Let’s discuss the mechanics of turbocharging. There are two ways to get "charged" air into the combustion chamber. Turbocharging and supercharging. As compared to the normally aspirated engine where the pistons are required to suck in the fuel mixture through the intake stroke, the "charged" motor has this mixture forced into the cylinder by the turbocharger or the supercharger.

Why the different size turbochargers? Porsche uses a variety of turbo chargers including the K24, K26, K27, K29, K31, and once there was a K36 (934 Turbo GT.) These numbers may look familiar to you. You've probably seen them advertised as "bolt-on" improvements. The numbers refer to the actual sizes of the fan wheels. A description of the different combinations and there effect is as follows:

The smaller the fan wheel, the more instantaneously the necessary RPM can be obtained to make the maximum boost. In other words, an ideal situation would be a small hot wheel and a large cold wheel. Exhaust gases propel the small hot-wheel. Exhaust gases are a force greatly dependent on the engines RPM, hence the nature of the compromise. At a lower RPM, the smaller hot wheel cannot drive the larger cold wheel to create boost until a certain engine RPM is reached. This creates turbo-lag. Turbo lag is the interval between the initiation of acceleration and the employment of power. In extreme cases you would have no power below 4,000 rpm and the next instant when the boost comes on full. Since the objective is to have usable horsepower, not only at peak rpm, but also at midrange and lower rpm, compromises in the size of both fan wheels are necessary. In a competition motor, the issue of low-end boost is of lesser importance because racing engines rely on a very limited high RPM range in order to create maximum horsepower.

One might be led to believe that bigger is indeed better but don’t be mislead...its not that simple. There are physical laws to be obeyed and compromises must be made. Depending on the design of the engine, one thing is certain: There is only so much boost an engine can absorb. Obviously, the quality of components such as pistons, cylinders, connecting rods, etc. is crucial. Other determining factors include compression ratio, internal cooling capability, and the ability to dissipate heat from the cylinder head area. Turbocharging creates horsepower, but it also creates tremendous pressure and heat. This pressure and heat are what can destroy your motor and under extreme boost conditions, and this can take place in seconds. Obviously, a competition motor has far superior cooling capabilities, both air and oil, than the average street turbos. A street turbo generally runs safely at approximately 0.7-0.8 bar boost (1 Bar = 14lbs.). A competition car typically will run at 1.4-1.6 bar boost for short periods. The difference in boost levels is what creates the horsepower.

With the 993 B-Turbo, the over boost regulation is done using a wastegate which is controlled by the DME control box. To increase the boost rate in a B-Turbo therefore, reprogramming of the DME unit is required. The 993 B-Turbo S-version, with 430 hp vs. 408hp, is a case in point.

An ideal solution would be to have two (twin) Turbos of a small to medium size. Porsche demonstrated this feature on the 1986 959 model by putting two (twin) turbos in sequence, minimizing turbo-lag. Unfortunately, to do it right was very costly, and it was decided not to incorporate this feature into production models.

Beginning with the 993 B-Turbo, two medium sized Turbos are used. Additionally, engine size was increased, compression was raised, and a more effective ignition and fuel injection was incorporated. This, combined with the ratios available in a 6-speed gearbox, virtually eliminated the turbo-lag.

There are, of course, many enhancements that can contribute to improvements in turbocharging performance. These include sport intercoolers with greater cooling capacity, fuel enrichment systems, and improvements to the flow rate of the turbocharger housings.

The bottom line is this: bigger in not necessarily better for your street turbo! Yes, there are turbochargers that are an improvement over stock, but bolting a K-31 on your otherwise stock-motor will do nothing for the driveability of your car despite the fact that it may add some horsepower to the top-end of your rpm range. To achieve this top-end you will compromise power in the low to mid-range power band and on the street this is where you need it. When was the last time you wished you have more kick at 140 mph?