[TR] 1980 Spit: vacuum retard

[Jim Muller]

On 2 Jun 2009 at 9:38, Tim Gaines wrote:

> Has anyone ever been able to repair a vacuum advance or 
> retard unit.

Nope, but I'm not sure it's worth the trouble.  Well, if it is rigged 
to produce a retard at idle then it is part of the emissions system, 
so that's a reason, more or less.  But without the rest of the system 
in place the effect is minimal.  When my '80 Spitfire's air pump 
seized a long time ago I disconnected everything, then ran it through 
state inspection with the vacuum retard hooked up and not hooked up.  
It was marginally cleaner with the retard, so I left it in place.  
But it was not so much dirtier that I would have worried about it if 
the diaphram was broken.

If the diaphram provides advance via manifold vacuum, it is for extra 
power at part throttle, sort of.  More specifically, it advances the 
ignition a bit when the intake air charge is less dense due to the 
throttle being not open fully.  This give you more power at part 
throttle, but it isn't really necessary because if you want more 
power you can always just open the throttle further.  When the 
throttle is fully open you have no vacuum advance at all, i.e. the 
same as if it was disconected.  So what's the point?  It gives you 
marginally better fuel mileage at part throttle.  Does that matter in 
a Spitfire?  Well, of course, but not so much as for a heavier car 
with a bigger engine.


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