49cc HS exhaust length too short??

[Aaron Brown]

I have a poo poo pipe on with no expansion or muffler part. So the straight pipe is about 18" total and tucked nicely.

Too short? Do I absolutely need at least 30"? All power ranges seem okay but maybe longer would help? Thanks!

 

[Old Bob]

Too short for the limited rpm MB engines run.There are two design considerations. 1 is the speed of sound, it relates to the pressure pulse traveling up and down the pipe.2 is the speed of the exhaust gas.The pipe needs to be long enough to prevent atmospheric pressure from flooding pipe before exhaust valve shuts, and the length long enough for pulse to assist intake during cam overlap period.pipe diameter needs consideration also.
Any of David Vizards books or Phillip H Smith book will help with the formula.

 

[Deleted member 12676]

30" is more than enough.
anyway the effect of an immediate change in cross sectional area from header size to open air does very little in the way of making a healthy long suction pulse.
also on piston port intakes (no reed valve) the suction pulse sucks onto the closed system of the crankcase and so does little good. With piston port intakes the only thing of any real benefit is the return baffle wave (a pressure wave) from a normal expansion chamber which increases the dynamic compression and reduces loss of intake charge out the exhaust port..
Please click onto my signature link to read about expansion chambers.

 

[Old Bob]

This is a four stroke conversion, take your nonsense some where else.

 

[Deleted member 12676]

same applies to 4 strokes. the rpm range of effect of an abruptly ending pipe is too small.
4 stroke manufacturers don't design the pipe lengths to be "tuned" to the engine.
racing 4 strokes do try to take advantage of that effect by tuning the length so the reverse suction pulse arrives back at the exhaust valve just before it closes at the engines top rpm.
but a high rpm racing 4 stroke is a far cry from these s**t engines made for bicycles.

 

[Deleted member 12676]

from the net:
"it would not provide as much of a boost as a tuned pipe on a two stroke but done properly it will increase the rpm 200-400 on the top end."

 

[Old Bob]

Go educate yourself on the subject matter, headers do add power if made to the proper length.

 

[Deleted member 12676]

OK then educate me since you think you know more than me.
Tell me something I don't already know.

 

[Old Bob]

Go buy the books I mentioned above.

"from the net:
"it would not provide as much of a boost as a tuned pipe on a two stroke but done properly it will increase the rpm 200-400 on the top end."

I forgot they can't put it on the net if its not true.

 

[Deleted member 12676]

you know Bob, I don't mind people showing me I don't know everything but what I do mind is people being sarcastic and insulting without any good reason.

these forums exist for the knowledgable to help the not-so. No ego or adversarial attitude is necessary.