Previous message:
> The signal that they use to position the needle is outside of the
throttle
> butterfly and, therefore, does not compensate for changes in manifold
> vacuum. Manifold vacuum is a primary indicator of engine load. An
engine
> at low rpm with the throttle wide open (low vacuum, high load) and an
So you are saying that throttle position has no influence on the signal
you mention? By opening the throttle more, this signal does not get
exposed to more manifold vacuum?>snip
Ulix,
SU carburators are called constant velocity but they are more accurately
constant depression carburators. Assuming no pressure loss in the filter,
the position of the dashpot and needle is adjusted to maintain a constant
pressure difference between atmosphere (the front of the dashpot piston)
and the venturi (the bottom of the dashpot piston). Both of these are
outside of the throttle butterfly so throttle position has no direct effect
on either signal. This signal (pressure difference) is generated
exclusively by the venturi effect of the dashpot cylinder and this is
dependant primarily on the mass volume of air moving thru the venturi.
At low speeds and high loads the engine will be at high volumetric
efficiency because, with an open throttle, the manifold vacuum will be very
low so the cylinders will fill more completely. However, due to the low
rpm, the total amount of air flow will also be fairly low so the dashpot
won't lift very high and not a lot of fuel will be delivered. At cruise,
the manifold vacuum will be higher so cylinder filling would be much less
efficient so, even though the engine is running faster, it is not consuming
any more air and the dashpot level would be the same as the low speed, high
load situation. The engine would recieve the same mixture even though it
could tolerate a much leaner one at cruise or demands a much richer one at
full power. You can only choose one setting so you should tune for worst
case (high load) which means that SU's cruise rich.
I am sorry if this dissapoints anyone (I mean no personal disrespect to
anyone but it does seem that anytime a shortcoming of British engineering
is pointed out people get defensive). Carburators are always a compromise
and even the best is only perfect in two or three narrow loading
conditions. As I said before I think SU's are a very elegant solution to a
complex problem. They have very few moving parts and provide excellent
driveability when properly tuned. Another carburator would only be trading
compromises. If you really want to improve efficiency, I am sure that
there are companies out there who would be more than happy to trade some
negotiable assets for a closed loop programmable fuel injection system and
the dyno time to tune it. Then your MG would run just like a M**ta ;-)
Regards,
Bill Eastman
61 MGA with character and modern iron with none.
|