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vacuum advance???

Bill Monk

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Just bought a distributor and ignition box from 4secondsflat. This is a quote from their tech instructions:

"Hot Rods, Muscle Cars, Classic Cars or nearly any performance application the Vacuum Can is connected to the CONSTANT Ma nifold Vacuum source. Doesn't matter what you read on the internet if you don't have any computer controls, Catalytic Converters or other emissions equipment it goes to CONSTANT. End of discussion."

This is contrary to everything I thought I knew. I have always plugged the vacuum advance to the ported hole at the top of the carb so that you get advance as vacuum increases. Throw some input at me!
 
I use manifold vacuum. This is a great debate so don't expect any definitive answer it's really personal preference.
 
I'm not using my vacuum advance... but when I did, it was ported/venture vacuum. Like 65-440 said... it would give you full vacuum advance at idle if you hooked it up to manifold vacuum and then no vacuum advance at wide-open-throttle.
 
I'm not using my vacuum advance... but when I did, it was ported/venture vacuum. Like 65-440 said... it would give you full vacuum advance at idle if you hooked it up to manifold vacuum and then no vacuum advance at wide-open-throttle.
Well, that's what I thought which is why I posted the question. You would still get advance with the mechanical advance at WOT but I can't get my head around why they are saying to hook it to manifold vacuum???
 
The vacuum advance on stock motors takes the place of the mechanical advance when the rpms are low. Once you speed up, it drops off and the mechanical takes over. The timing from the factory is without the vacuum advance. So once you reattach it, the advance will jump up that much more at idle.
 
i use ported only. i like the FBO guys, but i do take exception to their ignition curves. the only time to use full manifold vacuum is when engine vacuum is very low.
 
I spoke at lengths with Don at 4secondsflat while I was ironing out my motor and buying a Quickfuel. He's very knowledgeable and told me the same. Even though I didn't buy a distributor from him he told me exactly what to shoot for on my motor and the car has never ran better.

Here's what I have, my motor likes to idle at 30 degrees, any less and it's a dog but at 30 degrees it's a bit much for starting. My timing is set up with the initial being 20 and the vacuum advance set for 10 degrees of advance, motor fires right up and with 10 degrees advance from the manifold it pulls it up to 30 as soon as it fires then 40 fully advanced.
 
I spoke at lengths with Don at 4secondsflat while I was ironing out my motor and buying a Quickfuel. He's very knowledgeable and told me the same. Even though I didn't buy a distributor from him he told me exactly what to shoot for on my motor and the car has never ran better.

Here's what I have, my motor likes to idle at 30 degrees, any less and it's a dog but at 30 degrees it's a bit much for starting. My timing is set up with the initial being 20 and the vacuum advance set for 10 degrees of advance, motor fires right up and with 10 degrees advance from the manifold it pulls it up to 30 as soon as it fires then 40 fully advanced.

Exactly
 
I spoke at lengths with Don at 4secondsflat while I was ironing out my motor and buying a Quickfuel. He's very knowledgeable and told me the same. Even though I didn't buy a distributor from him he told me exactly what to shoot for on my motor and the car has never ran better.

Here's what I have, my motor likes to idle at 30 degrees, any less and it's a dog but at 30 degrees it's a bit much for starting. My timing is set up with the initial being 20 and the vacuum advance set for 10 degrees of advance, motor fires right up and with 10 degrees advance from the manifold it pulls it up to 30 as soon as it fires then 40 fully advanced.
Interesting stuff, that's what I was looking for is something that makes sense! Thanks
 
I spoke at lengths with Don at 4secondsflat while I was ironing out my motor and buying a Quickfuel. He's very knowledgeable and told me the same. Even though I didn't buy a distributor from him he told me exactly what to shoot for on my motor and the car has never ran better.

Here's what I have, my motor likes to idle at 30 degrees, any less and it's a dog but at 30 degrees it's a bit much for starting. My timing is set up with the initial being 20 and the vacuum advance set for 10 degrees of advance, motor fires right up and with 10 degrees advance from the manifold it pulls it up to 30 as soon as it fires then 40 fully advanced.
It just hit me that with the vacuum attached to manifold vacuum, you don't have 40 at full advance because the vacuum advance will decrease under throttle so you are likely running around 30-35 all the time. As mechanical advance increases, the vacuum decreases. Does that make sense or am I missing something?
 
I think every gear head should drive there car for two weeks with a vacuum gauge hooked up and watch it.
 
It just hit me that with the vacuum attached to manifold vacuum, you don't have 40 at full advance because the vacuum advance will decrease under throttle so you are likely running around 30-35 all the time. As mechanical advance increases, the vacuum decreases. Does that make sense or am I missing something?

True, with 10"Hg vacuum at idle you'd only have 40 degrees advance at idle and less once you hit the gas....but you might have almost 40 degrees advance at cruising speed (near max vacuum).
 
