As Lynn mentioned 2" and then into the
housing port where it's reduced, is good.
The nice thing about a 2" runner dia P Port is that if doesn't make power
at the desired rpm you could place a linner in the runners to reduce the
dia. A liitle harder to increase the runner dia without
disassembly. Thisof course depends on how many bends are in the
runners.
----- Original Message -----
Sent: Saturday, March 27, 2010 1:40
AM
Subject: [FlyRotary] Re: P-Port
performance
“that things that
work in one application (like Rx-7 racing) just great - may well suck in
another application.”
Or not suck
enough…. ;)
Neilk
Here is some more stuff to read, since none of you are building
right now.
This fellow fills in some cross section on the intake runners of
big engine street and road race bikes.
Even the factory people fall in love with maximum HP numbers to
sell bikes. The big numbers at the top RPM require big intake runners. Big
intake runners means high flow velocity will be at astoundingly high RPM.
This means the reverse will be available at mid and lower RPM. So, the rider
is beaten silly on the road course by some weekend nitwit with a smaller
engine, that appears to be stock. He obviously has less maximum HP, but
seems to be doing quite well with his setup.
Because.......
The smaller engine has good torque and HP at lower and mid range
RPM. Maybe he is not as fast at the end of the long straightaway, but he
seems to get to his top speed much sooner than the new bike with the
bigger engine. And in road racing and drag racing, it is the first person to
his top speed that wins, not always the highest top speed.
Think gear ratio spreads, and area under the curve when you graph
HP.
Racing engines and airplane engines are the same engines. Some
racing organizations even use rev limiters to equalize competition between
car brands to keep the racing interesting. So you have to imagine that your
club, be it hair club for men or the home built airplane club has a rule
about top RPM.
Of course the club has no such rule, but Mother nature does have
such a rule. It has to do with the tip speed of propellers. So once you
calculate the ratio of your reduction unit, you know how fast you can turn
up your engine. And it isn't very much RPM.
So now you are unhappy with the torque of the car engine that had
good torque at 2,500 RPM in the street car, and now you want the best torque
to be where? 5,000 RPM with best HP at 5,800 or 6,000 RPM?
Note that the car engine has short runners for high RPM and valving
to lengthen the runners for low RPM. You might even say that low RPM is
where the airplane engine runs all of the time. This should be easy. That
length thing has to do with tuned length, or a pipe organ effect.
And our motorcycle friend above shows us that in order to fill out
the mid range,(right where airplane engines run) you need to fill in part of
the intake runners.
So folks are making errors at both ends of the runner. The Throttle
body is so big that the last 1/3 to 1/4 of opening has no affect on RPM.
Fortunately this has nearly no affect on HP. At the engine end of the
runner just as in the stock intake manifold the runner needs to be smaller
just as it mates with the block. So the highest flow velocity is right at
the port face.
The rotary has a problem that involves the bowl shape below the
opening into the engine, valved by the sides of the rotor. In order to get
the valve timing we need, that bowl is too big (Too much volume). Some folks
fill this in a bit with epoxy products. Some folks make it worse by
increasing the port timing but that makes the bowl volume increase. The bowl
volume causes the velocity to drop right where we want the highest velocity.
Some folks make Periphery port engines with 2" tubing run from some
distance that seems to make sense. This is a big help at RPM above where you
can use it for anything. So at RPM lower than the ideal to take advantage of
the Pport, there is less HP than a side port engine.
The opening into the housing with a Pport is usually a round hole.
Not ideal. Later intake opening and earlier closing give you more mid range.
The 2 inch pipe flattened slightly, or a dart removed to reduce the ID about
15% or a bit more. would keep the velocity high, where it needs to be high.
The best HP should be just before the top speed, so you can lean away power
and heat to get down to cruise RPM.
Like the racer you want to be first to your top speed. If you have
to pull off throttle to stay at cruise RPM, add some prop and test again.
The engine will make power to well past 9,000 RPM in street trim, if the
breathing is available.
If you want to go real fast, cool the oil.
A 12-A can do 310 HP at 10,700 RPM breathing through two 44MM
holes. My side port 12-As can do 250 HP at 9,400 RPM breathing through two
38MM holes. My intake manifold gasket is stock. The runners at the port face
are the street diameter and shape. A 12-A is 2,292 CCs, a 13-B is 2,606CCs.
These engines are not dynoed below 7,000 RPM, because they have no power at
all below that RPM it is pointless to test there. They are towed to the
false grid with little tractors because they have no torque at all and
they idle at 2,200 RPM.
So if you keep building racing engines and puting them into
airplanes that operate below the bottom of the power band of the engine, why
do you do that? Doctor, Doctor, it hurts when I do that. Quit doing
that. Of course I could be completely wrong.
Lynn E.
Hanover