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Re: mechanical advance

To: ulix@u.washington.edu, jboatri@emory.edu
Subject: Re: mechanical advance
From: Ronsoave@aol.com
Date: Thu, 27 May 1999 09:19:39 EDT
Cc: spridgets@autox.team.net
Reply-to: Ronsoave@aol.com
Sender: owner-spridgets@autox.team.net
In a message dated 99-05-27 01:53:51 EDT, ulix@u.washington.edu writes:

> Jeff,
>  supposedly, your gas mileage won't be a s good.
>  I think that is the only purpose of the vacuum advance/retard units
>  (probably emissions too).
>  Ulix

Why:
The static advance can vary from 2 to 9 degrees BTDC on LBCs I've played with 
(I think Spridgets are 5?  No specs handy).  When idling at 900-ish RPM the 
mechanical advance is kicking in an additional 4 or 5 degrees of advance (it 
depends on the dizzy's weights, springs, and mechanical advance.  The trick 
is not to over-advance the timing to detonation knock or ping).   When 
running at 1000 rpm, the mechanical advance is adding another 4 to 5 deg 
advance, so assuming we have no vacuum assist the timong would be at around 9 
BTDC.  Each distributor has a different advance characteristic based on its 
weight , springs, etc.  So we got idle timing.  At high engine speed, say 
4500+ RPM I think the 1275 should see around 30 to 35 degrees advance, then 
it will typically stop advancing (the centrifugal dizzy on semi-permanent 
loan from Frank in my car keeps advancing well past 4500 RPM.  Bless you, 
Frank).  The advantage, or reason, for the vacuum advance is at the low speed 
where the dizzy is not providing a lot of "mechanical/centrifugal" advance.  
The engine can tolerate a lot more advance at light throttle conditions, such 
as when you first come off idle, or are doing around town puttering.  Vacuum 
advance provides the boost you need or can tolerate without the high engine 
revs.  If you have a centrifugal only setup, the idle timing setting is 
advanced well beyond what you'll have with a vacuum setup to accomplish this, 
and I've found I lose that "putter around" driveability, but the car does, as 
Frank said, feel snappier otherwise, especially when the dizzy keeps 
advancing at 5000 RPM.  What I had to do to get driveability was to advance 
idle timing so that the car would barely start, then back it off a few 
degrees. The thing to always beware of is DETONATION. My changeover to 
centrifugal-only came with carb and cam change, which changed the 
driveability (and added SPEED) anyway.  If anyone is running a centrifugal 
only dizzy with a more stock setup, maybe they've gotten away with lower idle 
settings.

Ron

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