Vacuum advance on 427/428 cars

[DanEC]

I doubt the 427 cars had any provisions for vacuum advance on the distributors but what about the original 428 street cars? Did they have a vacuum advance distributor?

Thanks

Dan

[FWB]

yes, i think they were adjustable diaphrams. as far as distributor casting numbers, no clue.

[DougD]

If you want a reliable and relatively cheap vacuum advance electronic dist, you can buy a Ford Duraspark rebuilt unit from just about any auto parts store (NAPA, Auto Zone, etc.) for about $75. As for a dist for a Ford truck with a 360 or 390 from 1975 or so.

[strictlypersonl]

Originally Posted by DougD

If you want a reliable and relatively cheap vacuum advance electronic dist, you can buy a Ford Duraspark rebuilt unit from just about any auto parts store (NAPA, Auto Zone, etc.) for about $75. As for a dist for a Ford truck with a 360 or 390 from 1975 or so.

Is it small enough to fit with a standard expansion tank?

[Cobra 29]

MSD makes a small billet distributor with vac advance for a 351W. Unsure of one for big block. If they do, be prepared to swap out the vaccum module with an Accel version to gain control over the amount of ingition advance from the vac unit.

[DanEC]

Originally Posted by strictlypersonl

Is it small enough to fit with a standard expansion tank?

Oh oh - that sound ominous. Bob, I plan on purchasing one of your expansion tanks for the motor. Are you saying a vacuum can will normally intefer with the expansion tank????

Dan

[DougD]

Originally Posted by strictlypersonl

Is it small enough to fit with a standard expansion tank?

I'm using the MSD myself, but the Duraspark is the same diameter - can't see why it wouldn't work with the small cap on it. I'm considering swapping out the MSD for a Duraspark to see if the vacuum advance yields better highway mileage.
As far as the vacuum cannister - 60's Fords used vacuum advance and the puke tank on FEs without a problem. Obviously the Duraspark wasn't around in the 60s.

[PANAVIA]

Vac advance will yield better MPG and cooler running at part throttle cruising.

There is a summit electronic FE distributor that is made by mallory--

IT may be worth a look.

[DanEC]

Originally Posted by PANAVIA

Vac advance will yield better MPG and cooler running at part throttle cruising.

There is a summit electronic FE distributor that is made by mallory--

IT may be worth a look.

I agree. I found this out with my 427 Vette that has a fairly radical motor. I put a vacuum advance unit on it and the summer running temperature dropped 25 deg before my eyes immediately. With L-88 heads, 12 pistons and a 252 deg @ .050 mechanical cam it will start from cold and actually somewhat idle on the fast idle cam. Gas mileage - well that's another subject. I didn't get much positive feedback out of Keith Craft when I started to mention vacuum advance.

Dan

[Excaliber]

KC not getting enthusiastic about a vacuum advance unit is no surprise. Pretty much the same way for all the major engine builders. The vacuum advance add's another level of complexity and risk to the engine. The concern is if you get to much advance you can damage the engine, so why risk THEIR warranty by introducing yet another variable? Same basic reasoning applies to hydraulic roller cams, thats a no brainer when it comes to "warranty", low risk, easy to install. It's the "kiss" principle at work.

BUT, if you know what your doing a vacuum advance is a good thing! And a flat tappet WILL outperform a hydraulic (now wait for the excuses about how modern is better). IS it? Not always.

[Rick Parker]

My understanding of vacuum is that it uses less initial advance to allow for easier starting (??) and pulls more advance in as timed manifold vaccum increases at cruise. Since most of the engines we utilize SB or BB seem to have a fair amount of camshaft in them, we typically use weak springs in a mechanical advance distributor to allow a faster advance curve. Isn't this just a different way of achieving the same thing by a different means??

[Excaliber]

Not really. As far a base timing and the addition of mechanical advance to that is "enough" by itself to gain max horse power and good overall engine performance. How fast you bring in the mechanical advance and how much you bring in varies with any given set up. The basics are 32-35 degrees TOTAL advance at some given rpm and that's it, your done. Vacuum won't have any impact at all on starting the motor, the only factor there is your base timing. There is such little vacuum available under WOT (wide open throttle) as to render the vacuum advance unit inoperable, same thing when you are cranking the motor to start it, no vacuum signal is present.

So why run a vacuum advance at all? A couple of reasons I do it. #1 is for better mpg, which means more RANGE in miles per tank of gas. #2 is a better overall idle speed, it can be smoother with a properly tuned vacuum signal, or not, depending on the cam, carb's, etc. #3 it helps or even flat out eliminates "run on" with engines forced to run a relatively high idle speed due to a wild lumpy cam. The vacuum advance helps to smooth that rough idle and allows for a lower idle rpm. This in turn allows the carb butterflies to be more closed so the carb doesn't suck up "air/fuel" after the ignition is shut down, thus preventing the "run on" or "diesel" problem some engines have when hot and a fast idle speed.

Under LIGHT LOAD conditions, think cruise rpm, flat land, steady speed, you can have a total advance amount of around 45 to even 50 degrees! Say 35 total of base plus mechanical advance PLUS another 10 to 15 of vacuum advance. 45 to 50 degrees advance will certainly damage your pistons and blow your motor IF that occurs under "load". But as soon as you "step on the gas" or put the motor under "load" the vacuum disappears and your back to your original 35 degree total. It's a kind of "fail safe" for running extreme advance timing, risky though, get it wrong and you WILL damage your motor. Get it right and it's sweet. Most engine builders won't take the risk of to much advance (and they probably never use starter fluid either).

[DanEC]

I left a few things out of my post. To get the best advantage of vacuum advance it has to be hooked up to a full time vacuum source below the throttle plate or directly from the manifold. This improves the idle quality and lowers the engine tempreature when sitting in slow speed traffic. Using a ported vacuum source defeats the purpose of using vacuum advance.

There is a problem that can arrive from an over-advanced situation at cruising speed - this occurred on my Corvette. I have a special vacuum can that works at low vacuum due to the old-school cam in my car (only pulls 8 inch max vacuum at idle). On the highway I had a constant light miss - it almost felt like a surge. This was from too much total vacuum and firing the engine too far from TDC. I solved the problem by configuring a stop on the vacuum can to limit the total amount of additional advance from vacuum to 8 degrees. That amount still helps my motor idle better and seems to work at keeping my engine temperature down. With 36 degrees mechanical and initial advance in the car - I have a potential max of 44 now at cruise if all the mech advance is in.

But your right - sometimes the vacuum unit can bring in too much total advance in cruising conditions. Not a good thing but since vacuum falls off as soon as you lean on the throttle I would think there is little danger to the engine.