I wanted to run my OS91FS with just an extension (no muffler), but was told by someone it will burn the exhaust valves. Any comments whether this is true or not. Thanks in advance.

I've never owned an OS of any kind but I can't see that happening. Saito Twins just have pipes on them and my ancient Saito 40 has no muffler.

I have known to many rc pilots who have flown their fourstrokes without mufflers because of accidents with the ground. The only thing I can think of is if you do get your engine hot enough and it backfires you may draw in enough cool air to chill the valve causing it to warp. you will have to compensate for not running a pressured tank, by riching your high speed needle valve screw. Have fun flying

Glow engines run to cool compared to gas engines to burn valves. Rolls Royce Merlin ran with short ex. pipes. Even on gassers, most common reason is lean mixture.

From what I've read, It's fine to just run a pipe or the pipe portion of the standard exhaust system.

But to not run anything in the head and just have the exhaust blowing out across the threads in the bare head, causes problems with burned and/or warped valves.

Enjoy,

Jim

Hello!
Somebody most be joking.......

Regards!
Jan K
Sweden

Bill:

Valve burn in a glow 4c engine? No. Warp? yes.

What happens, as Jim said, is the cold air hitting the hot valve. This can happen either at the time of a normal shut down with the valve open, or a backfire causing the cool air to be drawn in.

With a pipe at least two inches long you should have no trouble.

Bill.

You guys took the words right out of my mouth......I mean keyboard...............

Hello; My friend Ron Morgan broke the exhaust pipe off his new OS 120 in 1990, he has continued to use that engine in his huge piper commanche ever since with no problems. He has no pipe at all, just the exhaust port. The engine is loud, but Ron has a big cowl around it that tends to quiet it down a bit, plus it's so big that he usually keeps it quite high, and away from the field. That's 13 years with no problems.

The only problem that may arrise is the lack of a pressure tap for tank pressure. My old FS-90 ended up with a Perry pump. Its had a straight pipe on it since 1988..No issues.

The OS 160 Gemini twin cylinder uses two straight pipes with no mufflers, and my dad has lots of OS 91's and he doesn't use a muffler on any of them, just the straight pipe.