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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
 
Join Date: Jun 2008
Location: Detroit
Posts: 897
It is technically true that high massflow through the turbine makes for the most efficient turbo match, but why do all high performance road cars have wastegates?

A typical turbine map is shown below. As the mass flow parameter (just think of it as exhaust gas flow) increases, the efficiency also increases (red line).

But look at the range of efficiency. The bad is around 60% and best is around 70%.



That is small potatos for a gasoline engine. What is more important is turbo response. Getting up to the maximum boost as quickly as possible is the name of the game; efficiency is secondary. For best response, the smallest turbine that can be fitted, along with a wastegate makes for the best overall package in a street car. A K27 turbine is 68mm in diameter. A K29 is 82mm in diameter. That it a very big chunk of iron to get spinning. A K29 is not going to give an increase in response. The 10% in efficiency is no way going to offset the rotational inertia of that much iron. It will make more power, once you get there, but it will be a loooong wait. Even the lightweight Formula 1 turbo cars of the 80's used wastegates

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The only technical reason to increase the turbine size, is when response is secondary to another goal.

Very high power ratings for a two valve engine like ours can only be sustained when backpressure is as low as possible. In this regard, twin turbochargers are usually adopted because they allow for tuned exhaust plumbing. Efficiency is key. Above 800 HP on a two valve 930 motor the engine goes into a "death spiral". You have to add more and more boost just due to the breathing (volumetric efficiency) of the engine, but the backpressure is going up and up also due to the high exhaust flow and small turbine area needed to get that kind of boost. The only way to favorably impact this is to improve the volumetric efficiency (cams, and ports), and improve the turbine efficiency (big turbine). It will be a dog around town, but once it gets rolling, look out.

Botton line: a K29 is not going to be a good choice for most 930's on the street. If you are headed to the Texas mile, that's another story, but then you (hopefully)wouldn't be looking for advice in this forum!
Old 03-21-2010, 08:32 AM
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