From: doug@alliant.com (Doug Bloomberg)
Subject: Re: Aircraft Tuned Exhausts
Summary: Observered Results 
Organization: Alliant Computer Systems Corp.
Date: Wed, 23 Dec 1992 16:44:23 GMT

In article <19921223.065315.731@almaden.ibm.com> martinl@torolab6.VNET.IBM.COM writes:
>I have a question for you airplane engine experts - how do you design a
>tuned exhaust for a Lycoming IO-540? (6 cylinder). I know there are
>several tuned exhausts available for the IO-360s (four cylinders).
>These seem to mostly be the cross-over type (two front cylinders
>joined to one pipe, the two rear cylinders joined to another). As I
>remember, a Lycoming 4 banger has a firing order of 1-3-2-4
>(two left side cylinders fire, followed by the two right side jugs),
>so the exhaust gases would have longer time to clear the exhaust
>system. If this is correct, then what power advantage would this give
>over straight stacks? (Which brings up another question - how important
>is backpressure to the performance of an airplane engine?) Most Lycoming
>installations I have seen have 'Y' pipes - the two jugs on one side of the
>engine join into a common pipe. On my bird, I have all three jugs on one
>side into one common pipe.
>
>It is claimed that a properly tuned crossover system will net you 10%
>increase in horsepower. Comments?
>
>I have seen (including in this month's issue of Sport Aviation) several
>examples of each jug having it's own exhaust, with all pipes being
>about the same length. It is claimed that this is an attempt at tuning,
>yet all pipes exhaust into the open air. I might see the logic if the
>four pipes joined into one common collector, but what tuning could
>there be for the open exhaustsystem, execpt for back pressure?
>
>Of course, there is always the case of 'true' tuning. This is when
>the pipes are of such a length (same length for each cylinder),
>such that on the exhaust stroke during the valve overlap (when both
>intake & exhaust valves are open), the pressure wave in the exhaust will
>draw some of the fresh fuel/air mixture out of the cylinder into the
>exhaust pipes, and the pressure wave will then reverse before the
>exhaust valve closes, to pack the previously drawn out gases back into
>the cylinder. As I understand it, you need a 1/2 or 1/4 wave length
>pipe to have this effect. For airplane engine, running at 2600 rpm,
>this is quite long, and impractical to fit. (It appears to me that
>this is not the principle that the crossover system operates on, as
>the path lengths for the two jugs tied together is different - The
>common pipe is not in the middle of the engine, but rather runs down
>on side).
>
>Can anyone out there offer any advice? References? I have read quite
>a few textbooks & SAE articles of engine design, but never found
>anything really useful.
>

	Sorry to reprint all of the above but was necessary.

	A General answer to all of the above. The first thing you
	have to worry about in building exhaust systems for aircraft
	is clearance. Both clearing the engine, then the cowl and mounts.
	Next worry, is will the system hold together, heat expansion and
	vibration are exhaust killers. Finally, you worry about tuning
	and getting the stuff out of the engine compartment in an expeditious
	manner.

	Where I am coming from. I helped build 5 of the systems shown in
	the Dec "Sport Aviation". Originally, Larry Vetterman (my buddy)
	built a 4 pipe system for his RV-4 because he was tired of re-welding
	the SS Crossover system he had had on the plane. He thought that he
	was getting an improvment over the crossover with the singlepipe
	system. He flew with this for a number of years and after much
	begging from other RV owners he started building exhaust systems.
	He visited several performance engine builders, found what was
	considered by their experiance the optimum length for Lycoming
	320's and 360's.  He built up a set of optimum length for each
	pipe. (I must say it was mighty ugly as there were pipes of 
	various lenghts sticking out from the cowl) Anyway, a base line
	was done. [Note: the test plane had had a set of SS crossovers on
	it, test plane a RV-4 O320-E2D Pacesetter prop 68x68.]

	The original test with truely tuned pipes resulted in a 100 rpm 
	static gain and a approx gain of 75 rpm gain and a few miles/hr at
	top end at 8,000 ft (We live in Denver reason for the altitude)

	Because they were ugly, we trimmed the pipes down, in 1" increments
	until the final product. Now, from various builders of RV's many
	who have been flying their aircraft for some time. Including a Oshkosh
	Grand Champion/Lindey winner, another a notorious homebuilder/writer.
	Both siad they gained rpm at static and at top end. Also, they noted
	a lowering of oil and cylinder head temps.

	Basically the 4 single pipes work.  Now another observation. Larry
	is building a SS crossover system for RV-6A's, there is a serious
	clearance problem due to the nose wheel strut. Tony B has had both
	Larry's 4 pipe and cross over system on his RV-6A. He did not notice
	any difference in performance between either of Larry's systems.
	Though he did notice a decrease in performance using the other
	brand of crossover available for RV's.  Tony did see a slight
	rise in oil temp using Larry's xover system. We suspect it is due
	to the extra amount of tubing and heat retained inside the cowl.

	Cross overs should work better but why don't they?   See worry one.
	There is a definite way to correctly build a crossover.  Cyc 1 should
	cross left to and join 2 and exit on the left. 4 should run to the
	right and connect to 3 and exit on the right. Now if there were
	enough space and weight didn't matter. the pipe from cyl 2 and
	cyl 3 would be the same length as the pipes from cyl 1 and cyl 4
	are (to the junction of the pipes).

	Also, building exhausts you must use mandral bent tubing, most
	muffler shops CANNOT do this, if you look at the bends they have
	a reduced ID.

	Finally, design and construction technique are important. You should
	have expansion joints built into the design, you should have 
	adequate clearances, mild steel should be gas welded, SS tig welded
	with the tubes being welding filled with argon gas (this prevents
	crystalization of the area surrounding the weld).

	An Excellent book on exhaust design is "The Scientific Design of
	Exhaust and Intake Systems" (third edition) written by Phillip
	Smith and John Morrison.  pub.  John Bently, Inc., 872 Mass ave.,
	Cambridge, Mass 02139  1972   isbn:0-8376-0309-9


	a PS.  Backpressure is an important factor. Another local builder
	has tuned his Mustang II's exhaust. One item he has is a nozzle
	on each exhaust pipe tip. Reduction is from 1 3/4" to 1 1/4" in the
	span of about 2 1/2" Above 6000' he notes a definite gain in speed.
	At 10,000' he sees a gain of 9 mph.

	As you can see this is an Exhausting subject, I could go on and
	on.

	I wish all of you a Very Merry Christmas, a Happy Holiday season,
	and a New Year filled with Safe and Happy Flying.

	Doug Bloomberg  (doug@alliant.com)
	Littleton, CO

	RV-6 builder.