[TR] Vacuum advance/retard

[Jim Muller]

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

> > The ultimate power limit is still the same, VA or no.

> Right.  VA just gives better fuel mileage at cruise, and possibly
> somewhat improved part-throttle response.

Which that's some of wot I said in my first note, now didn't I?  The 
trimming process left out the description of why bigger engines had 
VA but little Spitfire engines didn't.

> Most if not all VA ports are 'timed', meaning they exhibit little
> or no vacuum at idle (so no advance at idle).  This is done by
> putting the port right next to the butterfly, so with the butterfly
> fully closed, the port is on the upstream side.

Ah!  That makes perfect sense too, thus preventing VR and VA trying 
to counteract each other at idle!  Idle is such an extreme running 
condition that it requires its own calibration anyway.  I've seen few 
if any cars, certainly never owned one (that I remember), that had 
both VR and VA.  It was always whatever it was - when a port 
disappears into a carb's innards, it goes wherever it goes.  Thank 
you for that clarification.

Actually, that VA off-at-idle behavior should make it even easier to 
distinguish whether a port is meant for VA or VR.  One should be 
obviously off at idle, going on quickly with throttle, the other on 
at idle, going off quickly with throttle.  If I absotively posilutely 
had to tell them apart, I'm sure it would be easy to test.  Anyone 
who hooks them up backwards would have to do so without thinking 
carefully.  Then again, lots of people do that, don't they?

Thanks again.


-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+