donzimatt

Hi All,

Been away for awhile, but still have my '78 euro 5sd that this forum helped me restore in 1999. Have collected some parts along the way and have a spare CIS engine that I want to put on the back of a Volksrod (fenderless Bug hot rod). I know this isn't for everybody, but I have 2 spare engines and they are really interesting to look at. For strength and gearing I'd like to use a 911 transaxle (915 version). Does anyone know if a 911 flywheel bolts to a 928 crank? It looks like a 911 flywheel has 9 bolts holding it to the crank like a 928. What about the bellhousing match up? I've searched for dimensions and cant find them, and have no trans or flywheel to look at here. I know I'll have to do some machining, but wanting to know what knowledge already is out there.

thanks, Matt

GregBBRD

The bolt pattern is the same on a late 911 crank and a 928 crank. Both engines even use the same crank seal.

The 915 transmissions are very robust. Some early weaknesses with the mainshafts, which was cured. Also sone weakness with the early shift hubs, which was also cured.The bearing races pound out the cases on the later aluminum units.
Pretty easy to build one to absorb V8 torque.

donzimatt

that's great news. when you say late 911, what year are you speaking of?

GregBBRD

Good point.....
The input shaft (main shaft) got updated in 1976. The shifting sleeves got updated in 1977. Aluminum transmission cases showed up in 1978. The "side cover" for the differential got updated in 1984.

The very best transmission that can be built is from a 1977 (Mag case with steel bearing inserts) with a 1985 (and later side cover.) Everyone in the business knows these are the best 915 transmissions....so they are probably tough to find.

I raced one of these transmissions in front of a 1969 Trans-Am Z-28 engine. Couldn't hurt it. After thst car was destroyed, I ran the same transmission (had to replace the broken case) in front of a 3.5 liter twin plug high butterfly mechanical 911 engine. Still could not hurt the transmission.

donzimatt

OK, so get a flywheel from the same year transmission? or is the right flywheel for the 928 crank from later on?

Any idea if the diameter is similar to 928 (will it fit inside the engine casting envelop?

Any idea if the bellhousing bolts up?

GregBBRD

That is where your project gets tougher. You will need a custom adaptor plate to mate the transmission to the engine.

What you need for a flywheel will be determined by what year 928 engine you use.

Fortunately, there's been a few 928 engines installed into 914's, so this has been done before.

See if Kennedy Engineering still exists and can be of any help.

Catorce

Uh, no they aren't. 915 transmissions in the 911 world are NOT known as being robust at all. In fact, there are many upgrades you need to run on that trans just to push 300 ft lb, and many of them don't work. Think billet side plates at the minimum and pump with spray bar.

Even with those things, 915s are well known to grenade often as can be evidenced by a quick search of these forums.

No serious 911 built engine uses a 915 for very long....the better choice is the 930 4 speed in either short or long bellhousing or a G50.

Just about any stock 928 motor has the capability to implode a 915.

GregBBRD

I should have prefaced my comment with "In my experience and in the 915 transmissions I built, they were very robust."

There is no doubt that the correct pieces must be utilized....the aluminum cased units were trash from the very beginning....as soon as the case got hot, the bearing races were .003" loose in the case....and they quickly pounded out, from there. Yes, certain side covers were inherently weak and this needed to be addressed.

7/31 ring and pinions from the 912E model (commonly used before " short ring gears" were available from the aftermarket) were available were junk and wore quickly.

And some of the aftermarket gears had very short life spans, due to poor quality.

And I'm betting that just like the 928 transmissions I see, age of the pieces has taken its toll. 30 year old pieces with hydrogen embrittlement and root tooth wear are going to be inherently weak.

Perhaps you are correct. Perhaps today, a decent 915 box can not be assembled.

James Bailey

Given that the O P is talking about a 4.5 cis 240 hp. 257 torque engine he just might be fine with a 915 gearbox. Besides with that much weight hanging out the back of his VW bug he won't be able to hit it very hard or the front tires will be in the air

Catorce

The most hp/tq the 915 was subjected to *from the factory* was 215/195lb ft in the Carrera 3.2, which is a lot less than even the crappiest 928 is putting out. Don't get me wrong - I have the 915 in my 83SC with a 3.2SS that I built. I think that car makes about 230-240 hp, and so far there are zero complaints from the trans, which I did not refresh or even touch. I changed the oil on it, and that car has 200K on the chassis.

I just would not consciously pick the 915 as a starting point for any custom build.

GregBBRD

Read my post #4. I put some serious horsepower and torque, through a 915 transmission, with zero problems....for many years.

Granted, that was 30 years ago and the pieces I used were "cherry picked" from the best years and were low mileage.

More to your point, Porsche obviously didn't think the transmissions were strong enough for the Turbos...otherwise there wouldn't be 930 transmissions, right?

Catorce

Greg

I wonder if the age of the components factors into it. When you raced yours, the trans was still practically new. Odd that Porsche would make an intentionally evil shifting and easily grenaded trans. Perhaps they just wear more and don't age as well as their other stuff. Curiously you never hear of issues with the 930 and G50 trans.

GregBBRD

I'm sure age is a huge factor...the "insides" of the 928 transmissions is getting to be pretty scary, in terms of gear "root" erosion.


I've never personally raced a 930, so I can't speak for those. However, they are very strong and the Porsche style synchros oil and cool very well, making the synchros very durable.

I do know that I had terrible issues with the G50 in my RS America, which we raced for many years. Getting oil into the inside of those Borg Warner design syncros was very diffIcult and the resulting "smearing" of the Moly surface was a constant problem.

Porsche made the pinion and mainshafts hollow in the 993 model, which allowed them to get gear oil through the shafts and to the synchros. This helped some, however we still had to service the synchros after every 45 minute sprint race. At the longer races, like 12 hours of Sebring and the 24 hours of Daytona, we knew the tranmissions would be "crunch-a-matics" three hours into the race. We just told the driver's to be as careful as possible and hoped for the best.