Vacuum measurement/Coil voltage

[Gary Schisler]

I have a 66 327/350 that I was checking for a vacuum leak. I would like to know from some of you what your vacuum readings are. I am getting 16 1/2" at idle with no accesories, when measured at the base of the carb for the port for the vacuum advance. I know that this is acceptable but is it high, low, or average?

Next question. My friend and I were discussing the purpose of the ballast resistor. He insists that this unit's purpose is to step down the voltage to the coil. I had always been told that it was designed to put a fixed load on the alternator for various reasons. How about you electrical whizzes out there? This question came up because my car wouldn't start yesterday and I was checking around and noticed that the aftermarket ballast resistor's coiled wire was intact but the white material (ceramic or asbetos?) that is inside the coil had broken down. The wire appeared to be unbroken, but the white insulating material (at least that is what I assumed it to be) was definitely not intact. I replaced it with an original one from my "bucket of bolts" and it cranked right up.

Thanks as always, Gary

[Mark Lovejoy]

Re: Vacuum measurement/Coil voltage

The purpose of the ballast resistor is to reduce amount of energy being switched by the points, hence making them last longer. The coil is nothing more than a boost transformer. The high voltage is produced when the points open, and the magnetic field in the coil of wire collapses creating what is known in the industry as back EMF (Electro-Motive Force). This is thousands of times greater than the energy it takes to create the magnetic field. Sooooo 12v or 9v in the primary side has little effect on back EMF. And that folks is our electrical lesson for today.

[Mark Lovejoy]

Re: Vacuum measurement/Coil voltage

The purpose of the ballast resistor is to reduce amount of energy being switched by the points, hence making them last longer. The coil is nothing more than a boost transformer. The high voltage is produced when the points open, and the magnetic field in the coil of wire collapses creating what is known in the industry as back EMF (Electro-Motive Force). This is thousands of times greater than the energy it takes to create the magnetic field. Sooooo 12v or 9v in the primary side has little effect on back EMF. And that folks is our electrical lesson for today.

[Jack H.]

Re: Vacuum measurement/Coil voltage

Vacuum 'head' pressure will change with CR, state of tune/wear of engine, leaks, gross engine displacement and specifics of carb and intake. Vacuum troubleshooting guides tell you not to worry too much about absolute head pressure and quote a range of 12-22 psi as generally acceptable. They council to note your head pressure with system loads (dist vac advance, pwr brake booster, wiper door/headlight on Sharks) disconnected then work backward (away from intake along vac lines) looking for any branch of the vac path or individual vac load item that presents a > 2 psi drop as leak potentials.

[Jack H.]

Re: Vacuum measurement/Coil voltage

Vacuum 'head' pressure will change with CR, state of tune/wear of engine, leaks, gross engine displacement and specifics of carb and intake. Vacuum troubleshooting guides tell you not to worry too much about absolute head pressure and quote a range of 12-22 psi as generally acceptable. They council to note your head pressure with system loads (dist vac advance, pwr brake booster, wiper door/headlight on Sharks) disconnected then work backward (away from intake along vac lines) looking for any branch of the vac path or individual vac load item that presents a > 2 psi drop as leak potentials.

[Gary Schisler]

Re: Vacuum measurement/Coil voltage

Thanks for the information on vacuum. I was trying to get a feel for how other engines stack up, recognizing that the variables that you mention need to be taken into account. As a matter of record, this is a 350 hp engine with around 2500 miles on the rebuild, with a mild cam (actually producing less than the original 350 hp), no vacuum accessories other than the vacuum advance, and a fresh tune-up with very little adjustment needed on the engine from the last tune-up. I am not even certain that there is a vacuum leak. I am just trying to eliminate an area of doubt while troubleshooting a possible carb problem that I have described on this board in the recent past. Gary

[Gary Schisler]

Re: Vacuum measurement/Coil voltage

Thanks for the information on vacuum. I was trying to get a feel for how other engines stack up, recognizing that the variables that you mention need to be taken into account. As a matter of record, this is a 350 hp engine with around 2500 miles on the rebuild, with a mild cam (actually producing less than the original 350 hp), no vacuum accessories other than the vacuum advance, and a fresh tune-up with very little adjustment needed on the engine from the last tune-up. I am not even certain that there is a vacuum leak. I am just trying to eliminate an area of doubt while troubleshooting a possible carb problem that I have described on this board in the recent past. Gary

[Jack H.]

Re: Vacuum measurement/Coil voltage

Well, Mark, a VERY good starting tutorial, but a few more words can be said. Within reason, an over-voltage supply to spark plugs doesn't hurt. Ballast resistor is in series with coil primary, and like Mark says, it drops B+ voltage to coil. But, the ballast isn't always there!

It's switched in/out of the system based on Start/Run status of the ignition switch and starter solenoid. It's out during cold crank to give you the hottest spark possible, then switched in after the engine turns and you release the ignition key from start position.

