Smooth?
Dave C said:
Hi All,
What is the smoothest sounding exhaust for the 360 lycs - not necessarily the quietest - just smooth? I've noticed several lyc powered RV's that have very different sounding exhausts. I know a lot depends on type of ignition, timing, rpm, etc; but generally speaking what is the smoothest sounding exhaust? 2 into 1? Crossover? 4 into 1? 4 straight? Dave C, Sacramento, RV 7, finishing
What do you mean by smooth? If not lower noise, than do you mean hearing or not hearing individual cylinders firing, like a dragster sitting at the start lights, you hear poppoppoppop.
I notice RV's tend to have a distinct pop-pop sound when flying over the house verses a Cessna or Piper. Here is my theory.
RV's are fast and have open pipes. RV's have that pop-pop-pop-pop sound as they fly-by at low altitudes with cruise power. I think its basically RV's are moving fast, so you hear the individual cylinder notes. Slower planes are more of a low roar and last longer (slower). I think of it as a doppler effect, like the train horn that's at high freq approaching and drops in freq or tone as it goes by. Of course Cessna's and Pipers have some kind of muffler and typically fly at slower speeds, so the sound last longer.
Would different pipes make a big different? I don't think so, straight unmuffled pipes are always going to be loud and allow you to hear the firing of the jugs. At high power and RPM it "smooths" out into a roar just because its happening faster, but still on the ground people will hear a bit of the pop of each cylinder.
On the ground taxing RV's have a unique sound, again no muffler but also because they are so close to the ground, the sound reflects off the ground more.
Your own subjective observations at the airport are valid, so what I suggest, go out and listen and find out what sound you like better. 4-into-4 separate pipes has a distinct sound. I find I like it, but its because its not smooth sounding. Individual pipes is best for hearing the individual cylinders which is cool. Don't you love the sound of a radial idling, blapblipblapblimpblipblap.
I would think a 4-into-1 is best for "smoothing" individual "pops" if for only one reason, there's one single source of exhaust noise, not 2 or 4. I don't think there is a lot of difference. However the length of the tail pipe does have an effect. The longer the better. Cylinder's individual "notes" mix and slow down before coming out into the open air with longer tail pipes. Most RV'ers are not willing to run several feet of tail pipe under their bird, its ugly and draggy.
Than there is the "Swiss Muffler", which not only uses a long tail pipe but fiberglass insulation around the long tail pipe to absorb sound. However just a longer tail pipe or pipes would make it quite in the cockpit as well as less noise to folks on the ground. You'd expect to lose engine performance plus add external drag. (Some Europen kit airframe designs work around the swiss muffler and have a tunnel to recess the pipe inside the airframe, saving some external drag. However loss of engine performance is still expected.)
If smooth = quite, consider a 4 into 2 cross over and add piccolo tubes (those cheese grader tips). It adds very little drag and loss of engine performance if any, and people on the ground notice the difference.
