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ORIGINAL: The_Pipefather

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ORIGINAL: britbrat

Uh --- Pipefather -- VE means Volumetric Efficiency. Don't you just hate when that happens -- you know -- suddenly the other guy's argument actually means something?

No it still doesnt mean anything. If VE is indeed volumetric efficiency, you dont get volumetric efficiency increases with an open exhaust as opposed to an untuned silencer, which is exactly what I said at the end of my previous post.

Kweasel, I have noticed that stock aircraft glow engines (atleast the couple I have had the chance to measure) have somewhat mild timing, on the exhaust side, compared with bigger engines. This may have something to do with the fact that actual port timing is less important than port time-area. Do notice that most glow engines have bridged exhaust ports which can be made very wide relative to the bore. So a milder timing can be used to make the engine more docile as far as idle and part-throttle efficiency is concerned, while still enabling a high gas throughput at higher rpms.

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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).

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ORIGINAL: The_Pipefather

Going back and actually reading ALL of his words, I found that this is what he said:

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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.

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ORIGINAL: britbrat

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.

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ORIGINAL: The_Pipefather

Is the muffler volume on glow engines restricted to provide reasonable fuel tank pressure? And folks who remove the baffle realize an immediate gain in engine performance?

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ORIGINAL: The_Pipefather

If that is the case then I bet using a stub exhaust will definitely be an improvement over both open exhaust & silencer?

That s how we tested our engines. A stub of tubing the same dia as the original header and about 6 to 8" long (on a 125cc engine) would remove the undesired reversion of exhaust gas during the upstroke (& the resultant messing up of jetting/carburetion), and also improve exhaust velocity over open condition. This was the baseline condition for checking changes in intake duration, intake length and carb dimensions.

Anyone willing to try it out?