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Re: 1978 MGB - Mallory Distributor & Thinking in a Vacuum

To: boballen@sky.net, CREICHLE@nsc.msmail.miami.edu,
Subject: Re: 1978 MGB - Mallory Distributor & Thinking in a Vacuum
From: barneymg@juno.com (Barney Gaylord)
Date: Thu, 10 Apr 1997 08:32:25 EDT
I knew this would start a lively discussion.  I haven't been quoted so
much in ages.  Start with a slightly vague concept that is not generally
intuitive in nature, and everyone looks at it in a slightly different
way.  I've combined several posts here, so if you can't take three pages
of technical chit chat, delete now.  And please, don't anyone quote this
whole post in response.

On Tue, 08 Apr 1997 23:28:31 +0100 Robert Allen <boballen@sky.net>
writes:
>Barney Gaylord wrote:
>>  <snip>
>> Ported vacuum is low at idle, comes on strong on acceleration at low
speeds, then increases slightly with engine speed.  Manifold vacuum is
high at idle, drops dramatically under hard acceleration, climbs again at
higher RPMs, and hits the highest when you close the throttle. .....
>
>Hmmm. I was thinking just the opposite. Perhaps I have my definitions
screwed up. Ported vacuum is usually taken just behind the throttle
plates of normal carbs. Vacuum only present at part throttle openings.
Thus *high* at idle, moderate at cruise, and negligible at full throttle.

Ported vacuum comes from a drill hole that is just ahead of the throttle
plate at closed throttle, but ends up behind then throttle plate when the
throttle os opened.  As a result, this vacuum is zilch at idle and high
at mid range.  This vacuum is also assisted by venturi action, as the air
velocity increases, so does the vacuum.  So, in a Weber carb with ported
vacuum, it will continue to increase with speed as the air velocity
increases.  In the SU carb with constant velocity near the jet, the
venturi effect is nearly constant.

On Wed, 09 Apr 1997 10:24:00 -0700 (PDT) "REICHLE, CHRISTOPHER"
<CREICHLE@nsc.msmail.miami.edu> writes:
>Barney, Are you sure?...
>
>**spark should _retard_ when you hit the gas WOT due to the increase in
turbidity and density of charge. As the name implies... vacuum advance
units, advance spark timing when there is a vacuum. At WOT vacuum is low
and therefore there will be less advance, retarded.

Ah, yep, pretty sure.  Note paragraph above on ported vacuum source. 
With a ported type vacuum source the vacuum increases with engine speed.

For good power output and efficient burning you need quite a lot of spark
advance, as it takes some time for the fuel to burn, and you would like
it all burned by the time the piston is just past TDC.  Problem is that
at idle the throttle plate is nearly closed, very little air gets in to
the engine, the fuel/air charge density is very low, and therefore hard
to ignite.  You have to wait for the piston to get nearly up to TDC to
increase the density by compression before the spark can start the fire. 
(This is why it's hard to start when it gets tires and looses
compression.)

With too much spark advance at idle you get misfire and lots of engine
shaking, so it's limited to about 5 to 20 degrees advance at idle,
depending on your engine setup.  Now, just touch the throttle and the
inlet charge increases dramatically in density, so you want the spark
advance to come up right now for good torque response.  However, too much
spark advance at high density leads to pinging.

>> ..... Total spark advance should be limited to no more than 32 degrees
on a stock MG 4-cylinder engine.
>
>**On an 80 B the vacuum advance unit can bring in a max of 25 deg
advance at 10-15 Hg(book spec). The amount of advance depends upon the
amount of vacuum applied to the unit, not the rpm of the engine. While
rpm affects the amount of vacuum applied to the unit, the main factor is
throttle position, 

Good up to here.

>WOT : low vacuum,  closed : high vacuum.

Only with a manifold vacuum source, not with ported vacuum.  If you
combine manifold vacuum with a vacuum retard unit you get approximately
the same result as combining ported vacuum and a vacuum advance unit. 
Two negatives make a positive.

>Mechanical advance is not linear. More advance is added at low rpms than
at high where the advance tapers off to 0.

Mechanical advance can be linear or not depending on the springs and
weights in the dizzy.  Usually the advance plot curves upward slightly,
so you're right, quicker mechanical advance at lower speeds.  The
mechanical advance does however keep on increasing with engine speed,
until it encounters a mechanical limit in the dizzy at about 3600 rpm. 
Beyond that speed the mechanical advance curve suddenly goes flat.  The
Mallory distributors have an adjustable upper limit, can be set of
maximum anywhere between 22 and 38 degrees as I recall.

>According to the book, an 80 B with all the emissions crap can have a
total of 63 deg advance at 4500 that's combining vacuum and mechanical.

Misprint?  Maybe should be 36?  I can't recall any engine with a total
advance much over 40 degrees.  Maybe I haven't lived enough yet.  I can
always stand more education.

On Wed, 9 Apr 1997 09:46:11 -0500 (CDT) todd@nutria.nrlssc.navy.mil (Todd
Mullins) writes:
> ..... Also note that my '84 Volvo gives the brake booster its vacuum
from directly aft of the throttle body (Bob-Allen-"ported" style).  .....
The airflow redirection flaps .....  also get vacuum from behind the
throttle plate, but they use a vacuum reservoir for low-vacuum
conditions.

Honorable mention to Todd here.  I know nothing about Volvo carbs, but
I'm willing to learn.

On Wed, 9 Apr 1997 10:54:20 -0500 (CDT) todd@nutria.nrlssc.navy.mil (Todd
Mullins) writes:
>Robert Allen writes:
>> .....  I believe that you'll find the most vacuum the closer you get
to the intake valves.
>
>I still don't see why this is necessarily true.  Perhaps instantaneous
vacuum levels are higher nearer the valves, but integrating  ..... I
would think the pressures would reach equilibrium.  Does anybody here
actually know? .....

I'm an engineer, so I know integrating.  Vacuum level depends on position
in the intake path.  At idle there is a nearly even high vacuum all the
way from the butterfly to the top of the piston.  At full throttle you
get less vacuum (higher pressure) but a high velocity air flow all
through the intake system, resulting in pressure drop as you proceed
through.  As the pressure drops the density decreases, the volume
increases, and the velocity increases as you approach the intake valve. 
Lowest pressure (highest vacuum) is inside the cylinder.

Except ...!!  When you have a long, unrestricted intake runner and large
intake valve, as the piston comes to rest at BDC after the intake stroke,
the incoming air continues to flow by the inertia of its own mass.  In
this case you can get a positive pressure pulse giving a slight
supercharging effect in the cylinder.  The length of the intake runner is
important for maximum intake charge at a given engine speed.  Race
engines tuned to run best at high speed will not do well at low speed. 
These pressure pulses can also play hell with a vacuum modulated
distributor.

On Wed, 09 Apr 1997 12:12:52 +0000 Robert Allen <boballen@sky.net>
writes:
> ..... As for the ported vacuum, it is my understanding that its source
is as close to the throttle butterflys as possible such that, at idle,
the port is completely on the 'engine side' of the carb and, yes, ported
vacuum and manifold vacuum should read exactly the same. .....

Oops, Bob,
Reconsider the rest of your reasoning.  On the SU carbs the ported vacuum
port is on the bottom of the throat, and the bottom of the butterfly
opens outward, so the little port is outside of the butterfly when closed
and inside when open throttle.

'Nuff fer now,

Barney Gaylord
1958 MGA

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