[TR] Vacuum advance/retard

[Jim Muller]

I sent this before but uncle mjb's size filter cut it off.

On 20 Dec 2007 at 11:23, Nolan wrote:

> While vacuum advance can have some running benefits, vacuum
> retard does not.

About VA, when you open the throttle the vacuum *diminishes*.  At 
wide-open throttle VA does nothing at all.  Ideally you set the 
timing for best full-power, regulated only by centrifugal advance 
which acts on engine speed, and that's the best it can be.

So why VA?  When the throttle is partly closed, air entering the 
cylinder is less dense so the flame front spreads more slowly.  So 
you can afford to advance the timing to get more power *for that 
particular throttle setting*.  Without VA the timing is delayed from 
ideal, but if you want more power at any time while driving you 
simply push the loud pedal more.  The ultimate power limit is still 
the same, VA or no.

Vacuum retard was for at-idle only, to improve emissions.  As Nolan 
wrote:
> ...nothing on the vacuum cans to indicate which...

True, but you can figure it out on the dizzy.  Note which on side of 
the diaphram the hose attaches.  That's the direction the diaphram 
will move when vacuum is present.  If it pulls the dizzy base plate 
the same direction as the dizzy shaft spins then vacuum will retard 
the timing.

At the other end, if the port is on the manifold or the base of the 
carb its vacuum will go down progressively as the throttle is opened, 
so it must be meant for VA.  If the port is right at the butterfly 
its vacuum goes away completely as the throttle is opened even a 
little, so it must be meant for VR.


Jim Muller
jimmuller at rcn.com