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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
Join Date: Jun 2008
Location: Detroit
Posts: 896
The N55 engine produces less power than the twin sequential turbo N54 engine it replaces. It is intended to give European customers, the majority of which pefer diesels, the torque and efficiency of a diesel at a gasoline price. I think many Americans would prefer the N54 engine.

The exhaust manifold IS NOT a log manifold, it just looks like it in the cutaway. It does in fact have some very short runners of unequal length. Equal length is not critical to twin scroll operation on any engine. There is a range of variance allowable, and it depends on the mass flow, cross-section, length, and manifold pressure.

The twin scroll has little effect on residual gas after the wastegate is open. Once the wastegate is open the benefit is mostly lost. Modern engines have very low residuals in any event, due to the superior combustion chamber layout, and pistons with extremely high and thin top rings. It is just not an issue any more in a modern engine.

For our engine, the twin turbo approach is preferable. The shorter length exhaust manifold provides a benefit right off the mark, and it has all the benefits of the twin scroll in terms of pulse utilization. I don't know if it will make more power (there certainly are more twin sized turbo's to choose from than twin scroll ones, as a practical matter), but it will be more responsive.
Old 06-07-2010, 04:07 PM
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