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Ringmaster Ringmaster is offline
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Join Date: Apr 2004
Posts: 298
[QUOTE=Speedy Squirrel;5130566]
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Originally Posted by Ringmaster View Post
Great thread! Here is a twin turbo set up for your consideration and feedback ]

This is beautifully made, and can take advantage of pulse conversion due to being a twin turbo. The problem is that the individual pipes dump into a pretty big collector, which lets the pulses expand before they get to the turbo.

Also, the individual pipes can and should be MUCH shorter and straighter.

Tuning individual pipes to a certain length is done for scavenging effect. For scavenging to work you need to have a fairly large amount of overlap in the valve events. The idea is that a low pressure suction pulse will travel back up the tube and arrive at the exhaust valve just as the piston has gone past TDC and is started on the intake stroke. The low pressure pulse will still let exhaust gas flow out of the engine, even though the piston is now going down. Obviously, such a situation can occur only at a very narrow RPM, and needs a cam with a lot of overlap to work really well. If you are like most, you have the classic turbo cam, which has exceptionally low overlap, so forgetabout scavenging.

The other reason that longer equal length pipes are used is for momentum tuning. In this concept the mass of the exhaust gas gets moving due to the high pressure in the cylinder on the exhaust stroke, and the low atmosphere pressure. Once the exhaust valve closes, the exhaust gas will still keep flowing down the pipe because of its momentum, as long as it doesn't run into any interfering pulses. It will empty out the tube and make the pressure nice and low for the next exhaust event. For us the problem is obvious, we have a turbine in the way, so instead of low atmospheric pressure, it encounters something quite a bit higher, which greatly reduces the momentum effect.

Make the pipes shorter. This will improve your time to make full boost. Since a setup like this can push a lot of air through the engine, the diameter of the pipes is OK, but no bigger. If your goal is in the 450 HP or less, they could be smaller. The diameter balance for you is: 1) too big and the exhaust pressure expands in the tube instead of the turbine at low speeds - no good. 2) too small and your monster motor will have too much backpressure.

Great post and thanks for the feedback. To quickly address a couple of your comments...

-The cams are large Schrick race units with lots of overlap
-The engine package makes over 700hp
-It was built as a semi-race engine designed to operate effectively between 4000-7500rpm

More later when I have some time to type
Old 01-17-2010, 08:06 AM
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