and a terrible way to go.

I _strongly_ recommend against it for your bus! I recommend either a 901 or an early 915... even using a 8:31 R&P in a late 915 seems like a poor idea (would make 1st 12.33:1 overall).

As a point of reference, a late bus had a 4.571x3.78=17.28 1st gear... so going to 9.50:1 cuts the reduction ratio almost in HALF from stock! Not a good idea.

Take care,

[Panelfantastic]

Thanks Shad. When you break it down into the ratios, it really becomes obvious which way I need to go.Now I need to spend some time in the archives learning about the swap problems.
Is the 7:31 R&P the common setup in the early 915?
I think I have a good local source when the time comes to get a solid trans and probably access to different gearsets too, a high school friend of mine wrenches on some of the really fast Porsches around here.

[Shad Laws]

Hello-

Thanks Shad. When you break it down into the ratios, it really becomes obvious which way I need to go.Now I need to spend some time in the archives learning about the swap problems.
Is the 7:31 R&P the common setup in the early 915?

Yes. All 2.4 911's had them (72-74). The 2.7 got the 8:31. Also, the 1976 2.0 912E had the 7:31.

Of course, all regular production 901's had the 7:31, too.

Take care,

[Wally]

Panel,
When you go look for one, be sure to check the type numbers also. For instance, a 915-12 is a 4-speed (yuck). 4 and 5-speeds 915's look exactly the same!
Also check for ouput shaft CV diameter. You want the 100 mm diameters, not the 110mm, since a porsche CV doesn't fit a VW axle...
Good luck,
Walter

[Panelfantastic]

More great info, Thanks!

Shad, glad you still have some time to post. Hope you can stay on as school permits.

Wally, I've already converted to 944 CV's and axles so I'm good there. Which series/type 915 am I on the lookout for...since the -12 is a 4spd, which 915-?? is the early mag case 5spd w/the 7:31?

[Wally]

Hi,
I have never come across a list of 915 type numbers :-( so I can only tell you that I am almost certain that I have a 915-40. The difficulty is that the type numbers are stamped on the lowest part of the case, which is also the part that gets in touch with the ground at speed bumps etc. on the 911's...
Cheers,
Walter

[Shad Laws]

Hello-

According to my books...

1972-73, 2.4L
Magnesium case
R&P 7:31
1st 11:35
2nd 18:33
3rd 23:29
4th 26:25
5th 29:22

1974, 2.4L
Magnesium case
R&P 7:31
1st 11:35
2nd 18:33
3rd 23:29
4th 27:25
5th 29:21

1976, 2.0L (912E)
Magnesium case, 901-style splines and clutch arm
R&P 7:31
1st 11:35
2nd 18:33
3rd 23:29
4th 26:25
5th 29:21

1975-77, 2.7L
Magnesium case
R&P 8:31
1st 11:35
2nd 18:33
3rd 23:29
4th 26:26
5th 28:23

All 1978-86 trannies are aluminum and have a 8:31 R&P. I also have the 4-speed ratios, but I don't think you care about them. I have incomplete data on the subtypes (i.e. -12)... not enough worth posting.

How much torque do you think the engine will make, anyway? I'll be willing to bet that you'll have a tough time killing a 915, even a weak one :-). What cruising rpm do you want?

Take care,

[Panelfantastic]

It will be a turbo EFI engine. In street trim we hope to make an honest 350hp. I know, I know, you asked about torque not hp but I don't have those figures yet :) . Still on the dyno trying to work through tuning issues.

At cruising speed (70mph), I would like to see 2800-3K rpm but that may not be realistic and also I am not that anxious to do top speed runs in a bus even though I have made many improvements to the handling (as much as you can to a brick:p ) so seeing 100+mph would be very rare and not a big requirement. I really like the 0-60 excitement best.

If I'm gonna convert to a better trans, I would rather step-up in the begining and do it right. Then be prepared for whatever beast of an engine I might evolve to down the road.

[Shad Laws]

Hello-

It will be a turbo EFI engine. In street trim we hope to make an honest 350hp. I know, I know, you asked about torque not hp but I don't have those figures yet :) . Still on the dyno trying to work through tuning issues.

Transmissions don't care much about power... they can spin 9000rpm pretty well and put through lots of power without much engine torque. What transmissions don't like is a lot of torque... given that you have a turbo setup, I'm guessing that you'll have a good amount!

At cruising speed (70mph), I would like to see 2800-3K rpm but that may not be realistic and also I am not that anxious to do top speed runs in a bus even though I have made many improvements to the handling (as much as you can to a brick:p ) so seeing 100+mph would be very rare and not a big requirement. I really like the 0-60 excitement best.

For that kind of 5th gear, you don't want a 1972-73 tranny.

You really need to know what kind of torque you're putting through...

The 7:31 R&P is weaker than the 8:31 variety, simply by design (the pinion is only 7/8 of the size!). However, given your large vehicle, it may be a very nice thing to keep around...

If I had to choose the *best* scenario for you, I'd do this:
- get a 1974 911 or 1976 912E core
- get a late aluminum 915 core (one with the second, inner annular rib of support on the sidecover) OR just a late, empty case
- get a Kennedy custom flywheel for the T4 with 911 pull-type clutch components
- get an aftermarket gear-reduction starter for a VW (fits the same as the "self-supported" starters)
- buy a shift fork jig
- make a mainshaft rearward nut removal tool (requires a non-plated, deepwell, largish socket, a spare mainshaft helical reverse gear, and a welder)
- take them apart
- inspect everything
- get any extra parts you need (noting that you should use the LATE style of 1st gear syncro/engagement stuff, post 1976!)
- put your masterpiece together
- sell off the extra parts to get back money

Or, if you don't want to do it yourself, then call up someone and ask how much it'd cost. The most difficult job is to correctly set the diff preload, the ring engagement depth, and the pinion engagement depth. Unforunately, when you swap cases and R&P, you have to do all of this all over again.

A simpler solution is to leave the case magnesium... this means that you can skip the job of resetting everything, but you do have a less-rigid case. The case doesn't usually "fail," per se. It simply holds the bearings and shafts in place with less rigidity, so under high load, they tend to wear more.

Get a copy of Excellence and start calling up all the people with little ads... you will eventually stumble across some people with cores lying around for a good price.

Take care,

[ppp]

vw gear reduction starter...Shad...which one would fit a 930 4spd with the wide bellhousing and wide ring gear?
just sold my 091 with 091 bellhousing reduction starter...oh...but i do have the stock starter...just no flywheel and ring gear to check engagement.

[jhelgesen]

From all my Porsche experience, THE guy to call about porsche trannies is Gary Fairbanks. He can build you what you need. It won't be bottom dollar, nothing in the porsche world is, but it will be right. His number is in excellence mag, or let me know and I can dig it up for you.

[Panelfantastic]

Thanks John. I'm not quite ready yet, but I wanted to be prepared in case I stumbled on to some core parts that I could use later on...knowledge is key.

[ppp]

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=34208&item=2435382064

[Shad Laws]

Hello-

vw gear reduction starter...Shad...which one would fit a 930 4spd with the wide bellhousing and wide ring gear?
just sold my 091 with 091 bellhousing reduction starter...oh...but i do have the stock starter...just no flywheel and ring gear to check engagement.

Honestly... I've no idea :-).

I know that the 930 uses a different clutch setup than the 915 (240mm as opposed to 225mm diameter, for one). I do not know how this translates into other differences...

Take care,