<VV> vacuum advance miracle

[djtcz at comcast.net]

Vacuum advance can have a several mpg effect on fuel mileage, but it's simultaneous influence on part-throttle response is usually overlooked or underestimated.

There are those who cry for "more advanced" timing, and give the distributor a slight twist.

I (and, by the sounds of it, your 110 engine) would prefer the "right" advance for a particular throttle setting, mixture, rpm, and temperature (when I can figure out what the heck it might be).

Then look at the kind of curve attainable with a basic mechanical centifugal advance curve - http://members.aol.com/DVAndrews/timing1.gif
Add 2 stage springs and we can put a kink in the basic curve. 
http://members.aol.com/blue317/advance.jpg
http://www.mgcars.org.uk/mgcc/sf/images/elect3.jpg

Here is an example of what an engine might REALLY like it's timing to be.   http://www.epicycle.com.au/images/map2.gif
It's not too tough to map out those 400 data points on the dyno and then make a look up table for the ignition computer to check every few milliseconds.  
Look at the curve along the 100 (% load, full throttle) line and disregard everything above 6000 rpm. 

Trying to simulate even that section of the 3D curve with mechanical advance's 1 or 2 straight line slopes means there are either going to be some over-advanced operation, or certain rpms that are significantly under-advanced.  
--
Dan Timberlake

-------------- Original message -------------- 
Date: Fri, 1 Jun 2007 06:22:08 -0400
From: "Secular" <rusecular at yahoo.com>
Subject: <VV> vaccuum advance...
  Hi:

  Recently I had to replace the vacuum advance unit in my 1966 Corsa 
  (the engine has been replaced with a rebuilt 110). 

  I found a unit locally which belonged to a 1962 102 HP engine. 

  Now, the car runs better than it ever has. Is there a book and/or article 
  so I could find out the  difference between the various vacuum advance 
  units used on Corvairs?

  Regards,


  Tony I. 
  Charlotte, NC