[Bryan Wood]

I've been looking at how much the exhaust sticks down into the airflow on my 9A and have been thinking about chopping some of it off. From talking to people I'm getting mixed thoughts on whether or not it is a good idea and was wondering if anybody on the group has done it to theirs. It just seems like round tubes of 1 1/2" or whatever it is sticking down into the airflow at least 5 or six inches can't be helping with the cruise speed. I'm hearing from people that the floors will take a pounding if I shorten them, and that the floor will get hot and the belly will always be dirty. I've also heard that it should be good for 2-3 knots more in cruise if they are shortened up some. Any real world experience appreciated.

Thanks,

 

[walter]

Good question.

Kent Paser writes in his book "Speed With Economy", a must read btw, that he gained 5 mph and a significant drop in engine temps by cutting back, 1/4" at a time, the exhaust pipes, until he saw a significant drop in engine temps. His exhaust pipes actually are forward of the cowl exit area, creating a "jet-pump" as he calls it.

At this point he had too much cooling and was able to reduce the cowl inlet size, hence reducing cooling drag and increasing speed even further.

My pipes only stick out about 1.5 inches below the lowest part of the exhaust opening, and of course they are angled out and downwards and they stick out way past the cowl exhaust exit. He also makes many other modifications including cowl flaps, modifications to exit flow area, etc. You should read it.

Kent makes no mention of extra vibration, sound or heat. But those are the things I keep hearing from folks when I ask them about this potential modification. But none of them has actually done it, at least the folks that I've spoken too.

I wonder if it's worth it.... I dunno, but since this plane of mine is experimental and will be coming home (KCPM) in the next week, I intend to do some experimenting over the long term...

 

[Sam Buchanan]

Here is a post I made a few days ago when this question came up in a different thread:

Several hundred hours ago I cut the turndowns off the pipes on my RV-6. Even though it was kinda cool to not see the pipes hanging out of the cowl, the mod definitely made the exhaust "sound interesting" and resulted in chronic gray exhaust stains on the bottom of the plane. Since I have insulation on the floor I didn't notice increased vibration but after a few hundred hours of cleaning the bottom of the plane I had had enough. Larry Vetterman sent me a couple of turndowns (he keeps them on hand for those of us who foolishly cut off the originals....) which I welded on the pipes. Now the engine sounds quieter and the belly of the plane stays clean.

As far as I could tell, the shortened pipes had no impact on the speed of the plane.

 

[Bryan Wood]

Thanks Sam

Thanks Sam, your information is exactly what I was looking for. My exhaust will stay exactly the way Vetterman designed them.

Regards,

 

[Harvey]

Cutting Exhaust

Hi Bryan,
Notwithstanding Sams comment, I trimmed the outlets on my O-320 RV6 to get rid of the down turned ends for all the reasons you mentioned. I did notice a slightly louder exhaust when on the ground, but no difference upstairs.

Then I trimmed the outlets on an angle (slash-cut) so that the outlets were more of an elongated oval than a circle, which brought the noise back to where it was with the downturned tips. If you were trimming the pipes to obtain a proper 'tuned length' then you would want a normal blunt end to give you a stronger return wave, whereas a slashcut tends to diffuse the return wave - hence the reason for my doing it this way.
My Rv is quite fast, very economical and light. It also runs very cool CHT's ( 310 deg F) in cruise. Whether it is the plenum or the short pipes, I dunno.
It worked for me, and I mention it to show that there is more than one way to skin a cat....
Cheers
Martin in Oz

 

[Steve Sampson]

If they are Vetterman pipes doesnt he adjust the length so they are tuned for max power? I would talk to him about that issue before I cut them if they were mine because I dont understand exhausts. Its a pretty academic amount of power, but if its there you might as well have it.

Steve
#4478
UK

 

[Highflight]

I ordered my exhaust from Vetterman just a few days ago because I deleted the standard Vetterman assembly that Van's sells in favor of the same but with mufflers (it's easier to just get the whole thing directly from Vetterman).

The subject of "exhaust pounding" on the floor came up because from what I have read, most people think that effect comes from the exhaust. However, when talking with Larry, he dismisses that "theory" and instead says that the pounding people refer to doesn't come from the exhaust, but rather, from the prop pulses.

Just thought I'd throw in some additional info about that here.