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copbait73 copbait73 is offline
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Join Date: Nov 2003
Location: Chesterfield IN
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A single turbo gives you a distinct RPM range for useful power production. All things being equal, you can improve when the boost arrives however by doing so you will cut off power sooner and vice versa. This is a function of effective turbine efficiency. Shoving all the exhaust into one turbine, causes backpressure, that causes the narrowed power range. For years the industry considered 350 HP the limit for a single turbo of roughly 3.0 diameter rotor. Its not by chance that Porsche moved from single to twin when planning higher HP for the 993 Turbo.

Parallel twin turbos give you a wider power range because the individual turbines will spool quicker due to lower inertia and the wastegates will open later. Unlike the numerous cobbled Custom single turbos the compressor wheel will match the turbine on each turbo and the overall effective turbine efficiency will be much better, thus lower backpressure.

Typically parallel twins are installed when the physical size of a single turbo needed for ultra high HP destroys drivability. However more modest twins matched to handle the burden of the 930 four speed and 500HP should have improved bottom end response and extended usable RPM range (top end power) and do this at a marginally lower boost level. Match like this feels more progressive and places less strain on the engine.

Of course, while not a direct apples and apples comparison there is the 993TT and that is where Id begin. Nice used 993 and 996 units are out there, sometimes for less than $550 a pair.

A couple of weeks ago I took a chance on a used 993 turbo and when I got it it was perfectly acceptable, $39.00.
Old 12-15-2009, 07:03 PM
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