Jason Jones

The BIG tranny swap:

915 or 930 transaxle in a 914 chassis? Decide for yourself.

By now, you have probably been well educated on the limitations of the stock 914 transaxle. If not, here is a quick recap…

1. Don’t use 1st gear period.

2. 300 HP and 300 FPT are the streetable limitations.

3. Shock loading will kill the tranny in no time.

4. No speed shifting or excessive abuse.

5. Have your 914 tranny converted to a four speed with taller gears by Renegade if the following scares you!

If 300 HP is not even close to what you had in mind for your 914, well…you might want to read what it takes to go really big!!! Don’t get me wrong… We have many customers that choose the upgrade after they have gone through a few stock 914 transaxles, but it take a pretty hefty contribution to the conversion God’s to afford this conversion done correctly.

Let’s first consider your transaxle of choice. The 915 is easily obtainable but it really only increases your HP and torque capability a little. I would push the 915 to about 400HP and 350FPT, before your odds of failure increase greatly. Plus, the stock 915 gear ratios are really quite short and very close together. For a V-8 conversion, your final drive (5th gear) puts your cruising RPM’s at over 3000 at 70 MPH in most cases…and your first gear is so low that it is virtually unusable.

We do have a taller 3:10 ring and pinion available for the 915, which makes that tranny really excellent for a V-8 conversion, but the cost is rather high, and the availability of that custom ring and pinion is minimal at best.

By the time you invest in the 915 with the taller ring and pinion, a tranny rebuild, the clutch components, and all the other bells and whistles, you might as well consider the 930 turbo 4 speed trans from a 911 turbo!

The 930 is quite the excellent choice for the 914 conversion for many reasons, but there is a lot to learn before you race out and grab the first 930 you find.

First of all, the good stuff. 700HP and 700FPT are not a problem with the 930 trans. The stock gear ratios are just about perfect for a V-8 conversion. Tons of custom gears and multiple high performance upgrades are available. The giant 240mm clutch components are excellent for high HP applications that still require "streetability." The ring and pinion can be flipped to run in any mid engine application. …And the list goes on.

The drawbacks are fewer, and most are easily rectified. With the following brief history lesson, you will see the pros and cons of this workhorse of a transaxle.

The first two years of the 930 turbo trans (‘76 and ‘77), the length was one inch shorter than the years to follow. The early 930 chassis was very similar to the 911 NA design, because the trannies were about the same in physical dimension.

Porsche used a typical pull type pressure plate, like the 915, with a greater surface area, and that was quite adequate at first. However, with the rapid increase of HP in the turbo motors, slippage became an issue by the very late 70’s. Because high performance materials available for the clutch disc surface were quite limited and rather temperamental, the only solution to getting more grip with smooth engagement was to increase the clamping force of the pressure plate. So Porsche added approximately one inch to the 78 and later bellhousings so that a thicker/stronger pressure plate design could be implemented.

This also made the chassis designers go back to their desks and look for some more space under the back seat for the bigger tranny. The compromise was to move the transaxle forward a bit, and skew the rear motor mount back a little, to accommodate the longer motor assembly. As Mike would put it… "It’s like putting 10 pounds of crap in a 5 pound bucket!" It was tight.

In the years to come after the introduction of the early ’76 and ’77 930 transaxles, the 911 NA racers started grabbing up any early boxes they could find and installing them in their 911 chassis. The later ’78 and up 930’s, were to long to fit in the 911’s so the box of choice was this first generation turbo trans. If you were lucky enough to find one of those two year only trannies today, you might find it to be quite tired and stressed.

Well, guess what? That short 76/77 early 930 box, is the only one that fits perfectly in the 914 with minimal increased axle angle. Since this tranny is not easily available, we have developed a method of shortening the longer (late model) bellhousings and the main shafts to accommodate the 914 mid engine application. But, there is a trick to doing this that can make this seemingly easy process a difficult mess.

