This will go on the web site as an introduction to intake systems.


"I have bad news for you. It takes HP to suck the combustion air into the
engine. This HP is subtracted from what you get out of the e-shaft. That HP
also burns fuel reducing the MPG your going to get. The intake system is an
aerodynamic device just like any other aerodynamic device such as an
airplane. The difference is it is inside out. The air flows through a pipe
rather than around an object. All the same rules apply. You want to reduce
the surface area the air is applied to as it could be going up to 400 MPH
and skin friction causes drag. Any bends or sharp right angle turns the air
is forced to make reduce the amount of air going into the engine. Side ports
are a good example of sharp right angle turns. Since there are at least four
side ports in a two rotor engine you can see the problem. P-port engines
have no sharp right angle turns and there are only two runners.  Four, 1.5
inch diameter runners, have four times the surface area and skin friction
drag as two 2 inch diameter runners if my arithmetic is right. Probably more
as the air velocity could be higher. This increases the pumping losses.

The high power RX8 six port/runner engine is even more of a nightmare for
our purposes. Mazda had to do it to get more torque at 2000 and 3000 RPM for
car use. The four port/runner RX8 engine is worse as the breathing is
becoming limited over about 6000 to 7000 RPM. Mazda had to do it for car use
but that is another story.

Unlike a car engine the aircraft engine runs at a high continuous speed
therefore the intake design is entirely different. Also you want to decrease
the velocity of the air flowing through the pipe to decrease the skin
friction drag. On the other hand decreasing the air velocity too much much
defeats the organ pipe tuning effect. The organ pipe tuning effect takes
advantage of a resonant system. Any resonant device will keep vibrating with
little input of energy. Think back to the days when you were a child and
your friend was pushing you on a swing. As you swung back and forth your
friend had to give you a little push now and then to keep you swinging. No
matter how hard he pushed you always swung back and forth in the same amount
of time. You just went higher. You were part of a classic resonant system.
The same thing happens in a intake system. At a given length and diameter of
intake pipe there will be one RPM where the energy required to keep the air
flowing back and forth is a minimum. Surprisingly the diameter of the pipe
also has a small effect on the resonant frequency. I am not sure why but I
think it has to do with the average speed of the air flowing through the
pipe. Mazda spent a huge amount of money back in the early 1980's
experimenting with different size intake pipes to determine the maximum
amount of air they could get into the engine and therefore the maximum
amount of HP they could get at any RPM. What they came up with was a single
pipe per rotor about two inches in diameter and a big hole in the rotor
housing.

It was called a P-port for peripheral intake port engine. Up until
the RX8 engine all mzda rotaries had peripheral exhaust ports. A p-port
intake system is easier to build as there is no flange bolted to the side of
the block. Just use the slide throttle design on the web site. Since that
time nobody has got more HP at any given RPM than Mazda. Fortunately Mazda
published the data in an SAE paper.  Mazda built a trombone system were the
length of the pipe could be continuously changed as the engine accelerated
up through the gears. This is unnecessary for our purposes as the aircraft
engine runs 99% of the time at one RPM. You can tune the fixed length pipe
for takeoff RPM or you can tune it for cruise RPM. The complication of a
trombone pipe is unnecessary for the vast majority of aircraft applications.

Mistral spent a lot of dyno time and money on the side port intake and they
got only 190 HP at 6000 rpm or so. No getting around it. Putting it another
way the max torque on the side port engine is about 160 foot pounds maximum
no matter what you do. It is, after all, only a 160 cubic inch engine.

People think the RX8 engine is a magic 240 HP engine all by itself
regardless of the intake. It WILL generate 240 HP at 8500 RPM with the right
intake system. That is 28 HP per 1000 RPM. So 7500 is 212 HP and 6500 is 184
HP. Since 7500 RPM is the max RPM you are going to see with a 2.85:1 gear
box and a reasonable diameter prop 212 HP is about all your going to get
with the stock or custom intake system using the side ports. Unfortunately
the stock system is too bulky and too high so people have to cut it down. If
you do cut it down all bets are off.

The p-port on the other hand is an easy 200 HP no matter how you mess up the
intake system. 200 foot pounds of torque is no problem. Moving the torque
peak up by shorting the length of the runners results in a lot more HP. 300
HP at 9500 RPM with 2 inch diameter runners and a slide throttle is no
problem at all. We demonstrated 240 HP at 6500 RPM on the dyno using about 2
inch runners and a carb no less.

Ironically there is no BSFC advantage with the side port. Probably on the
contrary as the pumping losses are higher with the side ports and multiple
runners.

Don't get hung up on low RPM. Forget everything you know about piston engine
RPM. The rotary is a different animal. It is more akin to an electric motor
than a piston engine. There are no parts that jerk back and fourth, fatigue
and crack. Forget the 2.17:1 gear box. It is ancient history. Go for the
2.8:1 gear box or even 3:1. Also go for a constant speed prop if you want a
lot of MPG as you can slow the engine to 4500 or 5000 RPM increase the pitch
cutting engine friction.

Paul Lamar ...No rotor no motor.

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