Fletch

I received this from Gary Kerr, it is a rebuilt 915 Transaxle w/ flywheel, clutch pressure plate & adapter plate for the Chevy SB V8. The final drive was swapped @ T & D Porsche and everything dialed in.

Endre (Andy) Bujtas I have a Porsche transaxle, but there is more to it than just having the trans. I'll talk about my entire running gear.

Transaxle: The transaxle was highly modified to accept the power of my 406. I purchased the trans and parts from Rennegade Hybrids back in 1998. They performed the modifications. This is what I have: Porsche Type 915 with magnesium casing Special heavy-duty "tall" ring and pinion (3.10:1 ratio) Ring gear flipped High-performance limited-slip differential Heavy-duty Sachs pressure plate and throw-out bearing (TOB) Special Kevlar friction disk

The 915 is a very good trans for a Small-Block Chevy (SBC) power plant. The trans is good to around 500 HP. Therefore, it can be used for most SBC applications in a Diablo project. It is a 5-speed with a standard H-pattern - first gear at the top left and reverse at the lower right. This was a change to the original Type 901 transaxle (early 911 and 914) that had first gear at the lower left - top left being reverse. As such, it will be familiar to most people - especially when you consider that most of the Diablo projects do not display the shift pattern anywhere.

If you plan on using this trans in the right-side-up position, you'll need to flip the ring gear inside. Otherwise, you'll have 5 reverse speeds and 1 forward speed. Else you could flip the entire trans upside-down. But I wouldn't recommend it since there may be a problem with oiling the gears if upside-down. The oil system (splash type) was designed to operate with the trans right-side-up.

If you plan on mating this trans with a high-performance SBC, then I recommend changing-out the ring and pinion with a set of "tall" gears, the reason to change to the "tall" gears is because you can't change first gear.. The stock gearing is about 3.88:1. This, in itself, is not an issue, but because the stock input shaft has first gear machined (not changeable) onto it, the engine may hit red line at about 30 MPH with this first gear/final drive combination. It may launch you, but you'll have to shift into second as soon as you let go of the clutch in first. I got my 3.10:1 gears from Rennagade and they installed it.

I also have a limited slip differential. The reason I got it was that I plan to do some racing with my car. These are highly desired by the Porsche "club racers". They would probably kill and rob me of it if they knew I had one. However, this is not necessary for normal applications and besides, they are costly. A good rebuilt diff could cost you around $1200. You'll need a heavy duty clutch with this to be able to hold the torque of a V8. A stock Porsche system will not do the trick. I had mine built by Rennagade to withstand the 490 ft-lbs my engine puts out.

Adapter:
To mate the Porsche 915 to a SBC, you'll need the following:

o An adapter plate to mate the porsche transaxle to the SBC block.
o A flywheel that will bolt to the Chevy crank and allow the Porsche clutch/pressure plate to bolt onto it. Typically, these are around 10 inches in diameter - much smaller than a stock Chevy flywheel. This is not bad since now the engine can spin-up faster with a smaller flywheel (less polar moment of inertia).
o A pilot bearing that will fit into the Chevy crank and allow the Porsche 915 input shaft to spin freely inside it.
o A starter ring gear. The starter ring gear is a separate component that gets bolted to the flywheel along with the pressure plate. They are usually sold separately.

Accessories:
In addition, you'll need the following parts as well:

o A high-torque starter motor. I got mine from Transaxle Engineering. They typically use a Lexus starter and are of the reduction gear type.
o A hydraulic slave cylinder. Unlike most clutches, which are the "push" type, the Porsche is a "pull" type. You'll need a slave cylinder and a method of mounting it to the trans body. Wilwood makes a "pull" type slave unit and you can get these from Summit Racing for around $56. But you'll still need to make a bracket. I have drawings of such that I gave to Rick Page.

All this, the trans, the work, the parts cost me around $7000. You can reduce this figure by not including the limited-slip diff, getting your own slave cylinder and making your own brackets. But you are still talking about a unit in the $5000-6000 range. You can save even more money if you can get a hold of a good 915 unit from junk yard cheap. Then sending it out for work, or doing the work yourself. Rebuilding a manual trans is not too difficult.

Some Assembly Required:
There is still some work that must be done to complete your transaxle unit before you mount it to the engine.

You'll need to modify the TOB arm to accept the hydraulic slave cylinder. The original Porsche unit, when used in a 911, used a cable to operate the TOB. The Porsche TOB arm was designed for cable operation. You won't be able to make it operate like this. Remember, the Fiero clutch system is hydraulic. See Rick Page's site for how he modified his. Or you can have one made up by Rennegade, Kennedy or Transaxle Engineering.

