How does Twinscroll Work?
Oct 15, 2008, 12:47 PM
#1
How does Twinscroll Work?
with so much misunderstanding/misinformation and detractors of Twinscroll on this forum, i had some things i wanted to post and clear the air. These are ALL facts, and NOT opinion. I/Full-Race did not write any of this.
there is a reason evo's came twinscroll dualwastegated from the factory
some good reading:
A twin scroll turbine housing uses dual side by side passages into the housing. When coupled with a pulse converter manifold that separates exhaust pulses as many crank degrees in the firing order as possible, a twin scroll or divided housing works to reduce lag, decrease exhaust manifold backpressure on the top end, reduce the potential for reversion, and increase fuel economy
The twin scroll is based off the same theory a tri-Y 4-> 2 ->1 header uses: keep spent exhaust gases out of an adjacent cylinder drawing in fresh air. At high rpm on a turbo car, exhaust backpressure is usually significantly higher than atmospheric pressure, and often higher than intake manifold pressure as well. A divider between each of the two volutes allows the cylinders to expel the exhaust gases without it interfering with the fresh air for combustion. Since there are two openings, each a smaller overall volume than a single scroll design, the exhaust velocity of each pulse can be maintained. This also spins the impeller more easily because lag is a function of the scroll area.
A single turbine housing opening isn't as efficient in the larger turbo applications since cylinders on the exhaust stroke of the 4 stroke cycle contaminate the cylinders that are on overlap with exhaust gas, thus a conventional turbine housing is not as effective in using exhaust pulse energy to help spin the turbine up to speed since it does not exploit the energy contained in the pulses as well.
The twin scroll is based off the same theory a tri-Y 4-> 2 ->1 header uses: keep spent exhaust gases out of an adjacent cylinder drawing in fresh air. At high rpm on a turbo car, exhaust backpressure is usually significantly higher than atmospheric pressure, and often higher than intake manifold pressure as well. A divider between each of the two volutes allows the cylinders to expel the exhaust gases without it interfering with the fresh air for combustion. Since there are two openings, each a smaller overall volume than a single scroll design, the exhaust velocity of each pulse can be maintained. This also spins the impeller more easily because lag is a function of the scroll area.
A single turbine housing opening isn't as efficient in the larger turbo applications since cylinders on the exhaust stroke of the 4 stroke cycle contaminate the cylinders that are on overlap with exhaust gas, thus a conventional turbine housing is not as effective in using exhaust pulse energy to help spin the turbine up to speed since it does not exploit the energy contained in the pulses as well.
some good reading:
Last edited by Geoff Raicer; Mar 17, 2009 at 02:51 PM.
Mar 10, 2009, 04:57 AM
#3
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On the 16G turbine housings there is a separate wastegate bypass port( or passage) for each side of the divided housing. The turbine inlet area is essentially divided into two chambers and a bypass port initiates individually in each chamber, by the time that the two passages exit the housing at the flapper valve, they are running side-by-side(siamesed), though still divorced.
Theoretically, the twinscroll turbine housing design has its advantages as noted in the starter thread, but it also has certain disadvantages, especially in the smaller A/R internally gated configurations(i.e. 9.0, & 9.8).
Theoretically, the twinscroll turbine housing design has its advantages as noted in the starter thread, but it also has certain disadvantages, especially in the smaller A/R internally gated configurations(i.e. 9.0, & 9.8).
Last edited by sparky; Mar 10, 2009 at 05:04 AM.
Mar 10, 2009, 10:52 AM
#4
A twin scroll turbine housing uses dual side by side passages into the housing. When coupled with a pulse converter manifold that separates exhaust pulses as many crank degrees in the firing order as possible, a twin scroll or divided housing works to reduce lag, decrease exhaust manifold backpressure on the top end, reduce the potential for reversion, and increase fuel economy
Mitsubishi however did build a pulse-converter style manifold for the evo.
Mar 10, 2009, 01:09 PM
#5
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Also, the fact that smaller AR housings exist only means more options for people. The ability to run a Green in the 9.0 or 9.8 housing (which has been proven by FP to result in minimal difference in turbine backpressure over the 10.5 housing because of the larger turbine wheel) would be considered an advantage to people looking to maximize spool/transient response
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Mar 10, 2009, 02:43 PM
#12
waiting on mitch... i got the setups completely built. just need the car to go to the dyno, get tuned and come back here
no problem if youre at the jersey shore this summer i can show you guys some of the setups, we are out at englishtown a lot in the summer time
no problem if youre at the jersey shore this summer i can show you guys some of the setups, we are out at englishtown a lot in the summer time
Last edited by Geoff Raicer; Mar 10, 2009 at 02:55 PM.
Mar 10, 2009, 03:00 PM
#13
But realize that regardless of the outcome, the dyno cannot reveal real world transient response. If all you do is drag race at the strip, this is not so important. If you are concerned about how quickly the car reacts to throttle changes on the street, it matters.
Mar 10, 2009, 03:11 PM
#15
I'm not sure why that should be so difficult to understand. It's the difference between moving the TP from 20%-75% and feeling the engine respond quickly, or feeling like one is waiting for something to happen. A dyno cannot resolve this. I feel anyone who can make their way out of a paper bag can understand it.