I must start this tech article with some background information. The fact is I'm not much of a fan for forced induction on an aircooled car, especially the Type 4. There are a lot of issues that have to be dealt with in designing a well balanced forced induction engine. This doesn't means that there aren't any powerful and reliable force fed Type 4s out there, I just feel there isn't enough information and parts availability out there right now to make forced induction easily achievable for the average VW and Porsche owner.
So, you as the reader must know I personally have never built or ran a forced induction VW engine. As you read this tech article, keep this in mind. I will do my best to provide a thorough discussion and give you all the facts. The points I've presented here are based on text book research, my analysis, and discussions with turbo/supercharged VW and Porsche owners.
Another thing to keep in mind: many books have been written about forced induction. There are many good ones, including Turbomania from CB Performance, and xxxxxx by xxxx. I found both books to be a wealth of information about turbocharging and turbocharging the air cooled VW engine. If you are seriously interested in turbocharging, I suggest that you read these books. That being said, it is my intent with this article to give you a brief overview of forced induction and the issues I foresee when used on a Type 4.
Once you have the facts, and have conducted your own research, I encourage you only then to evaluate whether force feeding your VW is something that's going to achieve your goals.
There are a couple of different ways of getting more power from any engine. You can increase the displacement (through the use of stroker crankshafts or big bore pistons), turn the engine to a higher RPM (through the use of a longer duration camshaft), or by forcing more air/fuel mixture into the combustion chambers (forced induction). All of these methods increase the amount of fuel/air mixture reaching the combustion chambers.
The more you can force in of the fuel/air mixture, the more power you'll gain. In the USA, the way of measuring how much pressure is being fed is psi (pounds per square inch). One psi is equivalent to 0.07 bar or 0.068 atm (atomsphere), which are other measurements of boost.
Forced induction is rather unique compared to the other options. The other options rely on the vacuum created by the piston as it travels down, whereas forced induction utilizes another device to push a denser fuel/air mixture into the chambers.
These devices run two different ways: the turbocharger uses your escaping exhaust gasses that exit the exhaust ports to propel a turbine, which in turn forces the air or the fuel/air mixture in the intake system.
The second device is the supercharger, often called a blower in American car circles. The supercharger differs from the turbocharger in that the supercharger is ran off a belt, which is usually activated by a specially made crankshaft pulley. The belt turns the turbine in the supercharger. The turbine pulls in the air or air/fuel mixture and forces it into the chamber.
Before you decide that forced induction is the way to go with your Type 4 plans, you need to understand some of the issues with the Type 4 cylinder head. These issues must be considered before even considering a turbo.
Any Type 4 cylinder head specialist will tell you the exhaust ports of any of the stock Type 4 heads are really restrictive due to their port design. Essentially the intake ports outflow the exhaust port. The opposite is true with the Type 1: it's exhaust ports outflow the intake ports. So adding a turbocharger to the Type 4 exhaust will cause a restriction, not something you want when the intake ports already flow more. Since the spent gases can't freely escape, the heat builds up and causes the problems related to overheated heads, like detonation, high oil temperatures, dropped valve seats, broken or burned exhaust valves, warped heads, excessive vavle guide wear, etc.
Additional heat can also be introduced into the engine bay thanks to the placement of the turbocharger. Your exhaust gasses can reach over 1200°F as it escapes the exhaust port. This heat is carried and transferred to the turbocharger. With most turbochargers placed in the engine bay, this will raise the ambient termperature in the bay.
With either a super- or turbocharger, you have a few decisions to address. Your turbo (in this case, either super or turbo) can be set-up in two different configurations. The first setup is probably going to get you the most power, a blow-through system. As the name implies, the turbo compresses the air and blows it through the carb(s). A setup with dual, twin barrel carbs (or throttle) bodies will probably yield you the most power, but it's more of a challenge to create ducting. The carbs also need to be specially prepared, as the increased air pressure can blow out seals and collapse floats.
The other setup is called a draw-through system. This system places the carb (or throttle) body on the end of the intake tract. So, as the turbo spools up, it produces vacuum that pulls the fuel from the carb. The carb is generally bolted to an adaptor to the turbo itself, but the tubing from the turbo to the intake port tends to be very long. My one reservation with this system is that I'm afraid that with the long runners the fuel will fall out of suspension before it gets to the cylinder head. Of course that's pure speculation on my part.
Another thing to keep in mind about using the turbo'ing the Type 4 is the cylinder head to cylinder sealing. The four head studs that hold each head sealed to it's cylinder is not in a square pattern. The left-right distance is longer than the top-bottom distance between stud centers. This means that the torque applied won't be evenly distributed. Once again, it's speculation on my part, but I suspect that this could lead to sealing problem with anything but trivial boost levels.
Once you've got the turbo plumbed and looking like business, you will need to address fuel and ignition management. The engine will need different fuel/air and ignition setting at low-to-none boost levels, and another group of settings for when the boost is increased. Using a specially modified carb and vacuum advance distributor can allow for a certain amount of tunability, but this case is where PEFI (programmable electronic fuel injection) and PEI (programmable electronic ignition) shine. Using a multitude of sensors, like an oxygen sensor, ambient sensors, and knock sensor, the turbo tuner can rest assured that your engine will not run lean or too rich.
A properly setup turbo engine will utilize a camshaft specifically ground for the Type 4 and the turbo system. In general, turbo systems like camshafts that have as little overlap as possible. Overlap is the time that the intake valve is open and the exhaust valve is open. With forced induction setup, overlap will cause the forced air/fuel mixture to go past the intake valve, through the combustion chamber and past the exhaust valve. You've just wasted all of your work in getting that mixture in there. Turbo camshafts also don't require as much duration or lift to make power as a normally aspirated engine. If you remember from my Camshaft technical article, more duration means higher rpms and harder life on the valvtrain. This is where the turbo excels.
Valve sizes are best matched to your intended application. I will recommend to keep the sizing conservative, and if you do go with bigger valves, make the exhaust valve size close to the intake valve size. The more you can help the exhaust port to overcome it's restrictions, and the turbo will not need as much increase on the intake side.
Now that you've read all of this, you might be asking "Shoud I do this?". Well, that all depends on what you want from your engine. A properly built turbocharged or supercharged Type 4 can be a blast to drive. A typical street setup can increase the power across the entire powerband. This means the Type 4 will be a rocket around town, on the highway, and on the track.
If you are adventurous and have the tools/skills to fabricate the header/intake, you can put together a turbo setup for a reasonable price. On the other hand, if you are like me, I don't weld or have the garage space, so I'd need to rely on a tuner for the pieces. As far as I know, there aren't any mass-produced turbo setups for the the Type 4 engine, stock cooling or upright. You'll need to contact an engine builder for a quote for the cost of the system.
You'll also need to address the issues that I covered in the previous section. No tuners have come forward with concrete findings of their experience. Many elect to keep that a secret, for their "competitive edge". Until we can get more information shared, any personally built turbo system will be an experiment.