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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Back to better understanding Twin Scroll.

Twin Scroll is said to be most effective on a 4 cylinder motor, there is some benefit on a 6 cylinder, and on an 8 cylinder it is not worth the effort (even on an even fire).

As an example, if we run a cam that has say 230 deg duration like a C2 cam:

Two cylinders of a 4 cylinder motor will have an exhaust valves open for 460 of 720 deg of rotation which leaves a fair amount of time for the pressure to drop between exhaust cycles and exit the header.

This would give us a lower average header pressure than if all cylinders were merged.

On a 6 we would have 3 cylinders per side with an exhaust valve open 690 of 720 deg. The time for each exhaust event to exit the header before the next is greatly reduced and the benefit lessened.

By the time we get to a V8 and have to put 4 cylinders together. We end up with exhaust valves open 920 deg of 720 deg of crank rotation. Now we an overlap of exhaust events with all exhaust cycles and much of its advantage has been lost.

Per above, all exhaust valves closed as follows:

4 cylinder, 36% of the time.

6 cylinder, 4% of the time.

8 cylinder, 0%. In fact we now have two exhaust valves open 28% of the time per side.

Cool stuff!!!

Now I better understand why a 4 cylinder sees more benefit with a split system.

Last edited by 911st; 06-05-2010 at 05:34 PM..
Old 06-05-2010, 03:54 PM
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