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
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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
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