Trimming Exhaust Length

[walter]

One of the things I remember about reading Kent Passer's book "Speed
with Economy" was that he trimmed his exhaust stack so that the end
of the exhaust was effectively right where the air exits the lower cowl.
This has the effect of ejecting the cooling air at a greater velocity
and hence eliminating a very small amount of cooling drag. Has anyone
done this with an RV? What effects were noted if so. I wonder if
this would help with CHT cooling. How much louder would the exhaust
note be in the cockpit. Would the floor boads rattle your legs off?

[redbeardmark]

Actually, I've been wondering about putting some little "cherry bomb" type mufflers on my exhaust. It is a very loud airplane and if the noise can be tamed fairly easily, well, it might head off any complaints from the airport neighbors. No complaints yet, but an ounce of prevention is worth a pound of cure. Also, I don't race, and am happy with cruising at 150kts.

Anybody tried itty-bitty little mufflers?

[pierre smith]

Mark,
A friend has a new RV7 with the Eggenfellner engine and a muffler. It sure is quiet. I bet that if you looked, you could find a pair of small glasspack straight thru mufflers.
Pierre

[robertahegy]

My neighbors love my 7A with an 0-360 and the straight pipes. I guess I would be reluctant to cut off any pipe. The drag can't be that significant. For 1 or 2 mph I'd rather not burn up the belly of my airplane.

JMHO
Roberta

[Sam Buchanan]

One of the things I remember about reading Kent Passer's book "Speed
with Economy" was that he trimmed his exhaust stack so that the end
of the exhaust was effectively right where the air exits the lower cowl.
This has the effect of ejecting the cooling air at a greater velocity
and hence eliminating a very small amount of cooling drag. Has anyone
done this with an RV?

Been there, done that; RV-6, O-320.

I wonder if this would help with CHT cooling.

I noticed no difference in CHT's.

How much louder would the exhaust note be in the cockpit.

Definitely louder.

Would the floor boads rattle your legs off?

Won't rattle your legs off, but you will feel more vibration. You are also treated to the frequent chore of cleaning lead deposits off the bottom of the plane.

After a couple of years of flying with the shortened stacks, I finally got around to ordering a set of turndowns from Larry Vetterman so I could correct my moment of madness.

In my opinion, chopping the stacks has no benefits; keep the hacksaw in the toolbox.

(I was, however, able to convince my hangarmate that my plane was four knots faster than his. )

Sam Buchanan

[penguin]

Originally Posted by walter

One of the things I remember about reading Kent Passer's book "Speed
with Economy" was that he trimmed his exhaust stack so that the end
of the exhaust was effectively right where the air exits the lower cowl.
... Would the floor boads rattle your legs off?

My experience on an O-320 6A is that short stacks cause a lot of vibration through the floor boards. In my case it was OK for half an hour, bearable upto an hour, and uncomfortable after that - pushing the rudder pedal in the cruise caused my right foot to go numb. Regular length stacks made it much more comfortable - but there is still some vibration there. Did not notice any change in temperature, speed or noise (but use noise cancelling headsets).

Pete

[osxuser]

I know a guy with a -4 that ran the exhaust out of the back of the cowl cheeks, through stainless augmenter tubes. It sounds a bit weird, but it's cool! Looking.