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kenikh kenikh is offline
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Join Date: Feb 2005
Location: North of Exit 17
Posts: 7,729
Twin scroll is best...period, where possible.

A fully split AND equal length exhaust system is the only (feasible) method for harvesting maximum exhaust pulse energy to improve efficiency and spool performance (there are theoretical methods for achieving this otherwise, which I won't bore you with). Just like wrapping your exhaust to insulate it and keep more heat energy (and minimize exhaust gas contraction and thus keep gas volume at its highest) in the exhaust tubes for best performance, maintaining the sine wave integrity of the exhaust pulse with a minimum of wave clipping (which destructively misapplies this energy in directions you don't want it), gets more energy to the turbine, as opposed to wasting in places of no or (negative) benefit to our purposes.

I have read a lot of technical discussions and documentation on these points, which is where I draw the above high points from. The Subaru and Mitsubishi enthusiast sites are full of deep and technical to light and anecdotal information on twin scroll; I'll post links here as I dig them. For now, here's a brain dump of "the state of the union of twin scroll" today:

There are numerous permutations of turbos that can be described as "twin scroll", all of which have different levels of 'discreteness' in terms of how clean they keep the exhaust pulses in pursuit of extracting the "free" additional energy in the exhaust stream. As a result, by designing the exhaust system to harvest unmuddled exhaust pulse energy to drive the turbine more efficicently, you get quicker spool.

This requires an equal length exhaust system that mates the cylinders that fire at even crankshaft degree spacing from one another. The Porsche 911 and Japanese 4 cylinder pocket rockets are perfect for twin scroll turbos in this regard. On the flip side, you will never get a twin scroll system to work on a 5 cylinder Audi (without some truly insane engineering wizardry), nor will you be able to use two twin scroll turbos with any positive effect on a 930TT. A V8TT on the other hand, would be able to.

The next step beyond simply keeping the exhaust discrete until it hits the flange are turbo designs that divide the exhaust stream past the flange, then comingle exhaust gases just before the turbine - maybe a few centimeters before the turbine. The longer the gas signatures stay discrete, the less energy is wasted.

Taken even further, the latest twin scroll designs not only have a divided housing, but actually have completely segragated cavities which ensure that the exhaust streams stay competely separate until the gases are actually in contact with the turbine wheels. The latest Mitsubishi and IHI turbos literally keep the exhaust streams separate until just a few thousandths of an inch before the vanes of the turbine wheel, so in reality aren't ever in contact until after they have crashed into the turbine, exactly where you want all available energy to be expressed (see the Mitsubishi Evo VII/IX/X or Subaru WRX STi).

The final (theoretical) permutation would be any flavor of the above twin scroll housing with variable geometry, whether single vane, dual, linked vanes (one for each scroll path) or even (although I have never seen anything like this) dual vanes with independent pitch control.

Net-net, more energy expressed in the direstion you want to apply force is always a good thing. This is what a twin scroll turbo designer's ultimate goal is.


- 1965 911
- 1969 911S
- 1980 911SC Targa
- 1979 930
Old 06-03-2010, 03:58 PM
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