What are the dimensions of the motor? ... w/ ancillaries?
What is the weight of the motor?
What are the motor mount requirements for the audi, and can they be met?
What is the radiator capacity/requirement ? (will need to be increased due to mid-engine arrangement).
Can the audi use the non-return fuel system of the Boxster?
Also, can the audi ecu interpret signals from the Porsche egas, speed sensors, fuel gauge, temp gauge?
What kind of exhaust arrangement will be needed?
How much change will there be to the center of gravity and to the weight distribution - Audister vs Boxster?
Is there adequate air intake to supply the turbos?
These are just a few of the questions you need answers to.
But, if these can be acceptably modified, the S4s motor is a pretty sweet powerplant.
