Need some advice - vacuum advance issue 454

G72Zed

Veteran Member
Sep 8, 2015
4,084
Canada
What is causing it to buck when hooked up to manifold vacuum is that you have it hooked up to manifold vacuum. Vacuum advanced are designed to operate on ported vacuum. You have a pretty good advance curve in it for a street driven big block. When you plug in manifold vacuum, it adds even more advance and screws up your advance curve. What you are doing when you plug in manifold vacuum is giving the engine full vacuum and centrifugal advance at cruise and no vacuum advance upon acceleration when the engine needs it.

FWIW, the last 2 BBC's I tuned lately responded positively to 39-42* timing for best HP and TQ, I think he may be light of the curve, but the 4 spd/3.08 overall gearing may be lugging and making things worse. I've driven undergeared O/A cars and they act the same.

The way I see "best timing" is it has more to do with chamber size/shape, stroke, intake tract design and trapped charge, some combo's just need more, the poorer the combo, the more timing generaly speaking.

And 90% of the time, warmed over or mild/well built performance engines respond better to manifold vacuum than grandpa's early 70's emision minded ported location. Specially when over cammed and under the required comp ratio.

@kdsracing, The last sentence, it would depend what "cruise" is, if it's under the centrifugals max rpm activation, you are not automatically getting full centrifugal, one is vacuum/throttle position related, the other is rpm, they operate on different platforms.

When you accelerate heavily and/or to WOT, you do not want vacuum advance, just the opposite, you want initial/centrifugal only.
 

450bench

Veteran Member
Feb 25, 2002
3,687
memphis, tn-shelby
I’ve tried both ported and manifold vacuum. Same results.
Timing is right at 18 and 36. All in around 2300.
I do have a run ready distributor that I could slap in there.
 

G72Zed

Veteran Member
Sep 8, 2015
4,084
Canada
I’ve tried both ported and manifold vacuum. Same results.
Timing is right at 18 and 36. All in around 2300.
I do have a run ready distributor that I could slap in there.

Some Holley "ported" bore channel hole is like .020-.030 above from the primary trans slot "sweet spot", so it takes only a few hundred rpm to introduce the "ported" signal. That's why the majority use "manifold", and the spark plug wires will thank you as well.
 

1980RS

Veteran Member
Jun 17, 2006
6,399
MN
MSD products are garbage. Try an old HEI dizzy and i bet your issues go away. A mild 454 like yours would want 32-34 total timing. install springs so that all your timing is in by 3000 rpm.
That's funny, I took the HEI out of my car and put an MSD billet distributor in with a 7AL2 car now will pull 7500 if I so choose, but it never did that with the HEI. I'll take good set of points any day over an HEI.
 

Bandit723

Veteran Member
Oct 1, 2016
4,274
Waupaca WI
That's funny, I took the HEI out of my car and put an MSD billet distributor in with a 7AL2 car now will pull 7500 if I so choose, but it never did that with the HEI. I'll take good set of points any day over an HEI.
I will defer this to your PCP for further testing
 

450bench

Veteran Member
Feb 25, 2002
3,687
memphis, tn-shelby
Much better after raising the float level but I haven’t hooked up vacuum advance. Ran good without it.

I wonder if the vacuum can on the distributor could be bad?
 




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