There are two types of vacuum advance - what might loosely be called
'British' advance and 'North American' advance.
The Britsh advance is non-existant at idle, maximum under cruising rpm at
low throttle openings, and negligible at high throttle openings.
The NA advance is maximum at idle, otherwise it is much the same as British
advance.
British vacuum is taken off the carb right by a throttle disc, NA vacuum is
taken off the inlet manifold.
Even so, British tuning instructions also state that the vacuum line should
be removed and plugged for setting dynamic timing even though the vacuum at
tuning rpm is negligble.
PaulH.
-----Original Message-----
From: MSLishego <mikesl@tartan.sapc.edu>
To: mgs@autox.team.net <mgs@autox.team.net>
Date: 05 January 1999 22:46
Subject: What IS Vacuum advance?
>...And what does the vacuum advance do?
>
>I've been wondering this for a while. This summer, I found a NOS
>vacuum advance unit for my 45DE dizzy. I figured that, paired with my
>electronic ignition, I ought to get some good results. I checked for
>leaks and found none, then put the unit in the car.
>
>After driving the car for a while, I realized that this unit was
>costing my car power! Even though it was hooked up to factory specs
>and there were no air leaks, it still caused my car to suck. My theory
>was the it was an emissions-related change that made the car run
>cleaner, but with less guts.
>
>So, since we've got so many great experts on all things automotive, I
>was hoping to get a dissertation from one of you about what a vacuum
>advance does, how it affects car performance, and why they were fitted
>to the car.
>
>And of course, I'll be willing to listen to arguments as to why I
>should hook mine back up... 8-)
>
>Thanks in advance for any info...
>
>Mike Lishego
>1974 MGB
>1986 Plymouth Turismo 2.2
>http://www.geocities.com/MotorCity/Speedway/3706
>
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