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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Speedy Squirrel & Dave,

You seemd to think the source I referanced above was junk.

In my effort to learn more about the new BMW motor that hits full TQ by 1200rpm (and near full boost). I found this:

Quote:

So what makes this twin-scroll turbo so special?

After searching for a long, in depth article on the subject of twin-scroll turbos, the best one was found over at Modified magazine written by David Pratte titled "Twin Scroll Turbo System Design - Divide And Conquer?"
See:TwinPower: Modified Shows How a Twin-Scroll System Works - BMW Monitor

He seems to think a bit more favorably of the quote and aritical. He went on and said the following that helped me further.

Quote:
The key to having an efficient motor, turbocharged or naturally aspirated, the exhaust gases need to leave the combustion chamber completely and smoothly. In a perfect system, the exhaust gas leaving the exhaust manifold will leave a low pressure pulse. The exhaust gases are being pushed out of the combustion chamber is in a state of high pressure, and is attracted to this low pressure area. This is known as exhaust scavenging.

In a single-scroll turbo system, especially one with an unequal length exhaust manifold, all cylinders exhaust gases interact with the flow of each other. Just as the exhaust gases have left one cylinder, it is just leaving another. The low pressure pulse isnít there.

This can cause the opposite of scavenging called reversion, where the exhaust gases go back into the combustion chamber which creates higher temperatures, less power, and more NOx emissions. To remedy this manufacturers have the exhaust valve open as long as possible before reversion occurs.

The a twin-turbo setup, especially on an inline six like the N54, has a better scavenging effect because the six cylinder exhaust system is split in two, with three cylinders for every turbo. This split creates the opportunity for low pressure pulse to occur in the exhaust manifold. But twin-turbo systems are complex and have a higher weight than a single-turbo system.

Last edited by 911st; 06-06-2010 at 06:45 PM..
Old 06-06-2010, 06:37 PM
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