Great tuning summation!  Lynn.

While not a racer, my experience in experimenting with intake manifolds on
the rotary for aircraft use tracks with your comments.  I initially tried
big, short tubes with my first intake and while it would have probably given
great HP at 8000+ rpm, the prop load prevented the engine from ever getting
to the magic land.  When I went to smaller diameter and longer tubes for the
intake - performance at 6400 and below improved considerably.

About the only thing I might add a different perspective to is the rpm
regime for the HP tuning goal.  Since I personally can't stand to see the $$
flow at high cruise power settings , I seldom ever hit top HP in flight for
longer than a few minutes (just to see how it does).

  Most of the time I throttle back to 7.5-8.5 gph for economic cruise.  On
the other hand, my personal priority is to accelerate rapidly on take off
and get a good initial climb rate to get my butt over and as high above the
trees as I can as quickly as I can.  My  (tongue in cheek) viewpoint is that
I don't care if you can hit 400 mph, if you don't clear the trees, it
doesn't matter {:>).

So for my priorities, I concentrated on trying to achieve best power at take
off rpm - around 6000 rpm.  The longer length and smaller diameter tubes
give the engine lots of oomph in that rpm regime.  Clearly if you wanted an
all-out air racer you would more likely choose shorter bigger tubes.


Thanks again for the great summation.

Ed




In a message dated 6/1/2007 8:20:04 P.M. Eastern Daylight Time,
daval@iprimus.com.au writes:
Hi Lynn,
since the Pport has a 'near lack of reversions' which are the source of
"organ pipe" tuning theory; would you say that Pport engine is a lot
less sensitive to "tuned length" than a side port engine?

Or does the Pport overlap between exhaust and inlet also cause a
pulsation which enhances 'organ pipe' tuned length.

I think this is what I wanted to ask :)


As you can in see in pictures of the Le mans engine, great care was taken to
have the correct length of inlet tract for each RPM. The engine was not used
even close to its maximum RPM or HP. And it had 700 HP.  Having poor
performance in the tuned area is not  no performance, and they went to great
lengths to get as much as was possible.

Larger diameter tubes gets a poor peak tuned effect but flows better from
less drag. A smaller diameter tube gives a more profound tuned effect but
flows less outside of the tuned RPM because of drag.  And they had big
tubes. But the Pport flows like a turbine and power is limited by the
strength of the pieces. The multi piece crank may have been the limiting
factor. They could have made a few changes and had 800 HP with good
reliability. But they wanted perfect reliability. The Lemans cars are forced
to have broad power bands because there is a low RPM 1st gear corner and a
230 MPH straight, so they went for the wider power band that all competitors
must have.

In a fixed length situation, you can only tune for one RPM, and that must be
for cruise and still have enough mid range to get the prop and plane into
the cruise speed range. Up on the cam, or on the pipe. It is not impossible
to have a killer motor that will not pull hard enough to get a dyno reading
without going up to RPM (Minimum used for racing) with no load on the dyno,
because the lightest load cannot be pulled by the engine. It might even be
that you cruise at or near peak torque and never get to peak HP.  The other
choice would be to cruise just above peak HP.

I gear the car to pass through peak power well before the end of the longest
straight.  So we are above best power RPM for much of the distance. The
driver could not get around that one. Once the drag is equal to the
available power the car goes no faster in any case.

It is the first car to its top speed that beats you to the other end, and
seldom the car with the highest top speed. Notice that dragsters that are
going for a top speed records use a taller gear than the normal get there
first gear (lower).

The highest HP is not the answer unless there is a variable prop system to
keep the engine near its best power. Not fun if it takes constant attention
on clime out to keep things going the right direction. The other end of the
scale is a broad power band that allows clime with no thought of engine RPM
and on a fixed pitch prop.

It is possible to stall a prop with a bit of extra power while sitting still
or while the plane is moving slowly in a takeoff attempt. Most people never
get to feel that one because at lower RPM there is not enough power being
produced. But the prop blade stalls the same as a wing stalls. Angle of
attack is the answer. The speed of the air passing through the prop disc
alters the effective angle of attack, on any prop fixed or adjustable pitch.
Of course it is more likely on a high pitched fixed pitch prop. It is also
counterintuitive to pull off a bit of throttle to stop the stall on a take
off roll.

So the tuning is not for the max HP but a broad band of power, mostly to the
south or lower than the maximum HP. A bit lower HP at the bottom of the
range is fine, and helps avoid the slipping prop (like spinning the wheels)
and as the speed comes up and more power can be absorbed by the prop the RPM
brings on more power.

So, straight tubes over curved tubes. Worse if fuel is injected before the
curve or with a carb at the very end. Longer tubes give a broader band
than shorter tubes. Smaller diameter tubes give a better effect over a
smaller range. A peaky cam effect. Long tubes work better at lower RPM.
Short tubes work better at higher RPM. Probably a tapered tube of medium
length would be fantastic, but difficult to manufacture. No inter connection
of the inlet tracts is used on Pport engines.
A smaller higher velocity port gives a wider band than a huge slower flowing
port.

A bigger port moves the peak power up the RPM band and is peaky. A smaller
port works over a wider range, and is less sensitive to tuned length. I had
a factory Pport housing and it had small "D" shaped ports with the flat part
on the bottom. So it opened quickly and closed slowly. Later aftermarket
Pports (Modified factory) had huge rectangular ports you could stick your
hands in. Those made the power between 9,000 and 10,700.

Lynn E. Hanover




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