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 ˝ ” 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 ˝” 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