[Herr-Kuhn]

Why Twin Turbos and not a big front mounted single?

I'm often asked why I chose to go with twin turbos in the 928 instead of one big single turbo. There are many reasons for this...the following are some of those reasons:

1. Packaging...the twins allow for placement of the turbos right off the manifolds for max exhuast energy. Additionally the twins allow for the intake air paths to be as large as possible. With a front mounted single turbo, one would have to mount the air cleaner somewhere remotely and be stuck with say, a 2.75" inlet for all engine airflow. Placing one turbo in front of the timing belt cover was not a good idea because of the heat it would place on the timing belt and associated covers up there. If you run a crossover pipe from one cylinder bank to the other, the heat loss in that pipe can be sizeable...additionally any long crossover pipe will experience severe thermal expansion issues, so an expansion joint is always necessary.

2. Spool Time...Two turbos can cut down on spool time as compared to one larger turbo. This allows for excellent turbo response.

3. Heat input...two turbos essentially "split" the heat input as compared to one larger unit. The thermal exhaust energy is effectivley split, thus halving it as compared to what you have with one turbo. This is important if you plan on placing the turbos in the 928's engine bay. This gets back to "where would one place one large turbo if you plan on keeping it in the engine bay"? The rear mounted system is an entirely different animal altogether...I'm not making my comparisons here to a rear mounted system.

4. Air Filtration...Two medium turbos allow for two medium sized filters to be fitted into the engine bay. This keeps the filters out of harms way and allows for easy maintenance.

5. Intercooling...I wanted the simplicity of air to air intercooling for the 928...the twins allow me to route the plumbing to two side mounted heat exchangers which do not impede the radiator airflow.

6. Aesthetics...With the 928's factory attention to aesthetics I wanted something that enhanced the design philisophy Porsche had when they designed the car. I believe the twin turbo system to be one of the nicest ways to augment the design of this vehicle.

There are a few more reasons, but the above are the big ones.