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"The concept of twin-scroll, while also something I like, is something which has bothered me for a long time for one simple reason: there's no scientific basis behind the idea that two small scrolls are more efficient than one large one.

However, there IS good scientific evidence that properly divided headers, similar to the Tri-Y, using a stopped-length technique described in "The Scientific Design of Exhaust and Intake Systems", DOES improve VE across a broad RPM range compared to typical 4-1 systems. In the case of the Subaru setup, there's a HUGE potential for improvement because of the akward cylinder-pairing; cylinder #3 breathes hot exhaust gasses and backpressure into cylinder #1's exhaust port during valve overlap. Similar happens with 4 and 2. A properly-divided exhaust system solves that problem through seperating the cylinder's exhaust pulses by 360 degrees.

What I think is happening is two-fold:

1) Person A put on twin-scroll header and turbo which slightly increases power and decreases lag over stock manifolds. Person A doesn't accurately compare improvement to similarly-tuned stopped-length Tri-Y headers and a single scroll which flow a similar amount. (many twin-scroll turbine housings flow less than their single-scroll counterparts and have lower efficiency ... it's even in the Garrett catalog) Person A concludes that Twin-Scroll is inherantly more efficient without looking further.

2) Person A then attaches Twin-Scroll turbo with improperly-paired cylinders and finds spool-up and power dramatically reduced. Person A concludes that this is further evidence for the efficiency of twin-scroll because the pairing of the cylinders affects power. Person A doesn't stop to think that improper pairing also affects power on Tri-Y headers, both turbocharged and naturally aspirated.

In Addition, turbines perform best with smooth, continuous, flow. The further you are from this, the less efficient they become. Ergo, it is only natural that they would perform best with evenly-spaced pulses and that an improperly-paired twin-scroll would perform poorer. However, a 4-1 single scroll is smoother than even the best 4-2 twin-scroll by virtue of having more pulses/revolution. When tuned properly, it may be more efficient.

My Hypothesis and Explanation:

A: 90% of turbo setups have runners that are too short to properly seperate the exhaust pulses to avoid blowdown during valve overlap. On 4-cylinder setups, you can avoid this by pairing them through the turbine: despite the short length, the interference can no longer happen because the adjacently-firing cylinders do not share exhaust paths. Ergo, there is an improvement in VE, especially in turbo setups with short paths, or which are poorly paired from the factory. (IE: 99% of production turbo cars ever)

B: The increase in VE allows the twin-scroll user to use a tighter, poorer-flowing, turbine housing which spools better without harming power. Result is more spool-up without less power ... which would also be true of someone using a properly-tuned 4-1, or 4-2-1 Tri-Y, single-scroll setup with a smaller single-scroll housing

C: The twin-scroll setup has less pre-turbo exhaust volume, if the pairing is done early, than a similar 4-1 single-scroll. That probably accounts for the reports of much quicker transient response. However, an early-paired Tri-Y single-scroll should have almost the exact same pre-turbo exhaust volume AND a smoother exhaust flow through the turbine. However there is, again, a caveat: the final merging of the paired cylinders into one pipe creates a scavenging wave which travels back down the exhaust pipes and the creation of that wave takes away energy from the initial exhaust pulses. Therefore, there may be less energy available to the turbine housing because that energy was used scavenging exhaust gasses instead via the scavenging wave. (which would, presumably, not form in a twin-scroll because the expanding section is AFTER a region of "choked flow" in the compressible-aerodynamics regime, which should not allow a scavenging wave to return back down to the exhaust ports)

That exchange of energy creates a dichotomy: is the energy best spent scavenging the cylinders or driving the compressor wheel via the turbine wheel? A better scavenging wave could result in better overall power. But a stronger pulse to the turbine could allow for a larger turbine housing to be used without sacrificing spool, which could allow for a better Exhaust / Intake" pressure ratio, which could do a better job improving scavenging than the scavenging wave itself. Which is better? It seems no one really knows.

Conclusion: based on the evidence in hand, I would conclude that the benefit of twin-scroll has absolutely nothing to do with the turbine housing having two scrolls, and is merely the result of the effects of the header tuning.

What Data is Sorely Lacking: a test with a properly pulse-tuned Tri-Y header against a nearly-identical twin-scroll setup which has a similarly-flowing turbine housing and uses the SAME turbine-wheel/shaft/compressor.

Normally, such an experiment is unpractical because of the extremely-long runner length necessitated by the high EGT (and thus high speed of sound) in a turbocharged car's exhaust system. But the stock Subaru setup offers sufficient room for such length and would be the PERFECT platform to carry out such an experiment.

The only problem is finding a a twin-scroll housing which flows the same as aanother single-scroll housing which bolts to the same CHRA.

Guess we'll hafta wait for the verdict for now ..."



Taken from rexnet forums. Thought it was quite intresting as im in the market for headers soon enough. Hopfully it isnt a repost!

Eric

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Honda Hannspree Edition CBR 600 RR - SOLD
Yamaha FZ1N - Purchased

read this before somewhere...NASIOC???

interesting.

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K.E.R.S!!!1

Yeah i believe the rexnet boys got it from NASIOC.

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Honda Hannspree Edition CBR 600 RR - SOLD
Yamaha FZ1N - Purchased

All i need to know is that they use twinscroll in the wrc, the spec c and evo's, that proves that it makes enough of a difference to me

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