- What is 75% power for
my Lycoming I/O 320 engine equipped with a 70 inch Hartzell constant speed
prop? I have made the following
adjustments:
By
definition, 75% power in the airplane world is any power setting at
which the engine produces torque at an RPM to generate a horsepower that
is 0.75 x the max rated horsepower. It has nothing to do wit your
prop - - whatever size it is.
By the
way, "percent power" numbers are largely over emphasized in the minds of
pilots.
>>
Different pistons were used to raise the compression ratio from 8.5 to
9:1. How was that done? Did it reduce my cubic inches or
somehow increase the stroke? <<
No. The change in piston height to go from 8.5:1 to 9:1 is very
small. In fact, the "swept volume" remains the same. We are
talking about changes in piston height on the order of 20 or 30 thousandths of
an inch.
>> A
non-filtered induction ram air system is used which raises the manifold
pressure 1.5” Hg at 180 KIAS (195 KTAS).
What affect is this on power charts or do I just artificially add1500
feet to the chart scale? <<
We teach you how to calculate power for your engine without reference to a
power chart of any kind - - (you can, ultimately, do it in your head) during
the APS course. Here is the theory. ROP - - power is almost
entirely determined by mass airflow through the engine - - - which in turn is
almost entirely proportional to RPM and MP. A 10% drop in MP will
get you a 10% drop in horsepower to a surprisingly close
degree. Thus, if you open the alt air ram door
and get a 5% increase in MP - - then you will get a 5% increase in HP - - -
when ROP, and all else the same.
>>
a. The added LASAR ignition system has 2 effects:
- A hotter spark, burning more fuel in
the cylinder. The consequences
are a 15-20 degree rise in cylinder head temps and a 90-degree reduction in
EGTs. This seems to lead to
increased torque, thus increased thrust HP because the prop pitch is
increased to retain the RPM.
This is seen as a sprightlier take-off run, a higher climb rate and
better general performance.
What is the affect on determining % power? <<
Huge misconception here.
"hotter sparks" - - means a higher voltage longer duration spark. DOES
NOT BURN MORE FUEL. That is marketing hype. The 15-20 degree rise
in CHT and the 90 degree reduction EGTs is due to ONE AND ONLY ONE THING
- - a bad thing - - at that.
If I take any stock IO-320/360/540 and if I
advance the timing a few degrees it will raise the CHT 15 to 30 degrees and it
will drop the EGTs by 50 to 100F. That is the ONLY effect you are
seeing. Repeat - - the ONLY effect - - - has nothing to do with
"hotter sparks".
MOST of these engines are already OVER TIMED on
takeoff with stock factory timing. Advancing the timing further -
- at least at sea level on a standard day - - is probably counter productive
with respect to HP... and it is guaranteed to substantially increase the peak
internal cylinder pressures in an undesirable manner.
>> ii.
At some RPM/MAP point, the “spark” is advanced, resulting in higher
power and more efficiency. How
does this affect % power at full power and best power? What is the affect when running
LOP?
- The addition of a
harmonic dampener, which for fixed pitch props generally, increases the
full power rpm by 100. Does
this improve my power also by increasing the flywheel affect (allowing
an increase in
prop pitch to retain rpm)?
<<
At this point, your curiosity is getting beyond what I can do without a
charts and graphs. When operating LOP at altitude, properly
advancing the spark timing (PROPERLY) will improve the Hp by as much as 3 to
5% - - essentially for free in that it does not require more
gasoline.
>> Another words, what are
the gross parameters I can use to operate LOP? What are the steps I can use when
operating ROP and how many degrees (EGT) rich at certain power levels. <<
We spend
about 6 hours on this subject and about 40 power point slides and throw
in a test stand run from the engine test stand - - to properly answer that
question.
>> With the above
listed modifications that appear to affect power, should I increase the
takeoff fuel flow to something greater than its current 15.1 gph? <<
Probably.
Almost certainly. But without more and better data, one can't say for
certain. You need a takeoff BSFC of up around 0.6 on the
Lycoming NA engines. Maybe higher with the high compression piston and the
spark advance.
>> Someone help us
little guys with answers to our questions! How else can we achieve harmony with
the universe? Huh? Huh? <<
Darn... harmony with the universe
? That is above my pay grade!
Regards,
George