You'll have to assemble the TOB onto the pressure plate. When you get the TOB it comes unassembled. And there are NO instructions on how to put it together. I have a drawing that shows how this gets assebled. It is a pain in the butt to assemble.
You should also purchase a Metric alignment tool. This is needed to properly align the input shaft to the friction disk and pilot bearing when you bolt the clutch assembly to the flywheel.

Shifter:
Since the shift mechanism for the Porsche 915 will be facing the rear of the car, you'll need a special cable shifter with mounting brackets. Rennegade Hybrids (and others) make a very nice cable shifter for this unit. The important thing to note is the cable length. The main shifting cable is quite thick, and thus very stiff. When measuring the cable length (from shifter to tailpiece) give yourself extra slop in the cable. It is best to use a heavy-duty extension cord to simulate the shift cable when measuring. Shifters like this cost around $750.

Running Gear:
Now that you have a trans and a way to shift it, you still need to transmit the power to the wheels. This requires axles (half shafts), CV joints, boots and hubs. For higher power engines, 930 CV joints are typically used. This requires some modifcation to the Fiero drive hub that gets bolted to the rear wheel bearing. The Fiero drive hub also is CV joint housing as well. Since I wanted 930 CVs, I had my Fiero hubs modified by having an adapter ring welded to it. The adapter is a 1 inch thick ring with the 930 CV bolt pattern machined into it. Rennegade did the work. I sent them my Fiero hubs and they heliarc'd the adapter to the hub. They were welded perfectly round and true.

I replaced the stock 915 output hubs with 930 hubs. This allows the 930 CVs to directly bolt to the trans side. This requires that the stock 915 hub have the same number of splines as the 930 hubs. Later model 915s do have the same spline pattern so that swapping the hubs is a snap. The 930 hubs are much larger (about 4 inches in diameter) than the 915 hubs and use 10 mm bolts vs 8 mm. I got my 930 CVs and trans hubs from Transaxle Engineering.

The axles or half-shafts must be custom made to fit your application. They must be of proper length to allow axle "plunge" as the wheels go up and down. And they must be machined (splined) to fit into 930 CV joints. Axles are typically made from either Chromalloy or 300M. For standard applications (300 HP or so), the Chromalloy axles are good enough. However, for higher output engines, I recommend going with the 300M (Indy car) axles. They are much more expensive, but you'll not have to worry about breaking them. I got my 300M axles from Transaxle Engineering.

The total cost of the running gear will cost an additonal $1000. So my entire running gear cost me around $9000. But not all at once. For example, I didn't get my shifter unitl I needed it - around a year later. And it took almost 2.5 years before I had my axles made. But you really can't do anything, or build anything, without the engine/trans unit in you hands.

The three sources for parts that I know (for builders of cars using a SBC and a Porsche trans) are:

Rennegade Hybrids

Transaxle Engineering

Kennedy Engineering

Andy

Rick Page The tranny I used is a Porsche 915 transaxle, modified 930 hubs. Flipped ring gear, ZF limited slip differential, taller gears and a Sachs High performance clutch. I changed the mechanical clutch to a hydraulic.
I got the transaxle in from Renegade Hybrids in Las Vegas. Scott pointed out how clean it was, but knowing what I paid for this modified transaxle I would expect no less. It has a 3.10 tall ring&pinion that has been reversed for V8 use and a ZF limited slip differential. This is an excellent example of what you have to watch out for in the kit car industry. Scott and I went over all the pieces I would need, and pricing. This included heavy 'core' charges as I didn't have a Porsche that I was putting a V8 in and hence no transmission etc. to swap for a rebuild. Scott sent the Sachs clutch package early and I noticed it didn't have the ring gear on it. I called Scott thinking it had just been delayed. Scott seemed surprised I would need a ring gear and didn't include it in the price, so I had to pay extra for a needed component. When the transaxle showed up, it didn't have the adapter plate for the small block chevy. Scott again was surprised when I called and said it wasn't included in the huge price I paid.
I got the Sachs high performance pressure plate, clutch, TOB and balanced flywheel.

The whole purpose of his company is mounting these transaxles on small block chevys and he knew that was what I was doing. Once again it cost extra, even though I carefully sent an Excel grid of all that would be needed and the price we agreed prior to sending a check. Scott tells me with all his customers, he can't keep everything straight that they might need. But as a consumer, I may not be the expert and know all that is needed or at least not included in a package.

My rant is over, just beware if you are a new builder. Scott was very pleasant to work with, and he is the new owner of Renegade. So I guess some slack is warranted. I'm sure the quality of his product is excellent.

