This page was last updated Friday, May 18, 2007

The 911 fan conversion has been popular for many years with the offroad racers and the European street crowd. It was felt that since the 911 fan could cool a 2.0-3.6L flat six cylinder, it could definitely cool a hot Type 4. The problem is, the conversions currently on the market lack some features and over look a design flaw. What's even more ironic is that the 911 fan conversions are becoming popular with the Type 1 engine builders, but do they have all the information? I believe they are sacrificing cooling performance for vanity.

We'll get to the problems with these 911 fan conversions, but let's look at the ins-and-outs of these conversions. Once you understand these points, you'll better be able to decide if the 911 shroud is for you.

These conversions have a custom laid fiberglass fan shroud (now carbon fiber is available from some vendors), a pulley for the crankshaft, and some feature a special adaptor for the alternator.

Let's look at the components that make up this conversion.

Fan blades

The fan design for the 911 is an axial design. This means that the air coming from the fan travels backwards, in a spiral path. The 911 fans are available with different number of blades. The early ones only had 5 blades, and as the 911 engine grew in size and tuning level, the number of blades grew. The most common one used in Type 4 conversions is the 11 blade.

Generally, the fan with more blades produced the most cooling air, but it also required more power from the engine to operate. So if you go with a fan that beyond the needs of your engine, you'll end up with less power than you could have as you'll be losing more power to operate the fan.

Fan diameter

Over the years, Porsche also used many different diameter fans and rings. There really isn't any to be gained from using one diameter over another. The size you need will be determined by which shroud kit you purchase.

External oil coolers

Most of the 911 fan conversions I have found in my research do not have any provision for an oil cooler in the fan shroud. These conversions will require that an external oil cooler be mounted elsewhere, out of the engine bay. This is usually done by full flowing the crankcase and oil pump.

I'd also recommend using an electric fan, thermofan switch and oil thermostat, all to control the heat. For more information about using an external oil cooler, check out my tech article on the oiling system.

Internal oil coolers

There is one option for those of you that want to run a 911 fan but still want the flexibility of the internal oil cooler. Sharpbuilt of Australia offers a fiberglass fanshroud that utilizes the 911 fan and the stock Type 4 cooler in it's stock location. This makes for a cooling at a stop sign or in heavy traffic at the local cruise night.

Fan height

Installation of the 911 conversion does present some difficulty if you plan on installing the engine in closed engine bays. Some fan shrouds may interfer with the bay cover, such as the decklid on a Bug. The FAT Performance kit does not require the breather tower on the case to be machined. They merely provided an adaptor that allows the 911 fan/alternator to sit on top. This makes the engine too tall and will negate you using the decklid on your Bug.

Sharpbuilt of Australia and BAS Ahnendorp of Germany sell conversions that require the breather tower of the crankcase to be machined down. This places the fan closer to the top of the case. This is almost always enough to clear the decklid. When you purchase the kit, they give you the instructions on how much needs to be removed.

Problems...

A lot of VW tuners these days think that the 911 fan is the answer to their prayers when it comes to cooling. Well, I believe that it isn't. Let me explain the shortcomings of this conversion and I'll let you decide.

First off, I mentioned earlier that the 911 fan is an axial design and that the cooling air is directed directly to the back of the shroud. If you've ever seen the inside of a real 911 shroud from a real 911, you know that Porsche placed objects in the shroud that directed the air to the hot spots. All of the 911 fan conversions I've ever seen do not have any form of directing the cooling air to the cylinders and cylinder heads. The cooling air hits the back of the shroud and bounces around. It may eventually end up at the needed hot spots, but only by luck.

Second point is that this design was created for a six cylinder, not a four cylinder. There isn't as much opening between the fins for the air to escape. For example, suppose you have a funnel and a garden hose. Barely turn on the garden hose and run the water through the funnel. The water quickly exits the bottom. Increase the water volume and it will eventually cover the opening, then the water exiting will equal the water being added. If you continue to increase the water volume, you will see the water level climb up the funnel, eventually overflowing. Get a bigger funnel (with a bigger hole in the bottom) and the water level will drop down.

So we see that adding more water doesn't necessarily make it come out quicker. This same principle applies to the 911 fan. It was designed to be used with a big funnel, so using it on a small funnel will mean it won't be the most effective.

The last word...

As you can tell, I'm not a fan of the 911 conversions right now. If someone designs a 911 fan conversion shrouds that addresses my points and has been dyno- and street- proven to cool a Type 4 engine evenly, I'll be first to save my pennies to try it out. Until then, leave the 911 fan shroud conversions to the showcars: you'll get better performance from the DTM.

If you must run a 911 shroud on the street, I recommend that you invest in the kit from Sharpbuilt of Australia. This is the only 911 conversion that includes ducting to the stock oil cooler.

Thanks go to Rolf Christensen and Jake Raby for their assistance on this article.