Vacuum advance question, GM 401 V-6 Engine in Uni Power Unit

I am replacing the old breaker point ignition in the 1110317 7A31 Delco-Remy distributor with an electronic unit from Pertronix, and while working on it I noticed the vacuum advance is not hooked up to a vacuum line. Should I attempt to put one on, or will it be unnecessary with a wide-open speed of only 2200 RPM? Or, put another way, does this sort of application even need to have a timing advance? Thank You for any input!

Lon

It probably was never connected from the factory. Many industrial engines came that way, cheaper to just use an existing distributor with a vacuum advance than to special make one without.

It does however need the mechanical advance. Be sure the weights are free and working.

Lon, likely Advanced Steve has it right, as that parallels my experience.

On the other hand, you are paying Pertronix a nice sum for the new distributor that they apparently list for your engine and they have a tech help dept./phone #. ( (909) 547-9058)

WHY not get your money's worth and run it buy those folks?

(quoted from post at 20:06:13 09/18/14) I am replacing the old breaker point ignition in the 1110317 7A31 Delco-Remy distributor with an electronic unit from Pertronix, and while working on it I noticed the vacuum advance is not hooked up to a vacuum line. Should I attempt to put one on, or will it be unnecessary with a wide-open speed of only 2200 RPM? Or, put another way, does this sort of application even need to have a timing advance? Thank You for any input!

Lon

Click to expand...

The vacuum advance was designed to change the timing as someone accelerated in most applications while the mechanical advance changed to timing for constant RPM's above idle. So if the engine in question is a power unit with a desired RPM setting and governor it most likely never used the vacuum advance. Couls be one of 2 things. As someone stated came that way (I doubt it, I've worked on more than one industrial engine/AG engine with GM distributers and none had the vacuum advance. It's also possible that a PO replaced the original dizzy with something they had or out of a junk yard.

Rick


Rick

Probably the factory timing was advanced to compensate for the lack of vacuum advance. For an industrial engine running at a constant speed and constant load all day long, the amount of vacuum advance would be small.

The vacuum advance serves two purposes. It provides a higher advance for an initial surge of acceleration, and backs off timing so that the engine will not ping under heavy loads. I would connect it if the load varies an lot, but not worry if it is under fairly constant load.

There are 3 components to advance, initial, centrifugal and vacuum.

The fuel mixture in the engine needs to complete combustion at around 10° after top dead center. At idle the engine needs little advance from about TDC to achieve the desired end of combustion at 10° ATDC. As the engine speed increases the time required for the fuel to burn remains somewhat constant. The crankshaft is turning faster so the piston arrives at 10° ATDC sooner. The mechanical advance moves the timing so the the end of combustion will still remain at 10° ATDC. Under light loads the throttle plate in the carburetor is only slightly open resulting in high manifold vacuum. The means that the cylinders will not receive as much fuel mixture resulting in lower compression. The lowered compression results in the fuel molecules being farther apart resulting in increasing the burn time of the fuel mixture. A vacuum advance is controlled by manifold vacuum and will increase the timing in addition to the mechanical advance again keeping the end of combustion at 10° ATDC. When the engine load increases the throttle plates open more causing the manifold vacuum to drop and allowing more air/fuel mix to enter each cylinder. This increases the compression in the cylinder pushes the individual fuel molecules closer together which in turns causes the fuel mixture to burn faster. As the manifold vacuum drops off the vacuum advance also retards the timing to again keep the end of combustion in the proper place.

That is the same explanation I have been led to believe on manifold vacuum advance systems. Now if you want to really confuse people start talking about venturi vacuum advance mechanisms.

Well articulated.