RappyPilot

Hi

I have a Seagull Edge 540 that I want to put an OS55AX in. Bit it is a small plane and I have to cut quite a bit of the cowl away to make room for the protruding standard muffler.

I was thinking of fitting a pitts muffler onto my engine.

Will a pitts muffler affect performance of the engine in any way?

Marcel

alan0899

G'day Marcel,
I use a Bisson pitts muffler on an OS 61FX, & an OS 160FX, they don't adversely effect either engine, they both run great, BTW they are both mounted sideways, so the exhaust outlets point down.

RappyPilot

Yes, this one is also side mounted.

Looks quite nice with two small muffler exhaust tubes sticking out the bottom of the plane

I was just curious because I have read a few threads where people complain that you do not have enough back pressure etc etc etc. And that idle and transition is affected.

alan0899

G'day Marcel,
Some people mistake back pressure for muffler pressure, back pressure in a 2 stroke is a function of the exhaust, as in 2 stroke motorcycles, they have an expansion chamber muffler, that helps to suck exhaust gasses out of the combustion chamber, thus allowing a better fuel air mixture, for the next cycle.
Muffler pressure generates a relatively small positive pressure in the fuel tank, to help push fuel up to the carburettor, thus allowing for aerobatics & differences in attitude of the aircraft, but, the engine should still run without muffler pressure, because the suction caused by the piston going up sucks air & fuel into the engine, if this suction is not strong enough, the engine will not run anyway.
Remember 4 strokes run best without back pressure, is in a car, straight through exhaust pipes, without any back pressure work best, but of course are noisy.

So any problems with Pitts style mufflers are usually because of insuffient running in, &/or bad tuning.

RappyPilot

Maybe the guy moaning just did not tune it properly.

On two occasions that I changed mufflers from stock to performance pipes on my heli's I had quite a bit of adjustment to do on the high speed needles ...

Anyway, slightly off topic but still relevant, will a Bisson pitts muffler fit inside the cowl without having to cut away portions of the cowl for the muffler, apart from a cooling exit hole?

The plane is a Seagull Edge 540 (46) Anyone have this one with a Bisson pitts muffler?

Marcel

Sport_Pilot

Today's modern engines have oversized carbs that will not run in the air without fuel pressure or a fuel pump. The fuel pressure is usually from muffler pressure. Not enough pressure and the engine will lean out in aerobatic maneuver. Many pitts mufflers do not have enough pressure for some engines. Some engines will run well with muffler pressure. I know the 61 FX is a low performance 60 so it may not have an oversized carb with large duration timing that would require more muffler pressure to run. However I think some have had trouble with the 160 FX.

landeck

My Feedback: (1)

Marcel, I currently have an OS 55 AX with a Bison Pitts muffler and it runs fine. I also currently have OS engines size .46, .61, and .75 with Bison mufflers and they all also run fine. On none of them did I have to cut the cowling for anything more than the exhaust ports, for cooling, and needle valve. Remember you will need at least 3 times the exit area as entry area for proper cooling of a cowled engine. Also you should use a three line fuel system with the third line having a fuel dot for filling/emptying the fuel tank.

Bruce

RappyPilot

Well, I called around a few hobby shops and not one here in South Africa seems to sell Bisson pits mufflers, a lot of hobby sell "home made" ones or cheap immitations ... So I ordered a proper Bisson Pitts Muffler from Tower. Not cheap but worth it I think.

But it just look so much nicer not having to cut a massive hole for a standard muffler. With a pitts muffler you onlt have to cut a hole at the bottom for the exhaust tubes to come out, obviously that hole will be much larger to accomodate the needed size if the exit cooling hole that you mentioned. This is not my first cowled plane so I did the math in working out the size of the exit hole :-)

Don't like a 3 line fuel system, you never know when the tank is full (Unless you can see it filling up) and by the time the tank is full and fuel leaks out your muffler the engine is flooded .... I use a fueler valve in between the fuel tank and carb, that closes the line to the carb when you stick in the fuueling probe, works much better.

landeck

My Feedback: (1)

The way I set up a three line fuel system is to add a "T" fitting to the pressure line to the muffler and run a line from it to a second fuel dot. At the "T" connect the line to the muffler to the 90 degree bend and the tank and fuel dot to the straight part to get most of the overflow to flow out of the fuel dot. Then when I fuel the plane I connect an overflow tank to the line from the "T" so I can catch and see the overflow when the tank is full. In this way I do not waste fuel or flood the muffler.

Bruce

NM2K

My Feedback: (14)

It truly depends upon the particular muffler manufacturer/model and other things such as prop choice, glow plug, fuel and your tuning habits.

Like the other poster, I've had great luck with Bisson mufflers, although I doubt if they would call it luck. 8>)

Years ago, there were some Pitts mufflers available that would actually cause engine problems because of the location of the muffler pressure nipple being located in not the best positive pressure location. That was a long time ago and I doubt that you could find such a muffler today without going to a lot of effort to find one.


Ed Cregger