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Dave,
Thanks for the info and offer. I want to take
you up on it sometime, but cant make it this week.
Mike
----- Original Message -----
Sent: Saturday, October 10, 2009 11:48
PM
Subject: [FlyRotary] Re: where's the
missing power?
Mike,
The last couple of inches of my throttle don't give me much power
difference either, for what it is worth. I can't believe thay you have
not been up in my plane yet. I have this coming week off if you have any
time...
Dave
On Sat, Oct 10, 2009 at 11:14 PM, <wrjjrs@aol.com> wrote:
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
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
--
David Leonard
Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net
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