Friends and others,
As you may recall, on the way to Osh on July 23 my exhaust system decided to depart
the engine leaving a lot of melted wires and charred cowling. I got it tacked back
together and raced home to Seattle from western Nebraska. Once home I
began the process of doing a permanent repair of the wires and developing a new
exhaust system. The wires were the easy part. The exhaust system has been a
long arduous process in which I pursued several options.
In the end I mocked up a design using 1 ½” and 2”
Sched 40 ABS plumbing pipe; not quite the exact dimensions for 2” and 2.5”
ID x .049 stainless– but close enough. I sent the model off to Aircraft
Exhaust in Princeton, Minnesota and a mere 4-5 weeks later I got
my finished exhaust system. I had them put a couple of “ball joints”
in right after the first bend about half-way to the merge. Also, the model was
designed, and I emphasized with them over the phone, that each leg was to be pretty
close to (exactly?) 21” from the flange to the merge (per Lynn Hanover’s
advice).
I received the exhaust 2 weeks ago. Getting it on the plane
was a breeze. Unfortunately designing and fabricating a support system for it, so
it is firmly braced to the engine, and fabricating a stainless heat shield took
24 or more hours! Since the airport is 50 miles away that’s a lot of work
and has taken 2 full Saturdays!
Anyway, my efforts were rewarded late Saturday when I
finally got it all together and began testing. I ran the engine on the ground
at low RPM (1200-2000) while the oil warmed up. After the oil was above 120
deg. I did a static test and was rewarded with another 200 RPM.
Oh, let me backup a minute. While the engine was down –
waiting for the new exhaust I decided to re-do my intake manifold. I have the
short Atkins manifold and had a plenum attached directly to the TWM throttle
body – with nice sharp edges butting up to the throttle body (hey I didn’t
know!). So I created some 11” long intake tubes that flare to a “sort-of”
bell mouth inside my air-cleaner plenum. I tested that briefly with the old
muffler and on the ground got another 100-150 RPM static and another 200-300
RPM in the air – up to 5400RPM and another 3-5kts airspeed (162kts). So
that was a win.
OK, back to the new exhaust – I was not getting
4400-4500 RPM static and after running for 20 minutes or so and checking to
make sure everything was still secure – I decided to take to the skies
before it got totally dark. There was a very noticeable improvement in takeoff
and climb – VERY NOTICEABLE. So I headed out west of the airport over Puget Sound and opened it up to see what would happen. It
shot past 162kts and settled in around 179kts and about 5950 RPM!! Which is
also pretty close to the design speed of the prop! So, overall it looks like I
have some significant performance gains!!
Around the end of the flight (or actually the cause of the
end), I thought I felt a little change in vibration and it looked like the EGT’s
went down – so I hightailed it back to the airport at reduced power (of
course descending even at reduced power meant I was still moving at 200MPH) and
had an uneventful landing and taxi back to the hangar. I didn’t have time
to pull the cowls – but everything I could see from the outside looked
OK. So, I have to see whether the problem was in my head or in the engine!!
Attached are some pics of the model and the new exhaust. The
“NewExhaust2” picture also shows the new intake in the upper right
corner.
Performance Re-cap:
Static RPM Flying RPM Max
Speed
Original 4100 5150(max)
157kts TAS
New Intake 4250 5400 162kts
TAS
New Exhaust 4500 5950
179kts TAS
Obviously the Tangential muffler sucks (actually blows) when
it comes to performance! WAY too much back pressure.
Regards,
Joe Hull
Cozy Mk-IV N31CZ (65 hrs - Rotary 13B NA)
Redmond (Seattle), Washington