Yes, a reduced voltage across the coil primary doesn't hurt under 'normal' engine run conditions and it reduces electrical 'stress' on the high tension circuit. But, it's real reason for being there is to increase point life in the situation where you've turned the engine off, the points wind up by random chance in contact closed position and you leave the key on (why 'weld' the points?).

For really high performance situations (high engine RPM), you want a 'hot spark'. Not because you need more voltage to ignite the combustion process, but because the electrical formula V=Ldi/dt exists, where V is the voltage across an inductor (coil primary), L is a constant associated with the specific coil, and di/dt is a touch of calculus representing the time rate of change in current flowing through the coil.

So, we see the 'back EMF' mentioned only gets generated when there's a change in current flowing through the coil. This is the reason points open and close to 'make/break' the current flow through coil and either build or collapse a magnetic field around the coil. The collapse of the coil's primary field winding gets coupled through the coils core laminations to the secondary windings and 'amplified' or 'reduced' by the winding ratio. This way you step up 12 VDC to a +20,000 volt level, but the higher voltage only exists for a very short period of time so total energy is conserved (don't get something for nothing).

Now, as the engine revs faster, points open/close quicker. The di/dt coefficient drops, and the amount of absolute voltage shuttled across the coil's primary/secondary windings drops. That means the spark falls in absolute energy and time duration. Hey, race guys want a 'hot' spark (voltage and duration) at high RPM....

So, you'll find Chevy had two different ballast resistors in that era ('black dot' ID mark with roughly 0.3 ohm resistance, and 'blue stripe' ID marks with a nominal 1.8 ohm resistance). The HP 'blackie' resistors gave less IR drop to the coil primary, made for a hotter spark at high RPM but gave less point 'weld' protection.

[Jack H.]

Re: Vacuum measurement/Coil voltage

Well, Mark, a VERY good starting tutorial, but a few more words can be said. Within reason, an over-voltage supply to spark plugs doesn't hurt. Ballast resistor is in series with coil primary, and like Mark says, it drops B+ voltage to coil. But, the ballast isn't always there!

It's switched in/out of the system based on Start/Run status of the ignition switch and starter solenoid. It's out during cold crank to give you the hottest spark possible, then switched in after the engine turns and you release the ignition key from start position.

Yes, a reduced voltage across the coil primary doesn't hurt under 'normal' engine run conditions and it reduces electrical 'stress' on the high tension circuit. But, it's real reason for being there is to increase point life in the situation where you've turned the engine off, the points wind up by random chance in contact closed position and you leave the key on (why 'weld' the points?).

For really high performance situations (high engine RPM), you want a 'hot spark'. Not because you need more voltage to ignite the combustion process, but because the electrical formula V=Ldi/dt exists, where V is the voltage across an inductor (coil primary), L is a constant associated with the specific coil, and di/dt is a touch of calculus representing the time rate of change in current flowing through the coil.

So, we see the 'back EMF' mentioned only gets generated when there's a change in current flowing through the coil. This is the reason points open and close to 'make/break' the current flow through coil and either build or collapse a magnetic field around the coil. The collapse of the coil's primary field winding gets coupled through the coils core laminations to the secondary windings and 'amplified' or 'reduced' by the winding ratio. This way you step up 12 VDC to a +20,000 volt level, but the higher voltage only exists for a very short period of time so total energy is conserved (don't get something for nothing).

Now, as the engine revs faster, points open/close quicker. The di/dt coefficient drops, and the amount of absolute voltage shuttled across the coil's primary/secondary windings drops. That means the spark falls in absolute energy and time duration. Hey, race guys want a 'hot' spark (voltage and duration) at high RPM....

So, you'll find Chevy had two different ballast resistors in that era ('black dot' ID mark with roughly 0.3 ohm resistance, and 'blue stripe' ID marks with a nominal 1.8 ohm resistance). The HP 'blackie' resistors gave less IR drop to the coil primary, made for a hotter spark at high RPM but gave less point 'weld' protection.

[G B.]

You're about right on vacuum

Gary -

I think your reading is okay for a stock 327/350hp engine at about 750 rpm idle speed. The same cam in my 400 small block pulled a little more vacuum - maybe 19 inches.

[G B.]

You're about right on vacuum

Gary -

I think your reading is okay for a stock 327/350hp engine at about 750 rpm idle speed. The same cam in my 400 small block pulled a little more vacuum - maybe 19 inches.

[Gary Schisler]

Re: Vacuum measurement/Coil voltage

Did I ever mention that I only got a C- in Electical Engineering in college? Thank you both for the information that you provided. I will work this voltage problem out with you help. Hopefully, this problem is resolved with the new/old ballast resistor in place. Gary

[Gary Schisler]

Re: Vacuum measurement/Coil voltage

Did I ever mention that I only got a C- in Electical Engineering in college? Thank you both for the information that you provided. I will work this voltage problem out with you help. Hopefully, this problem is resolved with the new/old ballast resistor in place. Gary