Unbeknownst to most Porsche enthusiasts, the 930 came in three versions…not just two. If you look at the bellhousing on a 78 to a very late 80’s 930 transaxle, you will not see an extra rib cast into the bellhousing approximately one inch back from the stock lip. This makes the 80’s version a poor choice for the modification without welding a lip around the mouth of the bellhousing and pockets for the bolts. If you have ever tig welded to a casting, you know that is a very difficult process.

The late model 930 bellhousings are a different story. At about an inch back from the stock bell, there is a rib that circles the mouth of the bell. This provides the lip that is needed to securely locate the tranny to the adapter plate. Here is the catch… The newer the 930 tranny, the more the price of the core.

But wait… There is more! A lot more.

The flipping of the 930 ring and pinion is quite the miracle if done correctly. Not only do you heavily modify the internals of the case, but you also have to add extra support adjacent to the main shaft which is evident just below the guide tube. There are very few Porsche tranny technicians that even get close to doing this process correctly and can maintain the high HP and torque capabilities. Let’s not forget the electric speedo modification to relocate the electronic pick up after the ring and pinion is flipped.

Enough about the tranny… How about everything else! It’s easiest to just give you a list at this point…and I am quite sure I will leave a few things out!

You have the Renegade clutch components with a high performance pressure plate, custom flywheel, Kevlar clutch disc, custom pilot bearing, throw out bearing, and hardened ring gear.

Then you need a Renegade custom cable shift kit that usually runs around 90 inches in a 914. It’s a particularly nice unit, but if you would like to finish off your interior around the gate shifter, a custom center console is recommended.

Then you have the Renegade custom pivot wheel and cable extension assembly, which allows the stock 914 clutch cable to be used with the 930 tranny. This usually attaches to the 930 clutch arm-broached finger-omega spring-bracket assembly, that never seems to come with the tranny when you find one. All of this stuff is strictly Porsche, and is very hard to get.

How about axles? You will need Renegade custom length axles with 930 inward CV’s and 911 (very rare) 4 bolt and two roll pin outward CV’s. Your stub axles are also 911, but they need to be the larger diameter face with the 4 bolt and two roll pin provisions, but still have the 914 axle spine. Don’t forget to have the rear hubs drilled to accommodate the five studs for your five lug conversion. We do it all.

Since the 930 has no ears on the rear housing like the 914, a custom Renegade rear trans mount bracket is the next on this list. The stock 914 rear tranny mounts are really unsafe at this point, so a pair of modified 911 sport style motor mounts actually work really well.

Your stock speedometer is cable driven, but your tranny has two wires sticking out the side of the case. It’s time to get a new 911 style electronic speedometer.

Some good news! Your current adapter plate will work as long as you have the thicker style…AND, if you have used a gear reduction style starter on your conversion before your tranny upgrade, it will still fit and operate correctly in the 930 starter pocket.

The damage??? Are you sitting down? You will spend over 10k on the tranny and related transaxle conversion parts, and that does not include ANY of the V-8 engine conversion parts! That is for just the tranny and all the related systems that make it work in the 914.

Back to the 915, you might save about $1000 at best over the 930. It’s not worth it. Go for the 930 if you plan to do it at all.

Don’t get me wrong. If you consider your 400+ HP 914 car, with the complete 930 tranny conversion, can out perform cars at 5 times your investment, this is quite a bargain. We have sold a ton of upgrade kits and 930 turbo trannies through the years with not one of our customers wishes they still had the 914/901 tranny.

On the other hand, if you can keep your motor down under 300 HP and 300 FPT, and your driving might be considered a little on the conservative side, the 914 transaxle might just be the box of choice.

This is the gospel on Porsche transaxles...

Mid Engine = When the engine is before the rear axle like a 914, Pantera, Indy car, etc. This puts the transaxle in the rear of the car.

Rear Engine = When the engine is in the rear like a 911, Corvair, or Deloreon. This puts the transaxle under the back seat.

Transverse = When the engine sits sideways and the transaxle is (for the most part) under the motor. This might be found in a front wheel (Honda Accord) or a rear wheel drive car (Fiero).

901 = Available from 65 to 69 in ONLY 911 and 912 rear engine cars. No need to use this box for mid engine applications because the 914 transaxle will do the job. (See 914) Good to 300 HP and 300 FPT if you do not use first gear and if you do not abuse it much.

