Ign Timing 101
Transcript of Ign Timing 101
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1 Courtesy of VW Trends March 2001
IGNITION BASICS
IgnitionTiming101GettingBeyond the 009
Part I:Ignition Theory and Distributor Operation
Y
On the left, the weights are all
the way in, like at idle speed.
On the right, the weights have
pulled out, like at high RPM.
Note the change in the
angleif you were precise
and measured it, then dou-bled the difference, youd find
the difference in timing from
idle speed to high RPM.
Here is a centrifugal advancemechanism. Note how the
weight assembly splits the
upper and lower halves.
STORY AND PHOTOGRAPHY BY SHAD LAWS
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2 Courtesy of VW Trends March 2001
FACTORS THAT EFFECT TIMING
AN OVERVIEW: TW O
TYPES OF IGNITION CON-
TROL
PROPER MECHANICALIGNITION TRIGGERING
Here are the qualitative plots of what our
approximations look like.
OK,now for a brief dis-
cussion on fluid (i.e.liq-
uid or gas) flow. Fluid
flow from one place to
another is highly depen-
dent on the pressure dif-
ference between the two
places.When the intake
valve opens,the cylinder
pressure is quite low.If the
pressure of the manifold
is also relatively low,there
isnt much flow into the
cylinders and you have a
low VE.How is the pres-
sure of the manifold con-
trolled? When the throttle
is nearly closed,the pres-
sure of the manifold is
quite low.
So,does it make sense
that manifold pressure
can be seen as a measure
of load? Low pressure =
low load and high pres-
sure =high load.
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ALTERING WHEN
TRIGGERING OCCURS
3 Courtesy of VW Trends March 2001
SUMMARY
Here are different types
of distributors:
Centrifugal-only (or
incorrectly mechanical-
only): has centrifugal
advance
Vacuum-only has vac-
uum advance
SVDA:has both vacuum
and centri fugal advance
(i.e. single vacuum,dual
advance)
DVDA: has vacuum
advance, centrifugal
advance, and vacuum
retard (i.e.dual vacuum,
dual advance)
These are different vacuum canisters, from left, a late model dual vacuum unit, a
late model single vacuum unit and a 36-hp fully adjustable single vacuum unit.
A close up of a 36-hp fully adjustable single vacuum canister. Note howmaximum
vacuum advance is controlledthe nut on the arm only allows the arm to move
so far before hitting the canister.
From left, you
can see how a
set of points is
opened by the
cam lobes. Note
the positioning
of the vacuum
canister arm and
the shaft and
how moving
them would alter
timing. An
assembled late
model stock unit
with a dual vacu-
um canister. An
assembled
009 unit that
lacks vaccum
advance. The last
distributor is not
advisable for a
street engine.
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