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Exhaust diameter for engine size
smart51 - 1/6/09 at 10:27 AM

Is there an optimum exhaust diameter for a given engine size or does it not matter once it is big enough? Is there a handy way of working it out?

tegwin - 1/6/09 at 10:31 AM

Yes there is, but I left my book at home!

I have been working out the exhaust for my straight 6 engine and the length and diamter of the primaries and secondary pipes (assuming a 6-3-1 arrangment) are vitally important to ensure that the pulses reach the opposing cylinder in the pair at exactly the right moment!

I would imagine a 4Cyl would be no different!

smart51 - 1/6/09 at 10:33 AM

How about for a 3 into 1?

tegwin - 1/6/09 at 10:38 AM

quote:

How about for a 3 into 1?


Not sure what you mean?

I figure my primaries need to be 38-42mm, secondaries 55-65mm and then onwards at something around 65-75mm through a non restrictive can...

smart51 - 1/6/09 at 10:45 AM

quote:
Originally posted by tegwin
quote:

How about for a 3 into 1?


Not sure what you mean?


A 3 cylinder engine.

twybrow - 1/6/09 at 10:48 AM

Smart are you playing with Trumpet lumps too, or is it closer to your username?

tegwin - 1/6/09 at 10:52 AM

What is the firing mode for a three cyl? I cant imagine there would be any oppertunity for exhaust gas scavenging using pulses....


This should help:

http://www.nightrider.com/biketech/calc_exhaustlength.htm

Dont forget that the length of your header MUST include the length inside the head as well!!! (distance from valve seat to end of pipe.... not exhuast manifold to end of pipe!)

twybrow - 1/6/09 at 10:53 AM

Mine is 1-2-3

smart51 - 1/6/09 at 11:01 AM

quote:
Originally posted by twybrow
Smart are you playing with Trumpet lumps too, or is it closer to your username?


No, A friend of mine has a suzuki 657cc turbo and is of the bigger is obviously better school. I'm more inclined to think that there is a "right" size for his new exhaust.

garage19 - 1/6/09 at 11:32 AM

He is right. On a turbo engine, after the turbo, bigger is better. Remember it is all about pressure difference across that turbine so you want the least pressure after it. There is no pulse tuning after the turbo as the turbine has removed any chance of that. The ideal exhaust for a turbo is a very short tappered out secction just to reduce turbulance as it leaves the turbine housing.

[Edited on 1/6/09 by garage19]

will121 - 1/6/09 at 11:45 AM

think there is a optimum main exhaust pipe bore size, ive recently fitted on a 2litre zetec tin top with throttle bodies a 2 1/2" bore single silencer system, while its very free flowing and producing 150bhp at 6200rmp think the torque has dropped off, ive got it booked back in for another rolling road run on Saturday and have refitted the 2" bore twin box system to see if the torque will come back or at what expence BHP/Revs, will update and let you know, mind you with such a short system length on a 7, might not be as bad

tegwin - 1/6/09 at 11:55 AM

If you had a large bore pipe from the turbo to the back of the car of an unknown length could you not stall the turbine if you get a bad pulse frequency at exactly that resonance length? I know its critical in gas turbine exhaust design!