One question that I continually am asked is what kind of transmission can I/should I run with a Type 4? Of course, this question is not easily answered. Each conversion is unique and requires some knowledge of your options and what you intend to do with the finished project. I'm going to assume that you will be building a Bug, but it will also apply to the Karmann Ghia, Type 3, and any car built on the Bug floorpan for street use. I will also assume that you will be using about stock diameter tires. The use of really short or really tall tires brings in other issues.
The first option we'll look at is the transmission you already have in your car: the Type 1 (identical to the Type 3) transaxle. The Type 1 transaxle is suitable for most of the Type 4 conversions, as it's a pretty strong design that can take a lot of abuse.
A pretty much stock Type 4 conversion, driven in mild manner will be able to use the stock transaxle. Care must be taken in not putting a lot of torque through the transaxle by accelerating hard from a stop sign or from downshifting and engaging the clutch. My recommendation is to drive your car with the stock trans and start saving the money to get a "beefed up" transaxle.
Of course if you are going to be driving the car in a spirited manner (pedal to the metal, power shifting, burn outs, etc.), the stock transaxle is going to fail. It wasn't designed to take this kind of abuse, that's why purchasing a "street" transaxle from a reputable builder is highly recommended. The generally accepted number of 200hp is the limit of a beefed Type 1 transaxle.
The late Joe Locicero of Oregon Performance Products recommended the following Type 1 transmission modifications when using a Type 4 engine.
"Use a super-diff, H.D. side cover and have the engagement teeth welded on third and fourth gear. On the '71-up trans case, use an asymmetrical bearinged Vanagon pinion bearing.(Same I.D./O.D. as T-1 unit) Transmission straps are out. Use a rear engine hanger with the stock rubber mounts- unless your racing. This results in five insulated drive train mounts. Best ratios found for Type 4 in Type 1 is a 4.125 R&P with a .821 4Th gear with a final drive ratio of 3.38:1. In a Type 4 in a Type 2 (early) is a 3.875 and a .89 4th with a final ratio of 3.44."
The above applies to '71 and later Type 1 transaxles with the guided throwout bearing. In the '67-'70 type 1 trans, the trans can be made to appear like the '71- up by changing the throwout cross shaft to a '71 Type 1 only throwout cross shaft. The sleeve on the input shaft is available from Carter's gear box shop in Big Bear, CA. The input shaft seal is removed and the sleeve is driven into position over the input shaft. The sleeve has an internal oil seal. Now the bell housing looks like a '71-up. Some have suggested the use of a Type 2 IRS input shaft in the Type 1 transaxle to get the longer shaft, but the larger area differential area of the Type 2 transaxle makes for a longer input shaft overall. In other words, the shaft is too long and will not work.
Joe's recommendations on the gearing make the Type 4 a nice freeway cruiser. If you are interested in more performance (as in drag racing), try sticking with the stock 4th gear or go to closer ratios to minimize the RPM drop from 3rd to 4th gear. The extra torque of the Type 4 makes the use of ultra close gears like with a Type 1 unnecessary.
Your other option with the Type 1 transaxle is the Gene Berg five speed conversion. The GB five speed conversion allows you to run close ratio gears in the first four positions and still maintain a tall fifth gear to keep the RPMs at a respectable level on the freeway.
One compromise of using the Type 1 transaxle is the bellhousing. The small and shallow bellhousing limit the use of the 210mm and 215mm flywheel, with clearancing. If you want to know why this is a compromise, read the tech articles on clutches and flywheels
If you are now adventurous and have access to a good machine shop, then you might want to look into this conversion to get the large 228mm flywheel into the Type 1 bellhousing.
Contributor Gregory J. McGee sent me his method of getting the 228mm flywheel into the Type 1 bellhousing.
A 228mm flywheel can be made to fit EASILY in a T1 trans with the following mods: Vanagon 228mm flywheel (forged) suggested. Forged flywheels are HIGHLY recommended for racing service, and they are usually required by most racing sanctioning bodies for many classes.
Machine clutch mounting surface back 2mm and machine clutch surface back 2mm.
The pressure plate is the clearance issue and this greatly helps reduce the interference. All 12V aircooled VW flywheels have the same ring gear size.
With a 1/2" trans<>starter spacer ring AND a self supporting (914/late bus) starter will then engage properly. A bug starter will die an instant, ugly death from the one way bearing hitting the pressure plate, which will try to slice the one way to bits.
I've never done this conversion myself, but contributor Rolf has and says this method does work. He stated that he's still had to clearance the bellhousing, in the same places that you would to install a 12V flywheel into a 6V transaxle. He also states that care must be taken, as reckless grinding will put a hole in the housing.
Type 2 IRS
A common alternative to the Type 1 transaxle is the Bus IRS transaxle. There are two different models of Bus IRS transaxles: the 002 and the 091. The 002 was used in the '68-'74 Transporters and the 091 was used in the '75-up Transporters.
