What's the smoothest sounding exhaust?

[Dave C]

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

[RV8N]

While I don't know if there is a difinitive answer to this question, I would think that either a 4-into-1 exhaust or a muffled exhaust would be the smoothest. Something has to smooth out the power pulses, which really pop on Vetterman's standard 4-into-2 exhaust.

Just my first hack at it,

Karl

[gmcjetpilot]

Originally Posted by Dave C

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.

[rv969wf]

Dave, this is my experience. Compression ratio will make a small difference on the sound along with cam timing. I started out with a low comp engine and a Vetterman crossover. Sounded just like the rest of the RV's. I tried mufflers, SUPERTRAPPS, VERY quiet but lasted maybe 10-12 hours before the guts were burned up. I took the burned up guts out of the Supertrapps and had megaphones, OMG, loud loud loud and they had a very deep rumbly sound. I later threw the Supertrapps in the dumpster. I even tried the Vetterman crossover with a two into one outlet. Kinda sounded like a 4 into 1 but not the same sound. Later I built a new engine with 10:5.1 comp pistons along with the Vetterman crossover. Yep it had a small but different sound than the 8:5.1 comp engine. Later I built my own equal length 4 into 1 / 1 3/4" primary tube exhaust. Wooosh, what a different sound. I first used a "Burns" 4 into 1 collector that was choked down to 2" at the collector then merged into a megaphone up to 3 1/2" then had reverse cone choked down to 3" for the final outlet size, can't remember the length of the collector, somewhere around 16"-18" long. This was the best tuned exhaust from computer data from "Burns" ,,,, but I didn't like the collector hanging back past the firewall another 12" +++ or so. Later I cut it all up and did a reverse design. The 4 into 1 collector remained the same but I opened it up too 2 1/2" at the exit, dumped that into a 3 1/2" reversion cone then tapered down to 3". What this did was bring out a very deep mellow tone. Yes I have a turndown. Without the turndown the bottom of the plane will look like **** and the floor vibration will drive you nuts. I've had many comments on how it sounded doing flybys as it is not like any other RV and that was what I was looking for, a sound that no one else has. I would say that it is smooth sounding, but it has a much deeper throatier sound and not like the whimpy sound that the crossovers have. I feel like the collector / size of the outlet is the key to having a smoother or different sound. It all depends on what you want. If you want a deeper tone, go 4 into 1 with around a 3" collector outlet about 10" long or so. If you want it to have a higher tone keep it smaller and shorter. Anyway this is what I've done and I'm happy with it.
Click on this link and go down towards the bottom of page one and look at pictures of my exhaust. http://www.vansairforce.com/communit...cooling+issues Hope this helps some. AJ

[SteinAir]

Hi Alan,

Did you ever go do some speed runs and plug in the numbers to the 'Kevin Horton' Spreadsheets?!? I've been itching to see what your plane really does with all those multiple radical (and beautiful) mods! It's got to be better than 220mph IAS; so do share - inquiring minds want to know what that gorgeous ship will really do!

Cheers,
Stein.

(Nice job on the exhaust...)!

[RV7AAAAAAA]

Originally Posted by GMCPILOT

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 is high freq coming and drops in freq or tone as it passes. Of course Cessna's and Pipers have some kind of muffler and typically fly at slower speeds, so the sound last longer.

Good theory George! RV's definetely have a distinct sound. That sound definetley gets me out of my house and looking up to see who's RV that is!

But what's your theory on the Rutan ships? Vari-EZ and Long-EZ. Don't these planes have the same Lyc. engines we are putting in our RV's?

When a Rutan ship flies over our house even my wife will 'name that plane' .
These things sound like 2 stroke engines @ 6000 RPM. It seems prop and exhaust would help make this unique sound. But it must be mainly due to the pusher design. Same thing goes for the velocity. Don't these planes have Lyc. 0540's in them? They have that 2 stroke sound.

