I am
apparently not doing a very good job making the point about the relationship
between throttle body size, throttle travel and engine power clear. So
here is another try at it - using extreme examples and no math. Let’s
assume your volumetric efficiency is 100% (no losses).
IF you put a
½ ” dia TB on you engine. You could have it wide open and your engine
(under normal prop load) probably wouldn’t turn 4000 rpm. That is because
even wide open, the ½” dia TB restricts airflow sufficiently - that the
manifold air density never approaches the ambient air density. Since we
know that the engine power is directly proportional to the density of air in
the combustion chamber – and this density is limited in this case to less than
ambient, you engine is not going to produce much power, certainly not full
power. So this bit of information tells us “Bigger Lithium Crystals,
Scotty!!” – i.e try a larger throttle body.
Now if you
keep enlarging the diameter of the TB you would find that at fully open -your
engine would be producing more power than it was, but perhaps still not the
maximum power it is capable of. This is because the air density in the
manifold has increase due to the less restrictive flow, but is still below
ambient. This is due to the better, but still restrictive effect of the
TB size on the air flow. Now if you continued enlarging the TB size, you
would reach a point where with the TB just reaching fully open - your manifold
air density is exactly ambient and your engine is producing all the power it is
going to.
Now if you
enlarge the TB even further, you will simply find that you can cause the
manifold pressure (air density) to reach ambient without opening the TB
fully. Its simply large enough that all the air the engine can use
(ambient air density point) is met at partial throttle opening. In
fact , you can certainly continue to advance the throttle thereby opening the
throttle plate even more – but, you are not going to increase the air density
in the manifold and therefore you will not produce any additional power for
that additional throttle travel.
Bigger
Throttle body’s result in more power only up to the point the airflow they
permit causes manifold air density to reach ambient. Beyond that point,
the only thing they do is provide frustration – by having all that throttle
travel remaining which does nothing to produce more power {:>).
Now if you
can somehow lighten the load on your engine, then engine can turn faster
providing more “suction” on the manifold volume reducing the air density below
ambient, now opening your “oversize” TB a bit more will produce more power
because you are increasing the airflow again to the point where the equilibrium
point between ambient air density in the manifold and rpm is again
reached. Lighten the load further and you can again increase engine power
by opening your TB more. Etc, etc.
Ah, ain’t
this hobby wonderful {:>)
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm