Vacuum advance question...

[BobT-47P15]

Would the vacuum advance on a 1957 Plymouth flathead engine be the

same as the vacuum advance on a 46-48 Plymouth engine??

[greg g]

Would it fit? most likely. Is it the same? porbably not. Would it make much difference? probably not. Later cars had different advance curves, cranking in more advance quicker. Probably due to better fuel and higher compression.

That said it will probably OK.

[james curl]

I have three different engines and the distributors and vacuum advances use different pulls rods to the point plate; the vacuum advance will not interchange

[greg g]

Jim is correct I forgot that some hace points on one side some points on the other. So guess there oare other differences as well.

[Don Jordan]

I hope you don't think this is a silly question but can they be fixed? Mine was not working (I think) so I got a used one on the bay. The brand new one I saw went for more than I could afford. My friend, after sucking on it, said the used one I bought didn't work either.

I know the purpose of the vacuum advance - will it hurt anything if they don't work? Won't it just affect the acceleration? If they can't be fixed is there some one that deals in them? I haven't had much luck with used ones.

[randroid]

Don,

You really shouldn't run an engine without a V\A, but it can be done. You'll get lousy performance and gas mileage compared to running with one and it can be hard on the engine to already have the piston on the down stroke when the gas ignites.

The only moving part in a V\A is a spring-loaded diaphragm that goes south by rupturing. They are usually crimped-in and nigh-on lonesome to try to replace yourself without Devine Intervention, but they shouldn't be expensive. I bet the guy who's rebuilt some vacuum wiper motors for other Gents in the forum can steer you in the right direction if he doesn't already have a spare.

I wish you the best.

-Randy

[Normspeed]

Terill in Texas rebuilds them for around $85.

I had one with a small leak and I fixed it with Permatex leak sealer spray from the parts store. I just sprayed some into the diapragm area from both sides, let it set an hour, sprayed in a second coat. A couple months later I bought a replacement on ebay. The repaired one was still working fine. The Permatex spray forms a tough flexible rubbery coating.

[Don Coatney]

The centrifugal advance curve is set by spring tension and counter weights. On a P-15 centrifugal advance should pull 3 degrees advance @ 400 RPM's and be full in 9 degrees @ 1300 RPM's. The vacuum advance is set by adding/removing shims to adjust spring pressure on the diaphragm. On a P-15 the vacuum advance should pull 2 degrees of advance @ 6 3/4 inches of mercury and be fully advanced 10 degrees @ 14 inches of mercury. The vacuum advance comes into play only while accelerating. Once at speed it does nothing. Many Mopar flathead 6 industrial engines do not have a vacuum advance on there distributor as they run at a constant governed speed. To answer the question of "will the pictured unit work on your distributor" it might IF the linkage arm is the same as your original unit. If you replace a vacuum advance unit it is very important to use a vacuum generator as pictured below to adjust the shims to the prescribed range for your engine.

[windsor8]

I've been running my 41 Chrysler Windsor for years without a vacuum advance with no problems. You just need to plug the vacuum source the goes to the distributor so your engine doesn't think it has a vacuum leak.

[Jim Saraceno]

Here's the specs on distributors from '41 to '55:

[Normspeed]

If you replace a vacuum advance unit it is very important to use a vacuum generator as pictured below to adjust the shims to the prescribed range for your engine.

Oooooops.

I re-used the internal hardware from my old one.

[Don Coatney]

Here's the specs on distributors from '41 to '55:

Interesting that the Plymouth spec sheet differs from the MoTors manual spec sheet.

[Jim Saraceno]

If there is any doubt, I think I would go with the Motors manual. The list that I had was from a Chilton book made for the home mechanic. I think the Motors would be made for the pro.

[martybose]

It's also worth noting that the first table is in distributor degrees, where the second is in crankshaft degrees. You would double the distributor degrees to get crankshaft degrees, and you would double the distributor RPM to get crankshaft RPM..

Marty

[martybose]

(snip)

The vacuum advance comes into play only while accelerating. Once at speed it does nothing.

Don,

You've got it backwards. The OEM connections were straight manifold vacuum, so there would be no advance when you were on the throttle and accelerating, but you would have the advance when the throttle was mostly closed and cruising at a steady speed. Having the advance generally improves gas mileage, but you can't run that much advance under full power.

Marty