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Mitigating the power-kill when muffler is used on SPI engines? Empty Mitigating the power-kill when muffler is used on SPI engines?

Post  balogh Tue Oct 26, 2021 8:10 am

SPI engines are known to lose output if equipped with a muffler, given that the crank-case depression will suck in spent flue gases from the muffler when the piston skirt clears the bottom of the exhaust opening.

I have been wondering whether with a sort of depression, created in the muffler with its silicon tube extension, the flow of gases near the piston skirt during SPI could be reversed, and instead of sucking spent gas into the crankcase, the depression in the muffler would boost depression in the crankcase and help suck in more fresh air/fuel mixture from the carb. (I guess tuned pipes act similarly, albeit more efficiently, with the periodic depression created by harmonic waves in the tuned pipe coinciding with the opening of the exhaust ports, thus boosting exhaust and sweeping of the crankcase with more air/fuel mixture...)

The  end of my silicon  muffler-extension on the Cosmic Wind points in a slanted angle towards the rear of the plane, and the slight depression at the silicon tube end, presumably created by a venturi effect of accelerating air flows around the opening, when the plane is flying at high speeds, may help create the said depression inside the muffler by the time the piston is at the TDC, i.e. well after the overpressure present in the muffler while the engine discharging  the exhaust gases, has disappeared.....

Mitigating the power-kill when muffler is used on SPI engines? Exhaus10




Just a thought trying to explain why the TeeDee09 spins the unloaded 7x3 prop near 20k in the air even if the muffler is used? Unloaded, true...but the negative SPI effect is also there in principle...maybe the slight depression at the muffler extension tip acts like I have assumed here? Huh...
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Mitigating the power-kill when muffler is used on SPI engines? Empty Re: Mitigating the power-kill when muffler is used on SPI engines?

Post  Yabby Tue Oct 26, 2021 4:29 pm

I have used the standard Cox round mufflers on my 049 TD engines and Surestart engines and have not found the huge performance decrease often mentioned. I do tap pressure from the muffler, and I do run 25,25,50 - nitro,castor,methanol with a 5x3 prop. I have also found using the timing shims I got from Matt and will soon be available from both Matt and Bernie to work very well indeed. Yes, tuned pipes work fantastically with SPI as used on Piston Ported Yamaha TZ 350 Grand Prix engines which made amazingly violent horsepower due to the tuning all being based so heavily on the piston as the input and output valve.

My experience, maybe pure luck, I dont know, but I dont find the huge power drop people describe, but then Im not trying to race my engines, I just fly them for pure fun. I would like to be able to get some nicer exhaust pipes for all of my 049 engines but what I have works quite well for me.

You may be onto something there Thumbs Up The base logic makes sense. cheers Ive been trying to make a decent pipe but get too busy flying. cheers

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Post  balogh Tue Oct 26, 2021 4:35 pm

Thanks. Yabby.. The biggest rpm loss in my experience is with TeeDee 049..051 engines that lose 1..2k rpm if a muffler is added. I try to offset that with turbo heads.
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