an "advanced" question (spark that is) mechanical vs. vacuum

When it comes to choices, mine always, um.... suck. So here I am faced with yet another choice, vacuum or mechanical advance distributor. So, here I am seeking the help of those compled to read this post. Please share your choice and why you like (or hate) it. Please include some details about your application. Mine's a carburated Chevy 60 degree V6 for passenger car use.

personially, i perfur DIS

why not get both?

a mechanical advance uses centrifical force to advance the spark timing at higher RPM

a vacuum advance alters the timing (i can't remember advance or retard) during crusing(closed throttle, miniual engine load) for economy/emissions.

You just caught me after an intense period of ignition tuning. I just got done tuning up a 454 and learning tons.

Basically, you should have both. Mechanical advance is strictly based on RPM. It makes sense since there needs to be more spark advance at higher RPMs. Vacuum advance is not necessary, but it sure is nice. Since the amount of advance can be greater under periods of low load, the vacuum advance responds to that need. The more foot you have in it, the less vacuum is created and therefore the less advance generated.

You can tailor a mechanical advance for the perfect curve and leave it alone, but you lack two very important things; 1) maximum efficiency. The lack of extra advance under part load wastes BTUs available from the fuel, and 2) the vacuum advance... provided its ported vacuum... provides very necessary advance during acceleration. On a dyno, the engine will make the same power with or without a vacuum advance since it measures power once the engine achieves RPM. In a car (street or race) the acceleration of the car will be affected by how accurately the ignition curve meets the peak requirements of the engine. Hence why vacuum advance is important.

In fact, on my 454, we ended up locking out the mechanical advance and running entirely vacuum. We discovered after serious testing that the engine responded well to 38 degrees all over the place. We could have left it at that, but by adding vacuum advance, we added as much as another 12 degrees at part throttle, which (in my case) added 3 MPGs to my cruise fuel economy.

good info curtis, alot better then i could explain.

I agree, curtis made excellent points. Vacuum it is... and as an extra bonus, there's one less vacuum port for me to cap off. :)

remember curtis said only after much tuning did they ditch the mechanical advance, i would still but one with both, then modifity it as needed. most distributors are in some way adjustible.

The "Chevypower" book shows a "race" reccomendation of 36-40* total advance for the V6-60.
Since the stock initial advance is set to 10* at idle, that leaves you with 26-30* mechanical advance to put in the distributor.
My feeling is to NOT lock out the mechanical advance for the street. You do need to start your car when the engine is hot, don't you?
Use the vacuum advance also, as much as the engine can take.

Since the amount of advance can be greater under periods of low load, the vacuum advance responds to that need. The more foot you have in it, the less vacuum is created and therefore the less advance generated.

Terrific post curtis, very informative and well put.
As you say, ported vacuum is the useful source for a distributor. But what you describe here is manifold vacuum, which is different and IMHO much less useful source.
Ported vacuum does the opposite as you describe, more throttle equals more (not less) ported vacuum.

Ported vacuum does the opposite as you describe, more throttle equals more (not less) ported vacuum.
---And then it drops off to almost zero as the throttle is opened wider, since the port is located near the throttle butterfly and not the venturi.

Terrific post curtis, very informative and well put.
As you say, ported vacuum is the useful source for a distributor. But what you describe here is manifold vacuum, which is different and IMHO much less useful source.
Ported vacuum does the opposite as you describe, more throttle equals more (not less) ported vacuum.

True, but the part of the vacuum to which I was referring was between 1/4 and full throttle. I just omitted that part. Vacuum can be used (and I have it in use on a heavily cammed Caddy 500) to help add extra crank ignition at idle to help smooth out the cam.

Ported vacuum on everything I've done doesn't increase as throttle increases, it just doesn't come on until you crack the throttle. On my latest Qjet, the idle vacuum is about 18" at manifold. The ported vacuum peaks at about 15" immediately off idle and then continues to drop to zero at WOT just like manifold.

Depending on the airflow over the ported vacuum's port, on some carbs ported = manifold almost immediately after you open the throttle and they parallel each other to zero at WOT. In the case of my most recent one, the ported hole supplies a touch more than manifold after half throttle but I think that's because of the angle at which I drilled it :)

So, you are right for very modest throttle openings. But for the whole range of throttle; manifold starts high and goes low. Ported starts low, peaks at small throttle openings, then goes low along with manifold.