Cliff Manzke wrote:
Dear Paul:
I've been quietly listening to the various discussions taking place on this
forum for a couple of weeks now and decided I must speak up on this one. I
have to disagree with you on the issue of throttle body size and manifold
size.
If you use a single throttle to control flow for the entire engine, the area
of that throttle should be about equal to the port area of only one rotor
and not the entire engine. This is simply because only one rotor breathes
at a time (no overlap). In the case of the side port 20B (Al Gietzen's) the
port area is 1.236 inē for the primaries and 2.356 inē for the secondaries.
This gives a total area of 3.592 inē per rotor, which equates to a diameter
of 2.138 inch. I assume the 13B's have about the same port area. With this
in mind, the 2 5/16 inch Nissan part should be more than adequate, and
perhaps even too large.
As for the intake manifold runners, bigger is not always better. The two
conflicting requirements are for high kinetic energy and low pumping loss
(friction). For a given runner length, kinetic energy is proportional to
1/dia^2 and suggests that smaller is better to increase ram "supercharging."
On the other hand, friction is proportional to 1/dia^5 and suggests that
bigger is, in fact, better. A small increase in diameter, however, results
in a huge decrease in friction and only a moderate decrease in kinetic
energy, so the best compromise exists somewhere in the middle. This is why
reducing the intake diameter can (and does) work in some situations.
A related subject here is that of Air/Fuel ratios. The ideal
(stoichiometric) air/fuel ratio is about 14.5 for gasoline. In reality, it
is impossible to burn all the oxygen in the combustion chamber with a
stoichiometric mixture due to imperfect combustion, imperfect mixing of air
and fuel, and both cycle-to-cycle and cylinder-to-cylinder variations in
air/fuel ratio (variations of 5% in both air flow and fuel flow are normal).
Maximum power is reached when all the oxygen is consumed, which requires
some excess fuel. There are many other factors involved here (cycle
temperature, volumetric efficiency, heat transfer, etc., etc.) but you get
the idea. The point I wanted to make is that of complete mixing of fuel and
air is important for making power. Increases inlet air velocity does
improve mixing and therefore plays into the optimum manifold size.
I apologize for being so wordy here, but I thought my two cents might help.
Just don't get me started on turbochargers!
Cliff Manzke
Mechanical Engineer
Engine Guru
Cliff how do you explain the excellent Mazda Lemans four rotor NA results? One full
size opening per rotor. Huge pipe diameter. Nobody else has come close to the
NA power of the Mazda Lemans four rotor engine. Power Sport is getting 215 HP NA
with one butterfly in each sammler diameter rotor intake runner. Not quite up to
the Mazda Lemans engine but getting there.
Not only that the Lemans engine had to have good throttle response as it
was running on a road course. True it had a variable intake pipe length
but I assume we would be using the maximum length for our application.
The skin friction drag in an intake pipe is proportional to the
square of the velocity. There is quite a bit of turbulence in the process of the intake port
opening to mix the air with the fuel. Also if the injectors are at the start of a long
runner this too helps mixing.
Why not Cliff. I would love to hear what you have to say on turbo charging.
Paul Lamar
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