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