This conversion makes a lot of sense, but not for everybody. The 002/091 conversion, with stock internals, is stronger than a stock Type 1. The bellhousing and clutch parts were designed for the Type 4 engine, so it makes logical sense. No modifying of the flywheel is required.
This all sounds great, but it only makes sense for specific applications. Both the 002 and the 091 feature a really low (higher numerically) ring and pinion. This was needed to help get a rather heavy Bus up the hills. The tallest ring and pinion available on a Bus transaxle was the 4.57, where as the shortest Type 1 was the 4.375. For a heavy Bus or a Baja/Buggy with tall tires, this low gearing is exactly what is needed.
For a street car, this low gearing is not desirable. The short ring and pinion will cause the engine to wind out on the top end and the torque of the Type 4 will mean you will be shifting sooner in each gear.
When it comes to gearing, think of a mountain bicycle; if you shift into the smallest front chainring and the largest rear cog (what's commonly referred to as the "granny gear"), you have a low gear. This low gear lets you start pedaling with relative ease, but once you get moving, you can't go very fast or for very long. If you shift into the smallest rear cog, it gets taller, but not enough for any kind of speed on the road. Think of the front chainring as the ring and pinion and the rear cogs as your gears. With a bicycle we can change chainrings on the fly, but we can't do this with a VW.
If the short gearing and ruggedness of the Type 2 transaxle are desirable to what you are building, then you will need to look into how to mount this monster into your car. We'll assume you have a Type 1 based vehicle, like a Baja Bug or a dune-buggy.
There are a few ways to mount a Bus transaxle into a Bug. The cheapest way is a kit available from most vendors. It's basically a strap/mount kit with shorter axles. It requires that the shift rod from the front be raised to on top of the center hump or use an adapter. My experience that this kit is marginal is terms of quality and in terms of performance.
My preferred method of conversion is the Gene Berg conversion. GB manufactures a intermediate housing that allows the use of the Type 1 nose cone. This conversion does require the disassembly of the transaxle and is not a cheap method, but it's the most reliable, accommodates power shifting, and retains the stock position of the shift rod and shifter.
Both of these conversions will require shorter than stock axles, but they are commonly available from Sway-A-Way in high performance configuration.
A few people have emailed me about using the transaxle from a 411 or 412. Although it seems like an ideal option, it's far from it. The only thing it has going is that the engine bolts up without any problems.
First problem is supply. Ever try finding a Type 4 transaxle? They are extremely difficult to locate, especially the manual four speed here in the US. They make finding a split window Bug look easy. If you do happen to have one, locating parts for it is another obstacle. Almost all VW transmission rebuilders do not stock the parts for the Type 4 transaxle. If you decided to drive your Type 4 engine "spiritedly" (with a lead foot), there are no parts available to make it stronger.
The second obstacle would be installation. The Type 4 transaxle has totally different mounts, so custom brackets and mounts would be required to mate it to your pan. The other area in installation would be the gear selector rod, or "hockey stick". The Type 4 hockey stick is towards the top of the trans, whereas the Type 1 is located towards the bottom. In other words, don't go through the hassle of installing a Type 4 transaxle: get something else that's better suited for you.
I must start this section out by saying that I have no first hand experience with the Porsche 901/915 transaxle, or mounting it into a Type 1 chassis. The information I've gathered here has been gathered from people who've done this conversion and from Porsche fan websites.
One of the alluring features of the Porsche transaxles is that it is a 5 speed. The common misconception that the addition of a fifth gear means increased freeway cruising, but this is not always the case. Some of the Porsche 5 speeds do not even have an overdrive gear in fifth; they have a "direct drive" ratio of 1:1. This means that all five gears are close ratio, which is great for racing, but not for long distance highway driving. The moral is if you undertake a Porsche transaxle conversion, make sure the gearing is what you want.
Of course other factors need to be considered if you are looking into this conversion. The first one is the cost factor. This is an issue with most of us. The Porsche transaxle parts can be quite expensive, even from third party suppliers. The extra expense is finding someone who can fix the transaxle, should it become necessary.
The 901/915 transaxle is not a direct bolt in for the Type 1. The conversion requires fabrication of mounts, modification for the clutch cable, and for the shift rod. All of the necessary modifications are illustrated in Keith Seume's book VW Beetle: Performance Handbook.
Mating the Type 4 to the 901/915 is pretty simple. This pairing requires the use of the 914 215mm flywheel, clutch, disc and throwout mechanism. The 914 flywheel is offset differently than the Type 2/4 flywheel, so it's not the same.
Thanks to Joe Locicero of Oregon Performance Products for the wealth of information concerning the use of a Type 1 transaxle and a Type 4 engine.