From xi8940@freenet.tlh.fl.usFri Feb 14 09:36:36 1997
Date: Thu, 13 Feb 1997 17:51:36 -0500 (EST)
From: David Raistrick <xi8940@freenet.tlh.fl.us>
To: me! <y21cvb@armory.com>
Subject: Our Most Holy Distributor (fwd)
--
Mo.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.o.oM
Keeper Of The Light:
Pushing Back The Darkness at
http://www.armory.com/~y21cvb/west/darkness.html
ex'74 Super http://www.armory.com/~y21cvb
ex '68 Beetle
'75 Retired Westy-Canadian Model, Cody
'69 Westy-Itchigo xi8940@freenet.tlh.fl.us
'71 Squareback-Lady
---------- Forwarded message ----------
Date: Wed, 12 Feb 1997 05:19:19 -0800
From: Joe Perez <pinhead@GROVE.UFL.EDU>
To: Multiple recipients of list VINTAGVW <VINTAGVW@SJSUVM1.SJSU.EDU>
Subject: Our Most Holy Distributor
On Wed, 12 Feb 1997, Robin Newberry wrote:
> I don't understand this vacuum / mechanical dizzy system very well, or
> *any* dizzy system very well for that matter. Can some kind soul take me
> from utter ignorance to a state approaching dizzy enlightenment?
Pull up a chair, and put down an ale. Get comfortable, for we are about to
embark on a great and hopefully enlightning journey.
As we know, the distributor has but one function. To make sure the
spark goes to the right plug at the right time.
Selecting the right plug is easy. You just put a rotor within 20 degrees
or so of the right terminal and zap it. Yes, there is actually a cool
light show going on inside your distributor. That's why transparent caps
are so neat- until they break.
Timing the spark is another ordeal. Theoretically, in a world where fuel
and air combust totally and instantaneously, the spark would occur just a
few degrees after the piston has crossed Top Dead Center(TDC), such that
the crank was now in the correct position to reap maximum benefit of the
power applied upon it.
But this is an imperfect world. It takes time for things to start burning.
In fact, the time between when the spark is lit and the fuel actually
begins to produce power can be several milliseconds. You hard drive gurus
know how tedious *THAT* can be.
So in order for the engine to be most efficient, we need to light the
spark some number of degrees *before* TDC, while the piston is actually
still moving up. This way, it will begin to do it's thing just as the
crank crosses TDC, and the piston will be pushed smoothly down, propelling
your car forward.
That would all be so simple if everyone ran their engine at a constant
speed all the time. In fact, that's basically what the hybrid folks have
been doing for years. Makes engines very efficient. But we have to run our
engines at different speeds.
The problem is that while the time it takes fuel to burn is pretty much a
constant, the time it takes the crank to travel a given number of degrees
is not. The faster you turn the engine, the less time it takes. In the
time it takes your motor to go 7 or 8 degrees at idle, the NASCAR guys
turning just went 40 degrees. Mabey more.
Herein lies advance. The faster your engine is going, the earlier you need
to light the spark.
Unfortunately, there is a dark and sinister side to this equation. Light
the spark too early, and the piston won't have crossed TDC when the fuel
begins to burn. Result: The fuel pushes down on the still rising piston,
causing poor efficiency, bad karma, and engine damage.
I've written a page and a half without even mentioning the words vacuum or
centrifugal. I'm gonna grab another cold one.
Ok, the distributor mechanicals.
Inside the distributor is a squarish cam. 4 lobes. A set of "points" rides
this cam. When a lobe hits the points, they open. When the points open, a
signal goes to the coil, causing it to fire. Lot of electronic mumbo-jumbo
about invisible fields and such. This is how we time the spark.
There are two ways of making the spark happen sooner. Measure the engine
vacuum, or measure the engine speed.
The centrifugal is the simplest. The distributor is geared to always turn
at half engine speed. As the engine turns faster, so does the distributor.
Inside the distributor, the aforementioned cam is coupled to the
distributor shaft by way of a movable plate. This plate also has some
weights and springs under it. As the plate turns faster, centrifugal
action pulls the weights outward. They work on a lever which then causes
the cam to move slightly ahead of the plate. The faster it goes, the
further the cam runs ahead, and the sooner the points do their little
dance.
Houston, we have advance.
Enter the vacuum distributor.
Totally different concept. Here, the cam rides on a rigid shaft, and the
points themselves move. As if life weren't confusing enough.
On the body of the distributor is a large vacuum diaphragm. One side is at
atmospheric pressure, the other side is at carb pressure. Which, relative
to atmospheric, is vacuum. This diaphragm has a little rod which acts
upon the plate the points are on, causing it to revolve within the
distributor. The more vacuum, the more the spark advances.
So, as you go faster, the engine draws more air, and the spark advances.
You let off the gas, the spark goes back. Nice and smooth.
Isn't this neat? There's more.
The later model distributors, those found on most 1500 and 1600 cc
engines, use a combination of both cantrifugal and vacuum advance.
It is my humble opinion that they work best. I've run 009 centrifugals on
a stock 1600, a 1600 with an Engle 100 and a Weber progressive carb, a
1776 with an Engle 110 and the same Weber progressive carb, and an 1835
with some BugPack cam and dual Dellortos. And something tells me that
the late-model stocker is a smoother choice. Mabey on a bigger engine.
No hard data, no engineer's notes, just a funny feeling. The Force is with
me.
There is of course more, but this is the basics. ;)
-=>Joe Perez<=-
-=>pinhead@grove.ufl.edu<=-
Our Most Holy Distributor R