pro 46 an slimline muffler
 

well after reading all about the pro 46 i bought her an installed her on a np ultimate 40 i didn't want a large muffler hanging out the side an after reading a reveiw in fly rc on a world ultimate i bought a slimline muffler problem was the tube were so long that i had to cut them down then when starting the engine she had no back pressure an i never could get her to richen up an she got really hot i then hoping to get some back pressure installed some wheel collars inside the tubes she now richens up an seems to idle well but don't know how much power she has lost
is there any one who makes a nice inverted muffler that will come out the bottom of my cowl an still give me lots of power, what kinda r's should i get on a 11 5 apc prop

RE: pro 46 an slimline muffler
 

G'day Mate,
It needs running in, before it will richen up, as it runs the piston & liner mate together & create increased crankcase suction, this is what sucks fuel into the motor, your motor should be able to run WITHOUT muffler pressure, if it won't, then it needs more running.
Back pressure, is not muffler pressure, it is a function of the muffler design, as in 2 stroke motor cycles, with expansion chamber exhausts. The expansion chamber, helps to suck more exhaust gas from the motor, thus improving its running qualities. But they don't have a tube from the muffler, to pressurise the fuel tank.
2 stoke motors need some exhaust backpressure, to run efficiently, 4 stroke motors don't need it, that is why, you have such small, straight through mufflers on 4 strokes. In fact, when I bought my FS 120 in the 80's I received a small stainless exhaust tube & a muffler, as well, with a pressure tap so I could use muffler pressure if I needed it.
I'm sure I'll get lots of disscussion about this, but whatever you read, muffler pressure only helps to keep fuel up to the motor, during aerobatics & so on, it is NOT required to run the motor, during straight & level flight.