Straight Pipes vs. Muffler

[jiott]

Not that I intend doing it, but I am curious if anyone has installed the Rotax 912s in a Kitfox with straight pipes instead of going thru the muffler? Many other aircraft do this and I am wondering what kind of power increase one would expect and how bad the noise increase is and any other potential problems.

Jim

[HuDuGuru]

Although I don’t have any evidence, I’m willing to bet that a 912 wouldn’t produce more power with straight pipes and would in fact lose some power.

Just about all modern engines depend on some back pressure to maintain proper cylinder scavenging. Many other aircraft have used straight pipes, but all the ones I’ve seen were either Lycoming or Continental engines, which aren’t exactly a modern design.

What I do know for sure is that my old big block Dodge produced an extra 25 HP on the dynamometer with open headers versus running dual 3” exhaust with mufflers. We tried the same thing on a 2.4 liter Honda engine, it made 13 HP more with a (semi) properly tuned exhaust with a muffler versus an open header. Many high reving motorcycle engines won’t even idle if you remove part of the exhaust. As a general rule the higher an engine revs the more it depends on properly tuned exhaust.

[ClickClickBoom]

http://www.uucmotorwerks.com/html_pr...torquemyth.htm


More:

Many modern engines use a “tuned” exhaust to improve the volumetric efficiency of a high-performance engine. The operating principle is simple in concept: When exhaust gases exit the exhaust port at very high velocities, a mild vacuum is created in the cylinder. Engine builders therefore use intake and exhaust valve timing to take advantage of this vacuum effect to increase the intake charge of air and fuel entering the cylinder.

Since most of the torque generated by the combustion process takes place by 90 degrees after top dead center (TDC), the exhaust valve usually begins to open before bottom dead center on the power stroke. The residual combustion pressure in the cylinder is then released into the exhaust port, through the exhaust system, and into the atmosphere.

In most naturally aspirated applications, the intake valve is timed to open a few degrees before the piston reaches TDC on the exhaust stroke. When the piston reaches TDC, both exhaust and intake valves are open, which results in valve timing “overlap.” The mild vacuum created by the pulse of exhaust gas exiting the exhaust manifold and header pipe will accelerate the air/fuel mixture entering the cylinder. This acceleration effect is called “exhaust scavenging,” and is used to a great effect in naturally aspirated engine designs.

[lostpilot28]

Just one data point (maybe)...I had a Pulsar XP with a 912 and I had both mufflers and straight pipes for it. I know it's not a Kitfox, but the engine doesn't know that!

Anyway, the straight pipes made a significant difference in performance...I could feel a faster acceleration during takeoff and climbout. But, the noise was horrible. Very loud. No, wait...very, VERY loud.

[Esser]

Exhaust scavenging in a simplified form is a continuous flow of air out of your exhaust that is uninterrupted. So us think of it as air coming evenly and constantly out your exhaust which actually causes a vacuum more or less. Extreme set ups can even help in the intake of fresh air on the intake stroke. It is a myth that you need back pressure for exhaust scavenging. In fact back pressure is the opposite of what exhaust scavenging is. You can get custom straight pipes that will take advantage of exhaust scavenging but you need to get a custom exhaust shop to make it for you. They need to figure out how to get high velocity without restricting your air flow and i believe there is something about frequency and pipe length in that formula as well. Goodluck but personally I like the quiet. I'm sure you could get a couple more HP out of her though.