Just bought a distributor and ignition box from 4secondsflat. This is a quote from their tech instructions:

"Hot Rods, Muscle Cars, Classic Cars or nearly any performance application the Vacuum Can is connected to the CONSTANT Ma nifold Vacuum source. Doesn't matter what you read on the internet if you don't have any computer controls, Catalytic Converters or other emissions equipment it goes to CONSTANT. End of discussion."

This is contrary to everything I thought I knew. I have always plugged the vacuum advance to the ported hole at the top of the carb so that you get advance as vacuum increases. Throw some input at me!
That is correct. The ported fitting is simply an early attempt at emission control. I run mine off of the constant manifold vacuum.
 
I guess the real question is: what is the purpose of the distributor vacuum advance.....not whether the question of its source ported source or manifold source.
My understanding of the need for vacuum advance is: at light engine loads and partially open throttle, is the mixture velocity is low (relative to piston speed) and therefore cylinder filling tends to be on the lean side. Usually lean mixtures tend to burn slower, requiring them to be ignited sooner, relative to piston speed. The distributor's mechanical advance mechanism will not be fully advanced to produce maximum power and therefore to optimize mixture ignition, the vacuum advance will suppliment the combustion process.
Which method is best??? Many variables come to play...compression ratio, cam shaft profile (degrees, lift, overlap) cylinder head configuration. One must make up their own mind......remember to ask the brother-in-law's, next door's neighbor's cousin who drives a chevy, for his opinion as to why....odds are he will have the wrong reason or just won't know....
Bob Renton
 
That is correct. The ported fitting is simply an early attempt at emission control. I run mine off of the constant manifold vacuum.
Seems like there's some agreement from Edelbrock on that. Using the manifold vacuum helps low speed/ light throttle running but also increases NOx emissions.
1406_edelbrock.jpg
 
I guess the real question is: what is the purpose of the distributor vacuum advance.....not whether the question of its source ported source or manifold source.
My understanding of the need for vacuum advance is: at light engine loads and partially open throttle, is the mixture velocity is low (relative to piston speed) and therefore cylinder filling tends to be on the lean side. Usually lean mixtures tend to burn slower, requiring them to be ignited sooner, relative to piston speed. The distributor's mechanical advance mechanism will not be fully advanced to produce maximum power and therefore to optimize mixture ignition, the vacuum advance will suppliment the combustion process.
Which method is best??? Many variables come to play...compression ratio, cam shaft profile (degrees, lift, overlap) cylinder head configuration. One must make up their own mind......remember to ask the brother-in-law's, next door's neighbor's cousin who drives a chevy, for his opinion as to why....odds are he will have the wrong reason or just won't know....
Bob Renton
Thanks Bob, I guess this is one of those things that I have done for 30 years without sufficient understanding of why I was doing it. Never to old to learn you know. I posed the question to Don at FBO and his response is in harmony with what you are saying. Here is what he had to say on the issue and it makes perfect sense.

"1ST off ignore the nonsense you read on the internet there probably not .2% of all those people who know how a distributor works and what advance and retard timing means.
New cars run at about 40* of timing at idle and 50 or more at part throttle cruise.
Emissions (OBD 1 and up) engines run Ported vacuum to get as much advance as possible without blowing them up, the computer reads all the sensors and retards the timing to keep it out of detonation and excessive combustion chamber heat. As the air speed increases through the injector air horn it generates vacuum to the distributor (Computor) and advances the timing, this is where the computer takes control and retards it to keep it from blowing up.
You don’t have a computer so Constant Manifold is where you need to hook to, as load (Gas Peddle) gets applied the butterflies open and the manifold vac is gone and so is the Vac advance. The mechanical takes over and only allows the timing to increase to the amount that we set it for… max power and Minimum (hopefully None) detonation."

There’s more in my book but that’s the basics
Thank You
Don
2016 Madras Dragstrip Pro Champion and King Of The Track
 
It just hit me that with the vacuum attached to manifold vacuum, you don't have 40 at full advance because the vacuum advance will decrease under throttle so you are likely running around 30-35 all the time. As mechanical advance increases, the vacuum decreases. Does that make sense or am I missing something?
40 degrees full advance is set mechanically with no vacuum advance, remember there isn't enough vacuum at that rpm to even pull in the added 10 degrees so it is running at 40 degrees. Now if your cruising at really low rpm's (low enough to pull vacuum) then your not at a high enough rpm for the mechanical advance to even be advancing so your back to 30 degrees. I had a vacuum gauge on mine with the timing light and it doesn't take much pedal before the vacuum advance drops off.

I'm by no means an expert and like you got by fine for years mostly because I was typically starting with a complete motor where the distributor was set up at least really close. With this build I was starting with a block! This time around it was a Mallory distributor with a generic one size fits all curve and my motor didn't like it. I tried my hardest to wrap my head around it and with the help of Don feel like I finally have it nailed down.
 
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