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When the TB is opened suddenly from idle or near idle, the the
very low pressure inside the manifold is replaced by a sudden inrush of
local air pressure. So should the local air moving through the manifold
not contain enough fuel to provide a viable fuel/air mixture, the
engine starts to die.
But, as you have noticed, the situation seems worse than that. Not just a stumble.
At very low throttle openings, the available air for cylinder filling
is far below the actual displacement of the engine. So, the amount of
air available to compress before combustion is also very low, so, the
effective compression ratio at idle is likewise very low.
Just fine for running smoothly at 700 RPM and little to no load.
So now we slam the throttle open, the inrush of air roars down the
runners and into the chambers with far less than an ideal mixture, and
what is the effect on cylinder filling for the next revolution?
Why it is outstanding. Maybe over 100%. And it is too lean to light.
So when you see that old guy in the Champ, who always kills the
mags and goes to full throttle at the same time, as he was told to do
back in 42,
what is he doing? He is going from a very low cylinder filling idle
with low effective compression ratio, with a carb that has no
accellerator pump,
or, very little accellerator pump, to great cylinder filling and a high
effective compression ratio and poor to well over lean mixture, and pop
pop. The engine stops in two blades with little to no fuel in the
cylinders.
And its the original engine, and it has good compression, and its no
use telling him otherwise, because he learned it that way, and that way
it will stay.
Same thing for the rotary. You come in a bit hot, with the power
(throttle all the way closed) off and in a beautifull forward slip, and
it is going well but a bit too well as it looks like touchdown will be
just a hair short of the numbers, with your moron friends watching who
will settle for nothing short of a great landing, you snap out of the
slip right on the centerline, and after waiting a bit too long you pop
open the throttle to extend just a bit, and you get nothing at all. So
then a bunch more throttle and nothing, and just as the mains touch a
bit too hard, the damn power comes on like a top fuel dragster, and you
fly down the centerline a hundred yards, to the same landing you
commited at the end of your first solo, 43 years ago.
The carburretors cure this with the accelerator pump and jet system, in
the same way a big hammer can kill a gnat. It cures it a bunch, with
enough fuel to go around the patch once shot into the hole in a long
hard stream. Mixture be damned, its way over rich and lasts longer than
is ever required for the lean low rev situation. The carbs with no
accelerator pump systems have other bleed air gags to do some of the
same thing. If you screw with the idle jet too much you can make them
act the same as the overlean carb.
At low revs an over rich mixture is a happy mixture. It lights easy and
burns slow. It reduces the possible detonation of the low RPM big
throttle setting (looks like over advanced timing to the engine). Power
comes on as the throttle moves, not after it is at the stop and about
to go into burner.
Lynn E. Hanover
Subject: [FlyRotary] Re: Throttle body size/ other "Paul" issues
Humm, that very interesting, Lynn. The primary
reason I removed the 75 mm TB and reinstalled the 65mm TB was that the engine
would "bog" upon rapid opening of the throttle with the larger dia
TB.
If this was not caused by the large TB then I guess
I'm wondering what was causing it as the rest of the induction system was the
same. My thinking (apparently in error) was that the sudden change in
"effective" area by opening the large TB had a momentary impact on the air
velocity in the runners thereby causing the
hesitation..
Ed
----- Original Message -----
Sent: Wednesday, May 02, 2007 1:58
AM
Subject: [FlyRotary] Re: Throttle body
size/ other "Paul" issues
On further review,
TB size should have no affect on throttle
response at all in an injected system.
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