Bill-
I've been deleting this thread up to now, as the first was about Jap bikes,
and in that I have no interest...
Has anyone mentioned "volumetric efficiency"?
>1) This is a four stroke engine. Exhaust "cycle" and intake "cycle" are
>mutually exclusive. Exhaust valve open, exhaust gases leave. Intake Valve
>opens, in comes the fuel/air mixture. So what does it matter that the
>exhaust leaves fast or slow (as far as mixture is concerned)??
To address this point, they are not exclusive. There is something called
valve overlap. The exhaust valve is still slightly open (depending on
cam), while the intake valve is opening. Headers are sometimes referred to
as "extractors". With a properly designed header system, the rapidly
departing exhaust gases create a slight negative pressure in the combustion
chamber, which helps with "cylinder charging" of fuel/air mix from the
intake side.
Without that negative pressure, some exhaust gas will be left behind in the
chamber. Hot gases mixing with the cool incoming charge prevents a full
"charge" entering the chamber. Also, the increased temperature decreases
the density of the charge. With lower temperatures, density increases,
more fuel/air mix to be compressed. that's also the theory behind ducting
fresh air from outside the engine compartment to the carb. (My Lotus
Cortina MKI had it's original air cleaner mounted in front of the radiator,
like the Volvo P-1800ES, for this reason, as well as for space limitation
reasons).
Think of the extreme case of changing the speed of flow of exhaust gas-
Did you ever put a serious dent into a tail-pipe? The car would barely
run, because it could not pull a good charge of fuel-air into the
chambers... The old "potato-in-the-tailpipe-trick" yields the same results
for the same reasons. Also, for those of us in the snow-belt, backing your
car into a snow-drift is similar.
Take the opposite extreme case, increase the speed (and volume) from the
intake side (Turbo), and you can theoretically end up with a gas turbine.
Or was this all covered in something I deleted... sorry...
Now as to the leaning of the mixture... I THINK that has to do with the
basic mechanical inefficiency of the carburetter itself. Because the air
portion of the fuel air mixture has a different density than the fuel
portion, with the increased speed of the air, MORE air is brought in, but
"not-quite-more" fuel is concurrently brought in.
Bob Woolner
Hillsboro, NH (snow-belt cinched tight several months of the year)
70 Midget (with velocity stacks and foam uni-filters taking in hot
underhood air)
65 L-C MKI (someday with colder outside-hood air intake)
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