To find the air velocity
in each runner we divide the flow in each runner by the area of that runner we
find that the average velocity of air in the intake is only on the order
of 110 fps or approx 75 mph. Now most induction theory says you
want an intake velocity on the order of 300 fps,
Ed;
Where does that number
come from, and what does it mean? Or maybe the better question - Where
should the velocity be 300 fps, and over what length? That is a very high
velocity, and if it occurs over a very long length of runner, there will be
pressure loss. As I understand it, in a piston engine with valves, it is
good to have that kind of velocity entering the combustion chamber for good
mixing and some interia, and maybe at the injectors (usually at the ports). In
the rotary, as I understand from talking to Paul Yaw, relatively high velocity (I
don’t know how high) is good as it enters the port for the same reasons, but
the port area is rather large giving velocities much less than 300 fps.
Velocity in the runner
contributes to the momentum as the air is flowing into the chamber, which can
be helpful, but the offset is that it stops when the port is closed and has to
be accelerated again. Between that, and frictional and turbulence losses in the
runner, I’d just guess that 100 fps in the runner is plenty high enough.
Maybe keep the average velocity relatively lower, and use the dynamic wave to
add some energy into the port just before it closes.
I have a 5” long,
44mm (~1.73”) dia 3-barrel TB on my 3-rotor; one barrel/rotor. At 6000
rpm that gives me a bit over 140 fps in the TB. Based on my limited dyno
experience, I think there would be a bit more high-end (5000 up) power if the
TB diameter were larger.
Or maybe the issue is –
the TB barrel area is 2.34 in.2; from there it is into a short
manifold where it divides into primary and secondary runners with a total area
of 3.88 in.2 which matches the port area on the housing fairly
closely. So the velocity is relatively high in the TB (where the
secondary injectors are), which is good for vaporization and throttle control, but
then slows to about 85 fps (at 6000) in the runners and entering the ports.
Let’s see; does
that all go into the category of rumination, of little interest to anyone? Well;
whatever, it is built and it works fine. I suspect I will never be as
ambitious as you, ED, and rebuild the manifold 2-3 times.
Al