Brian,
You stated "...my EGT temps- they show up to 2000 at times- and above
1800 most of the time in cruise." I too experienced very high EGT's
in excess of 1800* when I was using a muffler that was too
restrictive. Also, power was way down, but it was quiet. ;-) The
high temps ended up damaging the o-rings and I had to rebuild the
engine. Since replacing the muffler the egt's have dropped down to
1600 - 1650* range and power is much improved. I suggest you examine
and modify or replace your current muffler.
Mark
The fuel flow seems correct, and coolant temps seem correct for the
speed,
and HP, so mixture is close. The EGTs are way too high, probably back
pressure and perhaps too large a header diameter. And the probes are a bit
close
in. About 3 inches is the compromise between accurate reporting and long
life. Each header pipe should be exactly the same length to the
collector.
Put a shop vac hose down each pipe as you build up the system and mark
the
length with tape. Make the second pipe the same length. Shallow angle
into
the collector. Collector about 4 diameters long. Long collector broader
HP
range. Short collector higher peak HP but very short range. Header tube
ID
to match the exhaust liner ID as close as possible. You want high
velocity
right into the collector. Header length in any multipal of 11" to 12" to
the
collector. If the muffler is close to the collector that is the end of
tuning. And best to have a see through muffler.
The other problem is the 13BREW irons are from a twin turbo engine
and have huge port runners. You want the highest possible velocity at the
port face on the iron. That requires small runners in the manifold and
irons. Some builders do use turbo irons for NA road race engines, but
those guys
shift well above 10,000 RPM. That is because of the big runners that
still
flow well at those RPM. Those engines have no bottom end power and not
much mid range.
Dave Lemon blends 2 of the 3 runners and ports in a 6 port iron to make
his E production engines. This is the same idea as the turbo irons but
with
more material to play with. He shifts above 10,000 RPM with a really big
street port.
Read this: _http://mototuneusa.com/thanx.htm_
(http://mototuneusa.com/thanx.htm) Look at "High velocity porting"
So the selection of turbo irons for an NA engine is an error in my
opinion.
Daryl Drummond uses the stock intake manifold gasket on 250 HP 12A GT-3
engines. So the runners are amazingly small. (High velocity)
The Pport gets back runner velocity with very early opening of the intake
port. Lots of overlap and late closing of the intake port.
The bridge port opens even earlier than a Pport, plus you control where it
closes by moving the closing line at the top of the port. I think you are
stuck so long as the turbo irons are installed, unless you add a turbo,
which ends most of the muffling problems.
Rather than bridge port the irons, I would swap them for non turbo irons
or
add the turbo. Get on the "Nopistons" list and look at the swap and sale
items.
Lynn E. Hanover
The best muffler for a rotary is 3 to 8 feet long about 4 to 5
inches in diameter. That gives the lowest aero drag for a given
silencing. Lowest frontal area. Mufflers work by the exhaust
gases rubbing on the metal surface losing kinetic energy to skin
friction and losing heat to the metal surfaces. The cooler the
gases in a pipe the lower the gas speed. When high speed gas
exits a pipe there is a sudden expansion that causes the noise.
The idea here is to cool the gases before they get to the end of
the inner pipe. The inner tube is best done in .045 Inconnel
but there is a trade off with thickness. If you make the inner
tube 2 inches in diameter and use 321 SS make it .065 thick with
a bunch of holes drilled in it. The outer tube can be .030"
thick 321 SS as it will always run cooler. Also make the front
of the muffler look like this. Don't drill any holes for the
first 8 to 10 inches. The slip stream will blow cooling air
down between the coaxial tubes. At the back end make the inner
tube 6 inches shorter than the outer tube. That will act as an
ejector to draw cooling air between the two tubes. Patent
applied for:)
A 2 inch diameter SS tube .065 thick 72 inches long will weigh about 8.8
pounds.
A 4 inch diameter SS tube .030 thick 72 inches long will weigh about 7.50
pounds.
Here is a dwg.
A 2.5 inch inner tube and a 5 and 1/2 inch outer tube may give a little
less back
pressure for a weight trade off.
Paul Lamar
Mark ,
From what you say and from the performance of my plane, I'm
starting to think that the Hushpower muffler is causing too much
back pressure and that would explain the anemic performance and
high EGTs. What type of o-rings were you using and do you think
that the high EGTs caused O-ring failure and how so?
Right now I'm thinking of how I can replace the Hushpower with
the design that Paul just put forth. Its a bit tricky hanging
that stuff on an RV-4 and working it around the heat exchanger
and scoop.
Brian Trubee
It might be melting inside.
Here is what Mike Wills did on the RV4 although the front of the muffler
outer tube
is not open for cooling air. This is what gave me the idea for the
coaxial
muffler in
the first place.
The subtle point is; cooling the inner tube saves weight as there is a
high temp
metal durability trade off with weight.
For an example Perry Mick has been running an early cast iron thermal
reactor for
years. They are very durable but they are heavy.
For anther example Ed Anderson runs SS pipe hand rail with an .109" thick
wall and
it hardly discolors. However you don't need a motor mount as the engine
can be
cantilevered off the fuse with the exhaust tubes :)
Paul Lamar
Brian,
I was running TES o-rings inner and outer. The high EGT's exceeded the
temp limit of the o-ring, allowing it to ooze out between the mating
surfaces. I switched to the SS handrail exhaust with a DNA muffler (no
longer available) and haven't had any more problems, although the DNA
muffler is not much of a muffler (pretty loud). People comment they can
hear me before they can see me. I wear a Lightspeed Zulu headset.
Mark
Got an pictures of those O-rings? Are you sure about the section size?
Heated silicone can crack cast iron if there is not enough room to
expand.
http://www.rbwagner.com/pages/Rbwagner.html
Paul Lamar
Mark,
What kind of muffler were you using that caused all the back pressure?
Brian Trubee
Oh, and also, when you changed the muffler, what kind of performance
gains
did you get?
Brian Trubee
Brian,
I saw a big improvement, especially at WOT. Climb rate improved, top
speed
improved, and EGT's dropped significantly.
Paul,
Attached is a picture of the exhaust I'm running now, except for the
downpipe section has been replaced with a DNA muffler. Sorry, its the
best
picture I could find of the exhaust. I heard they quit making the DNA
muffler.
Mark
I see very little advantage. It looks trick and different and that some
times sells :)
For the new guys this is Mark's old engine non p-ported.
As Lynn says you need a "see through muffler". :)
Paul Lamar
Paul,
All I can say is the performance was significantly improved from the
tangential design. Maybe when I run out of other things to do I'll build
one of your design and hang it under the fuse and see if it performs any
better. On a side note, it sure would be nice to have a dyno to use to
test our theories.
Mark
If you have the room under the cowl you can use one of these tuned
versions.
If you go with an 8 inch can and 6 inch tube cl radii then overall will be
12 inches.
You should have room for that in a 48 inch cowl.
You will get max HP at 7500 RPM and good BSFC from 5000 to 7000 RPM with
the magic 32
inch runner length all in theory.
You can fine tune it be moving it in and out relative to the engine by an
inch or two.
If square tubes are used any cl radii is possible because it is made from
flat sheet.
No bends are required and it is easy to fab.
Paul Lamar
Paul,
I would have to take some measurements to see if it would fit. Yes, the
cowl is 48" wide, but there are quite a few other items that need to fit in
that space too. ;-)
Mark
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