Being on the digest, I am probably too late with this but here it goes.
Most distributors take there vacuum from a port just outside of the
position that the throttle butterfly takes at idle. In this fashion, there
is little vacuum at idle but once the throttle is opened slightly, the port
is behind the plate and sees full manifold vacuum.
Why not have full advance at idle? The engine runs smoother retarded. At
idle you have very poor fuel mixing and distribution. Rather than run at
optimum timing and have the good charges really push and the poor charges
fizzle, the spark is retarded to even things out- everything combusts
"poorly" although this is not the actual case since the piston speed is
slow enough to allow complete combustion even with the retarded timing.
The less efficient combustion also make tuning at low rpm's easier since
larger changes in air or fuel flow produce smaller changes in engine speed.
If you don't believe me, try a little experiment. Take your ported vacuum
advance and hook it to the manifold. The engine will speed up and it will
run rougher. I learned all of this (the hard way) when I converted a 76
Tr$$$ph from vacuum retard to vacuum advance.
A reason that MG moved away from ported vacuum to straight manifold vacuum
when they started instituting emissions controls may be that these controls
destroy low RPM vacuum making ported vacuum unnecessary. Or maybe they
changed timing specs to the point where the car wouldn't run at idle
withour a little help. Or maybe the combustion temperatures weren't right.
In any case, I am sure it was a band aid brought about by the emissions
tuning, not a sudden revelation that ported vacuum was a bad idea.
So, Barney, your position as one of the reigning MG guru's remains intact!
Regards,
Bill Eastman
62 MGA huddled against record low temperatures
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