[NSRCA-discussion] OS 120AX header?

[Bob Richards]

Jim,
   
  I have not run my 120AX enough to conclude anything yet, but the little bit that I have run it left me thinking there was not enough muffler pressure either. I tried with/without the internal baffle, but I think the best combo would be to leave the internal baffle out and try restricting the tailpipe as you say.
   
  There are a couple of guys in our club that have the 120AX running the Jerry Smith mousse can mufflers and get MUCH better performance, and more reliable running as well since there is considerably more muffler pressure. They also fly several Outlaw delta planes with .32 size engines and do incredible manuevers, using the same type muffler setup. The only way they can pull the Gs they do and still run is because of the much higher backpressure.
   
  With a large volume muffler (the larger the better) the more pressure you can run in the muffler and the engine still run fine. In fact, you will burn less fuel as well since less excess fuel/air mixture blown out of the exhaust during the exhaust scavenging.
   
  I did a lot of muffler/prop experimenting (sport planes) many years ago. :-) I learned a LOT.
   
  Bob R.
   
  
J N Hiller <jnhiller at earthlink.net> wrote:
        

    v\:* {behavior:url(#default#VML);}  o\:* {behavior:url(#default#VML);}  w\:* {behavior:url(#default#VML);}  .shape {behavior:url(#default#VML);}                16 x 10 should be no problem. I have run one all season with stock muffler. I didn’t like how many RPM’s the 16 x 8 turned in horizontal flight.
  This may be a little off subject but may be of interest to other 120AX users.
  I fought a rich midrange (poor transition) from day one in order to keep from running too lean on a long vertical pull, which seamed to get worse in cool air. I have two motors that have different carburetors and focused on this as a problem even though both acted similar eventually concluding that there wasn’t enough muffler pressure. I have never gotten along well with pumps so I tried restricting the muffler outlet using a short exhaust tube (3/8” ID) with a brass tube insert. I tried a couple different sizes settling on 13/32 OD, which is pinched on the end from cutting with a tubing cutter (.350” ID). Flaring the other end keeps it in the exhaust tube without clamps. I also tried an OS-“F” plug which I thing may be better than the #8. It will be 9 months before I can run it in hot weather but I was able to run leaner without starving in the verticals and the transition is immediate.
  Jim Hiller
  Spokane
   


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