My intake TWM intake from block to thottle body was the lenght of the
Racing
Beat Weber manifold - maybe 6-8", so a little short for the LeMan's
type
output at 6000 rpm. But sure would like to have that 240 horses.
Sigh,
guess, I'll just have to stick that turbo on, next..
Maybe others, like Al, will have better results than I did.
Ed
Its the longer intake runner Ed. Not the smaller tubes. People have
been talking
about this for years and every few years Mazda increases the port area
and
reduces the velocity and the engine generates more power.
Tracy went into a state of shock and disbelief when I first told him
the RX8
engine had 30% more intake port area than even the big port 3rd gen
engine :)
If you want a good book get Taylor's "The Internal Combustion Engine
in Theory and Practice." Also H. Heisler "Advanced Engine Tech" a book
I have mentioned on here many times. Intake air velocities can
approach
supersonic speeds. It is not as simple as it looks.
Wall friction in the runner plays a major role. The larger the runner
the
lower the wall surface area to volume ratio becomes and the higher the
flow.
The simple way of looking at it is the ratio of the circumference of
a circle to the area of a circle. The area is pi R squared while
the circumference is 2 R times pi.
Velocity cuts two ways. When the port opens the mass of air
must be accelerated. When the port closes the kinetic energy
in the air mass due to it velocity is converted to potential energy
in the form of a lump of high pressure air. See the rubber ball
analogy
in
an earlier post of mine. This then rebounds and comes back hopefully
just as the port opens again. Larger runner, larger ball,
lower wall friction losses relative to the size of the ball.
It's like getting hit with a Mac truck going 50 MPH verse a Honda
going 70 MPH. They both have the same energy but the Mac truck is
seeing
less friction. One of the rationales for using plastic intake runners
is they have smoother walls than castings. One of the reasons
I designed the manifold in small sections was so the insides
could be polished. Extrude Hone is an expensive process but it
can be used and is used to polish the insides of production
cast aluminum manifolds.
The problem with going on what shade tree engine builders say is they
cannot possibly duplicate what Mazda can duplicate with dozens of
dynos
running millions of combinations with computerized data acquisition.
Kenichi Yamamoto recognized this problem from the get go and invested
in
a state of the art computerized test facility way back in the late
sixties.
Imagine what he was confronted with. A brand new engine concept that
absolutely no one knew anything at all about. I digress.
Paul Lamar
At what RPM would you be getting supersonic airflows?
Chris C.
microcosman.com/aircraft
Good question. Lets say the displacement is
80 cubic inches per revolution.
Volumetric efficiency E = 100% Mazda rotaries can do better
than that by a bunch.
Never the less 80 times 6000 RPM is 480,000 cubic inches per
minute. Bear with me as I am thinking out loud.
Lets say the area of all four pipes is 8 square inches
for starters. Each pipe is 2 square inches.
480,000/8 = 60,000 inches per minute.
60,000 /12 = 5000 foot per minute
About 60 MPH Average. Peak speeds could be twice
that or more.
Still sounds low. Check my arithmetic. Perhaps I made a mistake.
Perhaps my approach to the problem is also wrong. I suspect it is.
Taylor says on page 203 Vol. 1 inlets average .5 Mach for high VE engines.
Another way of calculating it is to use the max speed of the rotor face
moving away from the port adjusted by the port area to rotor face
area ratio. It is really hard for me to figure the max rotor face velocity
right at the moment :) There are some charts for this in the book
"Rotary Engine" as I recall.
For a piston engine peak inlet air speed in fps is peak piston speed
in fps times piston diameter squared divided by port diameter squared.
Gas speed = PS X (D^2/d^2)
According to the book "Scientific Design of Intake & Exhaust Systems".
Page 78.
They don't say whether D and d is in feet or inches.
I suspect inches. I guess the ratio would be the same
so it may not matter.
You can figure the peak piston speed by knowing the stroke and RPM.
Peak piston speed inches per minute = stroke X pi X RPM
Note also this is not the speed of the pressure wave which does indeed
move somewhere around Mach one depending on a couple of factors.
Paul Lamar
Sonic velocity is roughly 1100 fps or 66,000 fpm I seriously doubt that
apiston speed and area ratio would approach that. As you say, wave velocity
is essentially sonic for tuning, but that is not flow only wave travel.
Vance J
According to Taylor .5 Mach has been measured in a production engine.
If the pipe or port gets too small I suspect close to one Mach may be possible.
Now the VE might be real low and the engine not generating very much power
but it might be possible... especially if over driven.
BTW thanks for not jumping in here and obfuscating the equivalence issue :)
I was quaking in my boots with the thought that you might:)
Paul Lamar
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