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# Twin scroll Brain teaser

### Tech Articles

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People say twin scroll turbo runners should be paired with 1 and 3 cylinders and 2 and 4 cylinders when used on a 4 cylinder 4 stroke engine with a firing order of 1 3 4 2 but the theory doesn't compare in practice because the firing order could be 2 1 3 4 or 4 2 1 3 or 3 4 2 1... it doesn't make any difference, they are all the same order but at different times in the 4 cycles of a 4 stroke engine. Question is: what is the correct explanation for a twin scroll turbo?.

So what they have asked for is that each scroll has two cylinders that fire in order (ie, 180 deg apart). Think about the pulses and how they would interact if they were 360 degrees apart?

Hey Tony, actually the correct runner design for a '1-3-4-2' 4 cylinder would be to collect the runners from #1 and #4 into one scroll and #2 and #3 in the other.

This gives consistent pulses into each scroll every 360 degrees of crank rotation. If it was #1 and #3 paired then you'd get two pulses 180 degrees apart and then nothing for 360 degrees. The same goes for the other scroll. I hope that makes some sense?

Cool thanks. Yes makes sense.

Twin scroll runner design where runners from #1 and #4 into one scroll and #2 and #3 in the other is efficient design for turbocharger. In this setup the opposite pulses from adjacent cylinders of firing order (Cylinder 1 and Cylinder 3 or Cylinder 2 and Cylinder 4) wont cancel each other. This provides more pressure available at turbine inlet for each engine cycle and hence efficient turbocharger. Pulse effect in case of twin scroll turbocharger provides more efficiency at low engine speeds (1000 RPM to 3000 RPM) as compared to single scroll turbocharger. This also helps in shortening the turbo lag (Time to torque).

Tony, convention and simplicity is why we start firing orders with #1. Ford used to use 1, 2, 4, 3 which caught a few apprentices out - I don't know if they still do.

The Yamaha cross-plane motorcycle engine uses, IIRC, 1, 2, 3, 4 - but it's a really weird exception.

 Seems the Yamaha has a revised crankshaft design and new firing order 1, 3, 2, 4

It becomes interesting when twin scroll turbo engines have EGR. It is likely that EGR pick up is from one of the two scrolls.