Transition from exhaust port to muffler

[bmwmaster]

I would need advice what brings best results and how the transition from my exhaust port to muffler should be designed.

Im building an bmw m20 engine, where I have 36mm exhaust valves, then commes the port which at its end has about 40mm.
Im using an race header with 45mm pimary pipes.

Now im asking me, should I match the end of port to the diameter of the pipes?

Or should I leave it like it is to have an edge? This edge could prevent reverse flowing gas to enter backwards to the engine? And would result in better performance?

What is your advice here?

[mag2555]

My flowbench numbers always tell me to have the inlet to the header or Exh Manifolds bigger then the Exh port outlet at the head flange and the running motor will confirm that to you with a better cleaner idle and also coming onto the Cam sonner.

Also don't forget that your minimum Exh port area is no greater then 92% of the valve size used and that amount of area is your port choke.

[David Redszus]

I would need advice what brings best results and how the transition from my exhaust port to muffler should be designed.

Im building an bmw m20 engine, where I have 36mm exhaust valves, then commes the port which at its end has about 40mm.
Im using an race header with 45mm pimary pipes.

Now im asking me, should I match the end of port to the diameter of the pipes?

Or should I leave it like it is to have an edge? This edge could prevent reverse flowing gas to enter backwards to the engine? And would result in better performance?

What is your advice here?

A small increasing step from port to pipe will have no measurable effect on reversion but will assist scavenging.

[BLSTIC]

Personally I wouldn't match it, unless your header is actually the size you need it to be. 45mm is really big for a pipe straight off the head (Exhausted's tables imply that 45mm off the head is good for 85hp/cyl if you have smooth contours and a stepped header, so around the 500hp mark for a 6 cylinder).

So if you're around the 300hp mark and running a header suitable for 500hp, port matching just reduces the length that your pipe is right for. You may pick up a cfm or two but lose out on a bit of scavenging and anti-reversion.

[digger]

Seems way too large for an m20. What power level is it going to be ?

[bmwmaster]

Sorry hast wrong Numbers in my mind, the ID of Primary pipe ist 38mm and they have an lenght of 520mm.

Putting this Numbers in pipemax Shows that im Not completly wrong. Hopefully.

Also measured CSA of exhaust Port and eben converted correctly IT IS 1.14 Squareinch.

I hope to reach 240 to 250 HP.

Capacity ist 2.8l
CR 12.3
292 degree Dbilas Cam.....
Aftermarket Ecu Emu black without the flap based Air intakte. But at beginning with Stock intakte.
When engine will have good characteristic, maybe rhd indtake with Open trumpets will BE mounted.

[digger]

Pipe max says you need primaries

1.375”od stepped to 1.500”od 30-32” long

Sounds like 1.375od would match the exhaust port size