The sound of a .91 engine...

[monsul]

...is like nothing I've heard before. I just maidened my raptor g4 with an os 91hzr. I love it, sounds...civilized . Nothing like my 55hz.

and the raptor is amazing of course (wind? What wind

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[Mr.Brown]

so long as its running plenty rich i could listen to that all day long ;like music to my ears.

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[Steve]

It depends more on the airframe than the engine I think.

Put your same engine, pipe and blades into a TRex 700 and it will sound very raspy and harsh. The pleasing noise is a sign of a good airframe. My Xcells all sound lovely.

Nothing much wrong with a 700 of course, it's just that it sounds different. Also, if you get a load of 600s all in one place, all with different engines, pipes and blades, they all kinda sound the same. Again, because it's the airframe and gear train noise that you're hearing rather than the engine note.

It's a bit like car noise on the open road - most of it comes from the tyres on the road, not the engine. If your car is noisy, try changing the tyres, it's surprising how much difference it makes.