Spark Advance

[steven d. ferguson]

Dave,
I too have fought this battle. I hate TVS. Period. I've eliminated
this system on every car I've had that uses it and gone to a ported
vacuum source. Using ported vacuum, which is from a source located
above the throttle plates, the advance mechanism in the distributer is
allowed to move to suit the demands of the engine. When vacuum levels
are high, as in part throttle cruising, you have max advance. When
vacuum levels are low, as when the engine is under heavy load, you have
little or no vacuum advance and the engine operates under initial
advance, which is what you set with a timing light. There is an old
trick that still works better than anything other than a dyno for
setting initial advance, (or timing as it's sometime called), and that
is to set your initial advance to the factory setting and test it under
maximum load. Usually a long grade or steep hill. If you detect engine
detonation, retard the timing a degree & try again. Repeat this until
there is no longer any pinging during your testing. This is the optimum
setting for your engine. The other side of this test is if there is no
detonation at the factory setting, add a degree and try again. Still no
pinging? Add another and so forth. Bear in mind however, that this
method only works when the TVS is by-passed and you are using a ported
source of vacuum for the distributer. Also, take a moment to test the
vacuum advance mechanism by using a hand pump (MityVac) or sucking on
the vacuum line to the distributer while the engine is idling and note
the increase in idle speed. If there is an increase with the
application of vacuum and subsequent decrease when you remove the
vacuum, rest assured that the distributer vacuum advance is working
properly. It's tough to accomplish more than this without using the
costly dyno method.
To answer your original question about the plate movement; With no
vacuum signal present, the mechanism rests spring-loaded against a stop.
The vacuum signal overcomes the spring tension and moves the plate.
Vacuum applied to the distributer will advance the timing, not retard
it.
There is one other advance mechanism in the distributer that adds into
the overall equation and it is called "mechanical" advance. I won't go
into that here since your concern seems to be with the initial and
vacuum side of the house and also because working with mechanical
advance requires the use of some specialized equipment.
BTW, a quick check for a "ported" source of vacuum is to use a gauge
to test all of the ports on the carburetor (above the base plate) while
the engine is idling. When you find one that registers zero at idle &
increases as you advance the throttle, you've found your source.
HTH,
Steve Ferguson

 

[gene]

Well done again. Thanks I am going to leave mine on ported advance and
post your descripton and go on the the next unkown.

gene

>Dave,
> I too have fought this battle. I hate TVS. Period. I've eliminated
>this system on every car I've had that uses it and gone to a ported
>vacuum source. Using ported vacuum, which is from a source located
>above the throttle plates, the advance mechanism in the distributer is
>allowed to move to suit the demands of the engine. When vacuum levels
>are high, as in part throttle cruising, you have max advance. When
>vacuum levels are low, as when the engine is under heavy load, you have
>little or no vacuum advance and the engine operates under initial
>advance, which is what you set with a timing light. There is an old
>trick that still works better than anything other than a dyno for
>setting initial advance, (or timing as it's sometime called), and that
>is to set your initial advance to the factory setting and test it under
>maximum load. Usually a long grade or steep hill. If you detect engine
>detonation, retard the timing a degree & try again. Repeat this until
>there is no longer any pinging during your testing. This is the optimum
>setting for your engine. The other side of this test is if there is no
>detonation at the factory setting, add a degree and try again. Still no
>pinging? Add another and so forth. Bear in mind however, that this
>method only works when the TVS is by-passed and you are using a ported
>source of vacuum for the distributer. Also, take a moment to test the
>vacuum advance mechanism by using a hand pump (MityVac) or sucking on
>the vacuum line to the distributer while the engine is idling and note
>the increase in idle speed. If there is an increase with the
>application of vacuum and subsequent decrease when you remove the
>vacuum, rest assured that the distributer vacuum advance is working
>properly. It's tough to accomplish more than this without using the
>costly dyno method.
> To answer your original question about the plate movement; With no
>vacuum signal present, the mechanism rests spring-loaded against a stop.
>The vacuum signal overcomes the spring tension and moves the plate.
>Vacuum applied to the distributer will advance the timing, not retard
>it.
> There is one other advance mechanism in the distributer that adds into
>the overall equation and it is called "mechanical" advance. I won't go
>into that here since your concern seems to be with the initial and
>vacuum side of the house and also because working with mechanical
>advance requires the use of some specialized equipment.
> BTW, a quick check for a "ported" source of vacuum is to use a gauge
>to test all of the ports on the carburetor (above the base plate) while
>the engine is idling. When you find one that registers zero at idle &
>increases as you advance the throttle, you've found your source.
>HTH,
>Steve Ferguson
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[steven d. ferguson]

Dave,
A manifold source is not ported, it is constant. All of my references were for,
but may not be limited to carbureted systems only. If the instructions call for
a "manifold" source, this is referring to a source below the throttle plates. A
ported is above the throttle plates. I do not know what your instructions call
for.
On one fuel injection system I installed (Edelbrock Tuned Port Injected
system) the advance was controlled soley by the computer with no vacuum hook
ups. On a Holley unit I currently have installed on a daily driver, I use a
ported source of vacuum for the distributer.
Can you provide the info from your installation sheet? As to whether or not
Emery is correct, I can tell you that I'd never disagree with him.
Steve F.

> Steve,
>
> Thanks for the run down. This clears it up for me. So Emery is correct in
> that TBI users who have eliminated the TVS should hook their dist. advance to
> a manifold source.
>
> Dave

 

[steven d. ferguson]

Dean Wrote:
They need rpm. Mondello does
recommend that they continue to be connected. I followed that advice and
mine
is connected to the carburator, not the intake manifold. I have checked
the
timing advance through the rpm range and it's right on with or without
the
vacuum line connected. Our distributors are spring advanced units and
do not necessarily ( some
will argue) need vacuum for advancement.
Dean,
Whether you wanted to or not, you've probably got your distributer
vacuum advance hooked up to a ported source. I haven't seen any Holley
aftermarket applications that have any other type, save for the PVC
hookup. Your Mondello distributer has three means of advance, all
adding up to your total advance. The first is your initial advance, you
set this at idle with a timing light. The second is mechanical,
strictly rpm driven at a rate governed by the shape of the curve of the
center post & weights and the tension of the springs so you're on the
mark when you say they need RPM. The third is vacuum advance, the amount
of which and rate is controlled by the mechanical design of the can.
Together, all of these should be coming in at about the "sweet spot" on
the cam and at a rate engineered into the curve based on a specific
application. Mondello knows what he's doing so this most likely is
around cruising rpm. Race-only applications rely on initial and
mechanical advance because they're operated mostly under acceleration
with the throttle wide open. If you had only mechanical (RPM) advance,
any RPM other than cruising RPM would be inefficient. Street driven
(non-computer) vehicles have to have all three types of advance in
order to deliver good performance under a fairly wide range of operating
RPM.
Does this muddy it up?
Steve F.