ORIGINAL: The_Pipefather
Going back and actually reading ALL of his words, I found that this is what he said:
A good tuned exhaust will typically increase VE and torque by 15-20%, over the open-exhaust numbers and by about 25-30% over a regular silencer (RPM by 10-15% higher).
I may be wrong, as my knowledge of English is rather elementary, but he is saying that tuned exhausts give a bigger improvement over the regular silencer (25-30%) than with the open exhaust (15-20%). That is, he is saying that open exhaust has better VE and torque compared to a regular silencer.
Have I used all the words & do my posts make sense? Never mind if they dont. Cause this discussion is serving no purpose other than to marshall more and more opinion against me, the poor newbie who is supposed to ask questions like "D-Uh, I got an OS and I dont know the glow plug from the backplate", as opposed to talking about gas flow and Doppler anemometry.
You are not wrong because of your English, but because you are partially technically incorrect.
An open exhaust can certainly have a higher exhaust gas velocity than an untuned muffler & often does. There may be fortuitous times in an engine's operating regime when an "untuned" muffler exhibits tuned behaviour, but on average this is not the case.
As you very well know, 2-stroke engines are extraordinarily sensitive to exhaust backpressure. A truly untuned muffler results in excessive backpressure, lowering exhaust header gas velocity (relative to an open header), thus reducing scavenging & VE. Both a resonant volume ("tuned" ) muffler, and a tuned length & volume pipe can rectify this problem in a specific RPM band, by either inducing low-pressure pulses at the exhaust port, resulting in higher gas velocity & improved scavenging, or by inducing carefully timed high-pressure pulses at the port, resulting in a pseudo-supercharging effect & hence, higher VE.
An untuned muffler can do neither. In an untuned case, it is more beneficial to have minimum backpressure, as would result from an open header. Dar is quite correct in what he said. He is also inadvertantly in error where fortutitous circumstances conspire to cause an untuned system to accidentally result in either a low, or high-pressure pulse, with the correct timing & magnitude. The chances of that happenning with any usefull regularity are quite slim.