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