Pressure fittings

[Bootlegger]

  Hello Fellow's
  I am looking for some suggestions on the best size pressure fittings on a muffler.
  I don't know if the 4-40 or the 6-32 fitting will be best.
  I'll be using them on 40 powered ships, and using muffler pressure is new to me.
     Thanks for the advice,
 Oh where do you get your fittings anyway ??

[Steve Helmick]

Why? You can cure the upwind/rich downwind/lean thing in other ways. I've seen too many cases where a leak in a muffler pressure system caused way more grief than that. Blown exhaust gasket, loose muffler tailcone, leaky muffler pressure line, loose muffler pressure tap...so far. Not many around here use it, and they're finding out that they don't need it. More power doesn't happen!

Technically, all your pressure line needs to do is refill the tank with (polluted) air, replacing the fuel volume as the engine burns it. While I'm not sure a 4-40 pressure tap would be enough (they're made for crankcase pressure), there's no reason a 6-32 wouldn't be plenty. http://flyinglines.org/McClave.LA40.html      Steve

[Wynn Robins]

I only run muffler pressure and have never experienced any of the above issues - all engine runs are the same - all good to the last drop of fuel.

cant help on the sizing tho - I just run OS LA46s and they come with the pressure nipple there.

[Jim Thomerson]

I run muffler pressure when I can.  I figure if I am going to lug the weight of a muffler around, it can do some work for me.  When I don't run muffler pressure I use one-way valves on the uniflow and that works fine also. 

[taildragger-j3]

I've had to replace a few pressure fittings, and I use 4-40. It seems to be plenty of pressure, and I have a 4-40 drill & tap. I've gotten a few taps at a LHS in Houston. I'm pretty sure that Tower Hobbies has them, too.