914 Tail Shift = Available from 1970 to 72 in 914 mid engine cars ONLY. Some people will call this box the 901, but it is truly a box designed for the mid engine 914 application. Even the gear ratios were changed for the 914 from the similar 901 box design. This is not a favorite box due to linkage issues with the tail shifting design. Good to 300 HP and 300 FPT if you do not use first gear and if you do not abuse it much.

911 = Available from 1970 to 71 in 911 cars only. This trans axle was a modified 901 box with the ability to accept larger clutch components that were later used in the 915 style transaxle (see 915). In addition, larger sized CV joint output flanges were installed in increase strength and reliability and to accept bigger CV axles. Good to 300 HP and 300 FPT if you do not use first gear and if you do not abuse it much.

915 = Available from 1972 through 86 (or 89...I forgot) in 911 cars ONLY. With two different case materials and numerous gear combinations, the 915 was the work horse for most 70's and 80's 911's. This box may be modified to work in a mid engine application by splitting the case, making some modifications, and flipping the ring and pinion. This will give you the five forward gears with the motor spinning the opposite direction as it is in a rear engine 911. This is one of two transaxles that many 914 owners will choose to use for bigger HP. Good to 450 HP and 400 FPT if you do not abuse it much.

930 Early/Short Bell = Available from 1976 to 1977 in 930 cars only. When the 930 turbo was introduced, the HP of the stock motor and the gear ratios required, made the 915 a poor choice for the big 930 motor. The 930 four speed transaxle was born with a much larger ring and pinion, larger main shaft, stronger gears, the ability to accept 240mm (giant) clutch components, and a stronger case. Furthermore, it had a much taller set of gears in a four speed pattern. Capable of withstanding lots of hard driving at 700 HP or more, this is a very good choice mated to an American V-8 for several reasons. This two year only box was also a favorite choice for racers throughout the late 70's and 80's. Many 911 owners were able to make only minor modifications to their chassis to accept the 930 transaxle when the 915 was not enough for their big motors. If you are lucky, you might find one, but most are hammered due to their fondness with racers. This box may be used two ways in a mid engine application... The first way is to do extensive case modifications and flip the ring and pinion...which Renegade can do very expertly, or the other is to run this box inverted. An inverted 930 rear engine type transaxle will run in a mid engine configuration perfectly with only minor modifications to the case. The only drawbacks are the box will be running "belly high" and you might loose your trunk in some cars, and the outputs of the axles may not line up with the hubs, depending on your situation.

930 Late/Long Bell = Available from 1978 to 87 (up to 94 in some cars) in 930 cars only. With the quickly increasing HP of the 930 turbo motors, and the lack of good clutch materials for the street, Porsche was forced to add an inch to the bellhousing of the 930 transaxle and change the pan design of the 930 cars, just to get a bigger pressure plate in the clutch assembly. Since there is more chance of finding a later 930 transaxle in good shape, we actually shorten the bellhousing approximately one inch (like the early 930) for most kit car applications. Since the streetable clutch materials have been improved greatly over the years, the 930 short clutch components can be used with much success. In addition, there is much less spinning mass in the assembly which aids in HP.

Answers - by Rick Page

Actually once I converted the final ratio from the 3:88 to the 10:31-3.10 ring and pinion, I felt comfortable leaving the rest of the ratio's in place. The torque curve of my stroker isn't significantly different than the Porsche so once the increased torque is compensated for (and ring gear flipped for the mid engine rotation) I felt the rest would be acceptable.

The other notes:

• Check on 930 hubs and CV joints. You must have a 43 tooth spline on your current hubs to make this switch.
• Make sure you upgrade the clutch and pressure plate to the Kevlar high pressure model.
• Sachs high performance TOB
• Jeff will help you with the 300M steel axles (Indy car axles) to handle that torque.
• Have an electric sender installed for current speedometers
• Obtain the Wilwood hydraulic Clutch slave

Andy did an engineering drawing on the slave cylinder bracket. I'm sure he could email it to you.. or I can probably find it if you need.