MERGE COLLECTORS,WHEN TO USE,HOW BIG A CHOKE AND WHY

[englertracing]

With a proper firing order like a 4 cylinder you can go real small like just bigger than a primary tube and then you can attach something like a megaphone and it will run good. You see abrupt large changes in area reflect strong scavenging effect over a narrow rpm range....

Small gradual changes in area can do it over a wider rpm range so you'd use a merge collector to preserve the area increace and spread it along the megaphone.

You only have 6.25x primary tube area to work with.

What do you mean by 6.25x?

Oh according to sources like Gordon Jennings regarding 2 stroke exhausts, and Gordon blair on 4 strokes on any diffuser or megaphone going any more than 6.25 * the primary area does not provide any useful reflections.

[ptuomov]

You only have 6.25x primary tube area to work with.

What do you mean by 6.25x?

Oh according to sources like Gordon Jennings regarding 2 stroke exhausts, and Gordon blair on 4 strokes on any diffuser or megaphone going any more than 6.25 * the primary area does not provide any useful reflections.

In a complex manifold, how would you compute that expansion ratio? Say in a 4-2-1, do you go downstream a single path and add up the area of side branches at collectors and then add the area at the final collector outlet? Or something else?

To be concrete, suppose there's a 4-2-1 manifold with 30mm (ID) primary stepping up to 32mm then going into the first 2-1 collector which has a 35mm outlet which steps up to 38mm before the final 2-1 collector that has 35mm choke and then a megaphone expanding to 55mm. How would you compute the area that is compared to the pi*(30mm/2)^2 primary area?