Thanks for the feedback. Most of your
comments line up with my understanding/thoughts. I didnt go into installation
details when I wrote this for a couple of reasons. One, I've said it before,
and two, many of the details now are pretty vague because my exhaust and
intake systems came together about 6 years ago. I'll recount what I
recall:
1) The engine is a Bruce Turrentine built. Turbo
housings and high compression NA rotors. No porting.
2) The exhaust is a true 2 - 1 header with
equal length primaries. I'm a little sketchy on dimensions but my recall is 1
3/4" primaries, 32" long and equal length into a Burns merge collector 2
1/2". Muffler is 4" behind the collector, 2 1/2" inlet and outlet, 28" long, 4
1/2" diameter. It's a spiral flow design, homebuilt, with a relatively large
straight tube through the middle. I dont believe it presents any significant
restriction. Internals are Inconel and it was in good shape when I had it off
the airplane a couple of weeks ago.
3) The intake starts with a modified 88 Mazda NA
lower manifold with all the excess metal cut away and port matched to the
turbo housings (blended with JB Weld). I used the first part of the upper
manifold to make the 90 degree bend over the top of the engine and from there
welded on aluminum tubes (1 1/4" primaries, 1 1/2" secondaries, about 10"
long. Plenum at the end of the tubes is small. Hard to describe so I'll
try to find a picture. The throttle body is as described previously - a
hacked Mazda tb with only 2 of the 3 original inlets.
I guess what really shocked me is how much
throttle was still remaining when I reached max power. I guess the real
question is, is my prop load limiting power output, is there some dip in the
power curve and my engine cant push the prop load "over the hump", or is it
simply a matter of a restriction in the system somewhere that is limiting
the airflow? As I said previously, I'm not too worried about it because
the airplane performance is pretty good. But who doesnt want more, especially
if you suspect its there and accessible with hopefully not too much
effort?
Mike
----- Original Message -----
Sent: Saturday, October 10, 2009 11:14
PM
Subject: [FlyRotary] Re: where's the
missing power?
Mike,
There are several reasons that an engine will produce max power at
"less" than full throttle. First, and this can sometimes be hard to check,
be sure that your full throttle position doesn't rotate the throttle plate
past full open. I've seen this several times in butterfly throttle systems.
If you are using a Mazda throttle body it is likely that the wide open
position is controlled with a stop. You shouldn't have a over rotation
problem with that. The second reason, and far more common, is that the
design has more intake than it can use with the tuning or RPM that the prop
limits it to. This isn't really a problem other than indicating that with
development the engine could make more power. Often with the design
constraints placed on us by a close fitting cowling or restrictive
exhaust limit the best airflow to less than the throttle body can pass.
The engine just can't "breathe" well enough to need a larger
opening. The length of the intake or exhaust can be too long/short and
the maximum amount of air that the engine can digest will be at the 3/4
throttle area or even less. Most engines can be tuned to best performance at
a specific length inlet and the diameter is also important. Same with the
exhaust. If either system is off a significant amount it can prevent you
from making any more power past a given throttle opening. This won't hurt
the engine at all, it just shows you that some portion of the system can be
improved. If your exhaust gas temperature is correct then the injection is
working properly and you need to check the other systems. Be sure to double
check your exhaust for obstructions or a pipe collapse. We have had too many
exhaust problems lately. To put your self at ease remember this, most modern
diesels have NO throttle plate. The speed of the engine is completely
controlled by the amount of fuel injected. This helps the efficiency at
maximum throttle, but obviously there is more air than the engine can use at
low speeds. A small amount of fuel is injected, compression ignited ,and all
burns up. If all is healthy your engine is drawing all the air you can use
by half throttle. You can probably make improvements and get more power or
lower fuel burn by tuning.
Bill Jepson
-----Original
Message-----
From: Mike Wills <rv-4mike@cox.net>
To: Rotary
motors in aircraft <flyrotary@lancaironline.net>
Sent: Sat, Oct 10,
2009 8:27 pm
Subject: [FlyRotary] where's the missing
power?
I noticed quite a while ago and have mentioned
several times here that my engine does not have a linear response to
throttle. It reaches its max power before it reaches fully open throttle. I
havent worried too much about this up until now because the airplane has
sufficient power as is, has slightly better performance than my previous
160HP Lyc powered -6A, and my wood prop is actually a pretty good match for
the current power level.
But I would like to understand what's going on
here and eventually address it. I was flying yesterday, my usual boring
holes in the sky directly over the airport. Decided to investigate just a
little so leveled at 5,000 feet at full throttle. Started reducing throttle
until I noticed a slight reduction in RPM and fuel flow. Then looked down at
where the throttle was actually set and was shocked to see it slightly below
half open. I dont have a regular manifold pressure gauge, just an industrial
type vacuum gauge ( I really gotta get an MP gauge). Anyway, the vacuum
gauge was indicating 4" of vacuum.
So I suspect I am giving up a substantial
amount of HP. I think the most likely suspect is my throttle body. For a
throttle body I copied Tracy's original design. Started with a stock
late 80s Mazda TB and hacked off the third port and all of the extra stuff.
The cowl in this area is very tight and I am unable to install any sort of
air filter or any sort of bell mouth on the TB. And I'm sure the cowl's
close proximity to the TB influences airflow into it as well.
Any comments on this or ways to test it are
welcome.
Mike Wills
RV-4
N144MW