My exhaust fell off during flight today I flew for at least 15min before I realized it was missing. My question is did I harm my engine in any way? This is my first four stroke so a bit of help would be great. Thanks in advance.

Anyone[&o]

I've got an OS 91 F/S that I've run many times with just the header pipe and no muffler. There is a guy at my club who has been running a Saito F/S without a muffler or pipe for years. (His threads are broken out of the head.) It seems to have had no bad effect on his engine, so I don't think that you've hurt your engine.
Cheers!

Nope, no problem. I have run my 1.20 for several years with just a header and one of my non pumped .91s seems to keep breaking the header off right at the head. As soon as the pipe or muffler comes off the non pumped engines you know about it instantly though, they like there muffler pressure.

Thats great news. From a technical point does a pumped engine need a exhaust to operate ? Could you just extend the header pipe etc. I come from a heli background and as you know exhausts are critical for performance. Thanks.

A pumped engine doesn't need to be pressurized from the exhaust because the fuel is delivered by the pump. You can run it with just the header pipe as long as the noise isn't a problem.
Cheers!

Thanks for the advice. I dont have a noise problem at all so ill fly on the header pipe only. Thanks again.

With just a header you can also make up some different size collector boxes and change the sound. They do tend to run a little better with a bit of back pressure.

Had a couple of flights yesterday. No problems Thanks guys.