Naturally I take on board all Lynn and Ed say, as they have helped me
understand how things work and Ed with the maths to give a ball park figure
rather than a WAG, on performance.
So I have condensed the figures Ed gave me a tried to make it as simple as
possible to show others - see attached. I hope this helps those willing to
look into the maths.
You will notice that the results strongly agree with what Lynn has
stated (in general terms). This in turn strongly agrees with
what powersport was using i.e. 40mm up to 6,000 rpm and 44mm up to 7,500rpm
(checked this with Bill). What Lynn has suggested is 50mm reduced 15% 42.5mm -
hello! I've seen that figure before - dia required at 7,200 rpm. I also agree
with what Lynn suggests on P-port shapes - I just find round easier to do, so
round is probably not optimum, but good enough for some.
Some of the Renesis figures I've increased hp in line with compression
increase and rounded off some end result figures.
Hope it helps - it sure helped me.
“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