Tracy Crook wrote:
Your experience is radically different than mine. I suspect something else
is throwing the results off, maybe the O2 sensor but not likely in view of
all the other inconsistant results. The O2 sensor (assuming it is after
the collector) assumes that both rotors are running the same mixture. Are
you sure they are? (EGTs same?) Is your instrumentation accurate?
Equalizing mixture is one of the most difficult things to do with carbureted
engines. I can run the mixture down (using manual mixture control on EC2
EFI) to the point where it is almost off the bottom of the scale (<.2v) and
the engine is still dead smooth. The EGT is well below peak and power has
fallen off. Your EGT of 1760 is almost exactly what mine reads at peak
EGT. This is normal and if you are melting your cowl, you need to go back
and re-think the system.
On the exhaust system, I am so impressed with how well the under the
fuselage muffler system worked functionally (never mind the questionable
materials I used to kluge it up) that I would now reccomend it. Power was
up noticably, BSFC was better (6.0 GPH vs 6.2 GPH for the same airspeed),
and oil and water temps fell by 5 - 10 degrees after getting the muffler out
of the cowl. Results should be even better when I get the new system
fabricated out of 321 stainless. The drag of the external muffler was not
noticable. Attached are some pictures of the old muffler and the temporary
"Swiss" exhaust made from thin wall mild steel and a Thrush glass-pack
muffler. Note the O2 sensor on the old muffler and on the collector of
"Swiss" system. This has been in service for several years and seen about
85 hours of 100LL use. Still works fine on a mixture monitor but would
probably not work on a closed loop EFI system due to slow response caused by
the leaded fuel. I highly recommend using a mixture monitor.
In the left side view, note the use of aluminum foil as a shield against
radiated heat from the exhaust system. Remember that these pipes are bright
orange in flight. Air flow alone will NOT protect things around these pipes
any more than a cool breeze will keep you from getting a sunburn at the
beach.
In the front view, note the scaling on the lower pipes which are mild steel.
This photo was taken after only a 30 minute test flight on the exhaust
system.
Thanks Tracy.
My O2 sensor is in the rear of my muffler - figured there'd be enough turbulence and
mixing occurring in there to even out the result. I have been thinking of mounting
two sensors - one on each exhaust header.
Probably would be easier for me to change over to your EFI than fiddling with the
carbs.
While mulling over my muffler failure the last couple of weeks, I too had a strong
urge to get the muffler out of the cowling - even short straight pipes out the side
of the cowling and damn the noise :)
More details, please. Pipe diameters, length? Is the collector Y 72" from the
exhaust port flanges? How far back is the muffler mounted? Do I need stainless
steel heat shielding on the bottom of the fuselage?
Total weight of system?
How long do you anticipate that glass-packed muffler to last? Cost? I though the
rotary melted them down in short order.
I was thinking about running the two 2" pipes from the exhaust ports together into a
2" pipe under the fuselage, punched full of holes (totaling at least 3.2 sq inches)
and closed off at the end, enclosing it inside a 3" pipe. The 3" pipe would be left
open at both ends, potentially acting as a venturi sucking out the exhaust gas. I
guess that the drag of such a 3" pipe might even be less than an exhaust plume
directed 30 to 45 degrees downwards. Someone at a Sun'n'fun forum mentioned that the
exhaust plume extends more than one foot and acts as if you have a thick rod
sticking out in the air. Ok, everybody - start shooting my idea down :)
Finn
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