I went out on a cross country from Everett, Washington to Walla Walla and
back the
last few days. Cruising at WOT at 11, 500 feet I had a MAP of 19-20 and
was making
about 140 mph true rpm was around 6200 with a catto 3 blade 74 x 88 pitch.
I need
some aerodynamic cleanup on my cooling scoop, something I intend to work
on this
winter. But I think that I'm probably only making about 150 hp or so- not
bad, but
not what I could make. I think that the NA 13BREW with four ports and 9.1/1
compression would not be a match for a 13 b with p ports and 9.7/1
compression. I'm
thinking that I might be building such and engine for an engine swap in
the future.
Oh, and by the way, it was a great day for flying.
Brian Trubee
Build another engine with a p-port Brian. It will really help up high.
A 4 port will never match a p-port. A good 4 port manifold is hard to make
as well.
I assume you have a 2.17:1 gear box.
Paul Lamar
No, I've got a 2.85/1 gearbox. I'll fly with the present engine,but I'm
thinking
about building a P-port for installation in the future. I was burning 8.2
gph on my
flight the other day, for an AIS of around 145 mph at 11,500 msl.
Brian Trubee
That's good. A p-port works better with a 2.85.
You might be able to make your current manifold a bit better.
Make the runners adjustable length and make the runners the same diameter
all along their length. If necessary put the bells in the plenum.
Also a ram cold air scoop. Do away with 90 degree the down pipe.
What is the ID of the throttle body?
You could also try a smaller diameter prop to get the RPM's up.
74" is a bit much IMHO. 70" would be about right. Check with CATTO.
The tips are 80% of the drag. Looks like you are over prop'ed. Sand the
tips down
until they are really thin.
Paul Lamar
--------------------------------------------------------
I commented on here recently about how I took a cross- country flight to
Walla Walla
from Everett, Wa recently and was only seeing 140-150 mph cruise at 11,500
feet. It
got me to thinking- I was getting about 6200 rpm on the engine which is
2175 at the
prop. So, I'm a little overpropped/ underpowered.
So, there's the old chestnut about how you can get a few more knots from
an airplane
in cruise if you go a little above your cruise altitude and then descend
to your
cruise altitude, thereby getting "on the step" . Some pilots swear by
this, others
dismiss it. I have a theory- if I climb to a cruise altitude and stay at
full
throttle while doing so, I will only get the engine to run up to a certain
RPM and
therefore a certain hp will be achieved, and no more and so the airplane
will settle
into an equilibrium at a certain speed- say at whatever hp/speed - 140 mph
at 6200
rpm- let's say x hp.
Now, If I were to descend to my cruise altitude in a shallow dive, I will
be
acheiving a higher rpm and hence horsepower, say 6800 rpm and horsepower
will be x +
9%x for a hp of 109% of the hp achieved at 6200 rpm, so I should be able
to achieve
and maintain a higher cruise speed if I descend to my cruising altitude.
( I picked
9% arbitrarily for the sake of argument) .
Anyone want to shoot me down on this one?
This is just an theory, If I want to go faster first I have to clean up my
drag. My
cooling scoop is effective, but obviously draggy. And It needs a flap on
it. That's a
project for this winter.
Brian Trubee
Yes! Over propped. That is what I said back in 06-29-2012. Try cutting 3
inches off
each tip.
Paul Lamar
I used to put my Bonanza on a step but I can't remember if I ever truly
checked if it helped. I probably did because I'm totally anal about fuel
consumption. Did same with my 180.
Van
One way to test for too much diameter or not enough HP is put the nose down and dive.
If the RPM does not increase a lot the fixed pitch prop is too big in diameter.
Paul Lamar
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