I talked to Steve about the body and it's alignment measurements. He called me right back and has been true to his word about phone help. He told me the back was set to high on the 'T's and it should only have about 1/2" clearance from the main ducts in the rear. I set them down and everything started to come in close enough that I feel I should be able to tweak it and get it workable. Kurt Mylar also talked to me and made me feel good about the fitment and how much adjustment is possible. He suggested I don't start setting the body until I have the wheels on and are able to make sure the wheel well align properly. So I have backed off on the body, and went back to the Renegade Hybrids transaxle.

The transaxle was mounted temporarily on the engine to check fitment. I decided to go ahead and install the clutch and make it mountable. On the subject of the transaxle, I wanted to make sure that I didn't unfairly give the impression Renegade was unduly expensive. In talking to other transaxle 'converters' my price from Scott was on the low end of the cost scale for these transaxles. Also, his turn time of only a couple weeks was very good compared to others that were more in the 4-6 weeks range and he did give me the adapter plate at his cost. He also shipped it at no charge after the omission of it on my original estimate. I will always attempt to make sure what I put on these pages is as accurate as possible. Now let's look at how this baby goes together. I was really grateful for Andy's detailed pics of his transaxle, this pilot bearing could go in two ways.

Flywheel is bolted securely in place. This is a late model SBC externally balanced flywheel. When working with the clutch assembly you need to be sure your hands are clean and just before assembly, wipe the pressure plate and flywheel with acetone or a good non residue solvent. Oil of any kind on these surfaces will cause serious problems.

The clutch disk is aligned on the flywheel then the pressure plate is bolted on, the TOB must already be on the pressure plate. This configuration of 'pull to release' is found inhigh performance clutches. Andy also sent me detailed drawings of the TOB assembly, which was greatly appreciated. It was not intuitively obvious to a casual observer.

Well it's back together and ready. The ring gear and pressure plate clearance is really tight, but I'm sure it's OK.

Craig Nadeau

Here are some shots of the 915 Tranny. The best of the 915 tranny is the 75 and newer as it has a stronger ring gear. I got the rebuild parts from Motor Meister.

Total cost is $1000

It is a fairly simple rebuild. Due to the need for special tools you need to have a shop adjust a flipped differential and adjust shifter linkages.

This question gets asked alot, what is the distance from the bell housing to the axle center. 7 5/8" The distance from the input shaft to the lowest point on the tranny is 8"

I did all the work to have the 915 rebuilt and ready to go. Here is the work that had to be done on it.

1) Drill the diff cover for the magnetic pick up. Use a hard sealing material to seal the pickup.

2) Machine the case to accept the flipped diff.
- In the case, where the original pickup was machine down about 0.05
- Below the input shaft there needs to have clearancing about 0.1 where the ring gear sits. You can see where the part is clearanced by the factory for the non flipped dif runs
- Clearance the retaining bolts on the diff. These are very large bolts with very thick heads, they are clearanced 0.05 - basically remove the lettering on the heads and a hair more.
- Install the diff
- shim the pinion shaft so that you have 0.015 gear spacing - best to let a tranny shop do this because you need the porsche shims, it takes only about half an hour.

3) Install the diff first! This is the biggest difference in the re-assembly. This is a bummer if you are thinking you might change to a different gearing later. BTW, changing the R&P means that you have to change the entire output shaft. This means pulling all the gears and bearings. A time consuming expensive job. The existing gears are 8:31 so roughly a 3.875 ratio at redline this combo does 44 MPH (7000 RPM) (and 38 MPH @ 6000). Andy's diff switch 3.10 is nice for sure, as it gives him numbers of 55MPH and 47MPH respectively. The other option is to switch out the entire gear set, this is very expensive but can lead to a perfect setup. The Diablo does 60.5 MPH in first gear with a very low 2.41 ratio. This is probably why it is not a great drag car running mid 13s. (check out Ron's Video section and compare the Viper and how far ahead it is by 40 feet.)

4) There are lots of options for a dry sump oil system and is a good idea for high horsepower applications. It is best to have a cool oil to a minimum of the 1st and 2nd gear sets.

5) The adjustment of the shifter arms is not really hard but it is easy to screw up so mark the rods with a score before you start. The detants locate the shafts in a neutral rest position. The selector rod should move smoothly in neutral between all 3 stations. The shifter forks should be located exactly in the middle of the two syncros when in neutral. The 5th gear should be 1mm away from the idler gear when in neutral.

Once a Porsche tranny is rebuilt, adjusting the shifter rods can make an improvement in shifting. Also - and this applies to all the manual Porsche Audi transmissions, flip the detents over so that there is a new surface against the shifter rod. This can be done after the tranny is back together so long as you shift into neutral first.

Tip, if the rear housing does not come off easily and comes out about .25, it is the idler gear holding it, you need to tap it with a mallet and a drift while pulling on the housing. It will come off easily. Do not pry the housing, it will damage the gasket surface.

Links:

Rennegade Hybrids