Originally Posted by
Glowgeek
Weird thing. I tried a Pitts muffler on my converted 180, muffler pressure was barely measureable and didn't rise with rpm hardly at all. I blocked one of the two exhaust outlets, pressure rose up to approx .25 psi but same problem, no pressure rise above 1/2 throttle. I guess I'll have to restrict the one outlet until pressure comes up further. Hoping for at least .50 psi but I suspect I'll not see much of a pressure rise with rpm. There are two nipples on the muffler, one for tank pressure and I suppose the other for smoke, even though there is no nozzle that hole is located close to the exhaust port. The Pitts may not be a good candidate for this gas conversion program.
I typically see around 50 mbar (something like 0,6~0,7 psi) on basically all of my mufflers (most stock, a few tuned pipes) so I would indeed assume that Pitts muffler is not the most suitable.
What I find particularly strange is that you report a pressure rise from idle to half throttle, but no more rise after that. Virtually all of my engines have a very small pressure rise in the lower part and a much steeper increase in the 2nd half of the throttle range.
Is there also a lack of RPM rise? That could indicate either overpropping (suppressed RPM) or a too large carb throat diameter