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I know I know, no back pressure is the best. BUT what ball park numbers is the rotary starting to hurt at high RPM (8500)? Anything I read is your typical piston engine. (4000rpm = 4 to 8 psi)
I'm adding a restrictive pipe at the end of the exhaust to combat the noise. The exhaust end is capped off, and a bunch of small holes drilled along the pipe. I'll keep on drilling more holes to reduce back pressure, which will increase noise though. So looking for a happy medium.
Maybe I'm oversimplifying this, but is there a reason you can't put a regular automotive muffler or resonator in your aircraft's exhaust? I have no idea how it would fit with the aesthetics but, it would take out a lot of guesswork?
The size & weight mainly. Plus we are usually 80% power for 95% of the time. We don't have much length runs of piping to help dissipate heat. Most automotive grade products fail in the exhaust area for us.
I'm getting somewhere with getting the exhaust noise down, the prop is actually louder now
You just need to add more of that end piece with enough holes of appropriate size to disperse noise with minimal back pressure. You’ll likely just have to experiment with it. There are likely some photos on here still of a cut open OE muffler. That’s essentially what it has inside it for most of the silencing; a perforated plate.
wrt back pressure, at elevation atmospheric pressure is decreasing. So reducing exhaust back pressure has more importance than down here with us ground pounders.
Yeah I figured I'd have to experiment between back pressure vs noise, and find the good balance. Got a 0-15psi gauge, so I hope it's in that range to start with, and I'll drill more holes as I go. Will update my main thread with results, just wanted to quickly throw this out there to see what numbers people get with oem setups.
Don't think I'll get to it in time, but I'm thinking of adding an electric cutout. During takeoff, open it up, then close it up during cruise. I had so much fun in my 20's with my turbo GrandPrix and the cops, with the cutout. Loud as hell, then quite as a mouse. They never could tell it was coming from me.
These pictures are from Coax muffler. Note the inner pipe is capped off at the end, so the exhaust pulses hit the end and bounce forward to get out the little holes.
Another guy capped the end completely, and made the inlet, the exit. He said with only 16" length, he got really decent results.
I'll google around to see if I can see a stock muffler opened up. Usually they are all the same, bunch of baffles and holes. It's more of trying to find the total area of holes.
Yeah, that last option makes sense, but like you said; extending the main pipe through the end of the collector cap with a cutout valve on the end of it makes sense for takeoff when you need to wail on it.
01-22-2020, 06:43 PM
