Converted SBC distributor - Vacuum advance or not?

tubman

tubman

Well-known member
I hate to bring this up, but some of the threads (both on here and the H.A.M.B.) have got me thinking. I went out of my way to get a SBC Mallory distributor with both mechanical and vacuum advance that I had converted to use on my 8BA. There is a thread here that seems to say that a mechanical advance only distributor actually works better than one with vacuum advance added. It seems that setting up the vacuum advance can be a little complicated, but disregarding that, if properly set up , does a distributor with vacuum advance have an advantage on these engines? Any real world experience would be appreciated.
 
Ask this question. Do you know of any car built since the early 30's, that doesn't have a vacuum advance? Probably a good reason or the factory would waste the money. Also the Malory isn't v very good choice anyway,
 
Ol' Ron said:
Ask this question. Do you know of any car built since the early 30's, that doesn't have a vacuum advance? Probably a good reason or the factory would waste the money. Also the Malory isn't v very good choice anyway,
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Why is a Mallory made for a Chev. with adjustable mechanical and vacuum advance that has been converted for a flathead not a good choice??
 
I would expect it is for the ease of changing the mechanical advance weights and/or springs that are positioned just under the rotor on the Chevy distributor. Also, there is an adjustable vacuum advance canister that can be had for the GM distributor, but there are other methods of limiting vacuum advance that will work too.
 
tubman said:
if properly set up , does a distributor with vacuum advance have an advantage on these engines? Any real world experience would be appreciated.
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I think Richard in Florida has documented the advantage of vacuum advance. If your engine only likes 18 degrees advance under open throttle and you adjust it for that, that is all you will get with mechanical only. Since a lean mixture at cruise can use more advance, the vacuum can provide that for you. What you need is adjustability of the vacuum can for total vacuum advance and rate of advance, thus the "hard to tune" concept. Crane has a vacuum can for Chevy that does this, I don't know about Mallory.
 
My approach is to not overthink this question or many other Flathead questions. My
focus with the sbc/8ba distributor is to supply the advance springs and weights and
limit bushing to provide 20 deg mech at 2000 rpm when setting the initial at 4 deg and
an additional 6 to 8 deg with the can, ( or real close to that ).
First hand experience with my stroker powered '41 1/2 ton started with one of my
own sbc/8ba distributors (of course). All was well without the can except when
hauling up a couple big hills, because I'm running a true 8.5 to 1 compression I did
experience some detonation so I changed the springs to a bit heavier, that took care
of that. I kept the initial at 4 deg.
When I hooked up the can (intake vac) and headed out to the freeway my seat of
the pants feel was a noticeable smoother running motor above a steady 55 to 70.
That's my 'real world'. I have not felt the urge to touch the dist in 1 1/2 years, it
wound my motor to 5 grand in 2014 and still does today.
Charlie ny
 
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Thank you Charley. What people don't realize is that the engines are tuned for cruise (90% of the time) The power valve and mechanical adv take care of Power. The Vac and main jets are for cruise. What I don't understand is we go over this at least once a month for the past 10 years.
 
Just to keep all involved in the write up informed.
Was yours a stock oem Chev dist or a Mallory for a small block there is a vast amount of difference for changes depending what the donor was. Some easier said than done.

R
 
I have a MSD Ready-to-Run distributor with vac and mech advance. I noticed immediately when the vac can failed, a lot less responsive driving around town.
 
Ol' Ron said:
Thank you Charley. What people don't realize is that the engines are tuned for cruise (90% of the time) The power valve and mechanical adv take care of Power. The Vac and main jets are for cruise. What I don't understand is we go over this at least once a month for the past 10 years.
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Yup more than that Ron. Also some don't bother with the search function "just sayin"

R
 
mechanical advance does not start to work till rpm's are up, vacuum advance starts to work as soon as the throttle is opened
 
Ronnie said:
Yup more than that Ron. Also some don't bother with the search function "just sayin"

R
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The reason for the post is that a search turned up a couple of very recent posts saying that the mechanical advance alone was good enough. All of the threads about this seem to deal with the vacuum advance only peripherally. Given the conflicting opinions about this, I thought that a thread the concerns this specific question might be welcomed. Also, I like the look of a Mallory distributor over a stock Chevrolet one.
 
Could someone explain what the vacuum advance does that the mechanical doesn't do? At 2000rpm is the vacuum + mechanical advance advanced the same as a mechanical only advance?
 
The look factor vintage Mallory vs sbc is a slam dunk in fav of the Mallory, the sbc
tune factor wins hands down in my humble opinion.
Charlie ny
 
mrtex,
Vac adv comes in at low load once the car is rolling and at speed (high vac ). The engine vac increases and the vac can pulls the advance plate ahead usually about 8 degrees.
The motor under normal around the town driving ( decreased or low vac )would exhibit detonation with the extra 8 deg
adv because it is loaded with stop and go and hills etc. Open cruising under low load is where the vac can pays off, better mpg and lower temps.
Charlie ny
 
Much good info stated above. Bubba has a real liking for the flat top Mallorys (as I do too) would be interesting to hear his take on this also.
 
mrtexas said:
Could someone explain what the vacuum advance does that the mechanical doesn't do? At 2000rpm is the vacuum + mechanical advance advanced the same as a mechanical only advance?
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No. The mechanical advance is controlled by rpm only. If you were just cruising at 2000 rpm with a light load on the engine(high vacuum), the vacuum would add more advance above what the mechanical advance was.
 
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