Anyway, I know this wandered from the original post but this is something I've thought about over the years. OH! and I suspect that all of you out there will run not walk out of the house when you hear that distinct sound of a round engine.
I do anyway.
Jim
RV7A

[Lars]

Originally Posted by Dave C

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

Dave, there's a guy in the Sac RVators with a -6A that has a different exhaust. I want to say it's a 4 into 1 but I haven't actually crawled under and looked. In any case it sounds a lot different. Smoother, if that's a good word. Sort of how a 4 into 1 kills the individual pulses of a VW engine. When he flies overhead it's easy to tell it's him.

[rv969wf]

Hi Stein, I flew Tuesday and also today Friday from my location in Oklahoma to Wichita, KS and back twice. I was at 8,000 ft and pushing the sunset each time so I did not get to plug numbers in on a 3 leg run. Not sure where you got the 220mph IAS?? Did you mean TAS??? IAS today was 191mph at cruise / 8,000ft, 23" x 2,450 rpm in 50F air. True airspeed depends on many factors like what Kevin Horton provided to me and my indicated air speed could have errors just like any other aircraft. I have not had a chance to find nice weather, a calm day or time to do a 3 leg GPS run down low wide open. When I do I'll let you and others know. One thing I can say is that there is a 20 mph speed gain in IAS between running at 23" x 2,450rpm and 27" x 2,680 down low around 4,000 ft. I'm no where close to Tracy Saylors -6 with the 180HP. I have some rigging issues and I'm very heavy, but I'm gaining with trial and error. My plane is for travel, comfort and not racing. If I could do it all over again, I'd keep the comfort end out of the equation and keep it light as I'm not at this moment. AJ

[SteinAir]

Originally Posted by rv969wf

Hi Stein, I flew Tuesday and also today Friday from my location in Oklahoma to Wichita, KS and back twice. I was at 8,000 ft and pushing the sunset each time so I did not get to plug numbers in on a 3 leg run. Not sure where you got the 220mph IAS?? Did you mean TAS??? IAS today was 191mph at cruise / 8,000ft, 23" x 2,450 rpm in 50F air. True airspeed depends on many factors like what Kevin Horton provided to me and my indicated air speed could have errors just like any other aircraft. I have not had a chance to find nice weather, a calm day or time to do a 3 leg GPS run down low wide open. When I do I'll let you and others know. One thing I can say is that there is a 20 mph speed gain in IAS between running at 23" x 2,450rpm and 27" x 2,680 down low around 4,000 ft. I'm no where close to Tracy Saylors -6 with the 180HP. I have some rigging issues and I'm very heavy, but I'm gaining with trial and error. My plane is for travel, comfort and not racing. If I could do it all over again, I'd keep the comfort end out of the equation and keep it light as I'm not at this moment. AJ

Hi Alan, I thought I saw in one of your posts somewhere a 220mph quote, but then again my brain barely keeps up with daily thoughts so I might be thinking of something else.

I'm just guessing that when you get that thing all cleaned up it should really scoot! There is something to be said for comfort, and definately something to be said for the looks of your bird, it's freakin gorgeous!

Anyway, on a different subject & regarding exhaust I have used Vettermans on all my RV's (or customers) and used everything from straight "4" pipes to my current setup on both -6's with the standard 2 pipe...I have the small crossover on one -6 and the larger 2" crossover on the other.

That being said, on the -7 we're building now, I'm going with a 4 into 1 similar (but not nearly as scientifically advanced) as Alan's. Larry Vetterman and associates will build me a pretty nice 4 into one that should do pretty well....I hope! This plane is going to have a little bit hotter engine from Performance Engines so I'm going a bit more hog wild (at least for my cheap @#$@) and doing the 4 into 1. We'll see how it sounds compared to my crossover dual pipes and the previous 4 pipers!

Cheers,
Stein.

[Kyle Boatright]

Someone mentioned the unique sound of a canard.

That has to do with two things:

1) The prop spinning through the exhaust pulses.

2) The prop spinning behind the turbulent air off of the main wing.

These issues cause the canard guys prop problems. Ever notice how there are no metal props for canards? Metal props can't take all of the vibration.