I want to use the header and can muffler that was designed for the 60-75 engine application in the Topflight spit fire. I have a new 91 OS FX that fits nicely and gives me the added margin of power that I want. The stock muffler, while huge actually has a smaller outlet pipe than the muffler I want to use. If I blow throught both mufflers, the stock muffler has far more resistance as well. The header pipe that I need to use has a smaller oval than that of the engine exhaust but there is plenty of meat on the header so I can easily make the two ports match with I high speed rotary cutter. The header connects to the muffler via a 3/4" silicone flex tube.

Now here is the question, the muffler says not to use on any engine bigger than a 75, for fear of overheating the engine. If that is true than the stock muffler would be a real problem I would think. I can weld on a second outlet pipe if neccessary but my other 91FX has a slim line exhaust with twin outlets that have reducers at the exits that have less total area than the exhaust outlet on this muffler and the engine runs great.

The problem is that a muffler for the 91 requires more internal volume than the muffler for a 61.

Check your internal volume?

With a lack of volume, the engine will detonate and get hot.

When I recently bought an OS91 from a LHS, the owner almost immediately warned me that the 61FX muffler would fit but that it would simply kill the 91. He saw fit to warn me of something most people wouldn't even think of, so it must really have been something he thought was important to pass on. He said that the 61 muffler wasn't big enough.......

I use the Tower 75 muffler on my 91 fx. It has more volum than the stock OS muffler and also boosts power. The 91 fx muffler is long but narrow so it really don`t have as great volume as one might think. It holds about 185 grams of water and the tower 75 muffler holds 222 grams of water. Try to fill up the exhaust system you are going to use and weigh the water on a digit scale, then you`ll know if it is smaller or bigger than the stock muffler

The slimline muffler I am using on my other 91 fx has about the same volume or less than the one I plan to use especially when you conside the header and the 5 inch x 3/4" connector pipe. I am not sure how volume comes into play engine overheating.. I can see it affecting the sound and possilbly the tuning... can someone explain the volume issue for me?

Best regards,
Randy

I have a friend with a Slimline pitts on his 91 FX. The engine detonates at the bottom of the downlines. It sounds like the ignition is miss firing. He's used a colder blug and 10% nitro but it still makes that sound.

Another friend has one inverted with the original OS muffler in a UCD3D and it's the most beutiful sounding two stroke you every heard.

They are both the original model with the metal body needle valve.

w8ye,
Thanks for that observation. I've been planning to use my 91 on a Hobbico Su-31. The cowling on that ARF does not have the extra housing along it's bottom like is on the fullscale airplane. I had planned to mount the 91 at about 8o'clock so that the stock muffler would be positioned in that cowling bulge. Since receiving that ARF and discovering the lack of the bulge, I've been debating using a Pitts muffler instead. It would save me building the bugle myself and fitting it into the bottom of the mfg'd cowling.

The sticking point was that I didn't like the looks of the Slimline pitts muffler for the 91 nor it's price. And I'm getting a bit tired of using Pitts mufflers to solve cowling problems.

It actually shouldn't take much effort to layup that bulge, and now that I've read your observation, I'm actually a bit pumped on doing it. I've got good reason to stay with the stock muffler now. It's always bothered me to throw those things in the parts box. Seems a waste. And I kinda like the looks of that strange bulge on the bottom of those cowls. Was sorta disappointed when I saw they'd left it off.

Now I got good reason to do that modification and am looking forward to doing it..... thanks