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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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