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Old 09-25-2011, 07:10 AM
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Vacuum advance differences / how do they work

I have been searching threads and posts for days on this topic. I have seen many different thoughts and theories on how to tune and adjust for different apps. I am trying to figure out how different settings effect running so I can understand what will work best for me.

My dist is giving me aprox 18*-20* of advance. I have been told and read in many other posts to limit it to 10* of advance with aprox 36* total or until I get "pinging" then to back down.
Question on this .... what difference in performance is there between the 20* of advance vrs the limited 10* ?

Next is the vacuum on the dist and the adjustment inside it using the Allen wrench. I believe the more the adjustment is backed out the quicker the advance starts coming in or maybe just the opposite.
Question on this ... Again what performance difference is there between coming in early or late and does this depend on the power curve of the cam?

Lastly the "all in" rpm and the curve it takes to get there. I'm all in around 2500-2800. My dist is brand new so it's not worn but it has the stock springs. Changing the springs out will change the curve.
Question here .... this changes when the "all in" is maxed out and the time it takes to get to that point correct? And if anything soaked in with my reading when I hear pinging I have it advancing to quickly. Now at this point would the Allen adjustment on when the advance kicks in make a difference ?

I'm just trying to figure out how these different functions work and if any over ride the next so I can experiment and see what works best for me with my app. What throws a wrench in everything is I have seen time and time again that vacuum advance only effects mpg with highway driving

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Old 10-07-2011, 09:09 AM
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Distributr advance

Try Nelson Specialties 704-662-6600
He does everything from Sprint Cup to Antiques. He knows everything there is to know about ignition advance
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Old 10-07-2011, 10:47 AM
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Quote:
Originally Posted by Stubby686
My dist is giving me aprox 18*-20* of advance. I have been told and read in many other posts to limit it to 10* of advance with aprox 36* total or until I get "pinging" then to back down.
Question on this .... what difference in performance is there between the 20* of advance vrs the limited 10* ?
If you want 36º total, and the distributor mechanical is providing you with 18º-20º, the initial (w/o vacuum advance hooked up) will need to be 16º-18º.

Where the problems can arise, is if the engine doesn't need that much initial timing to run best, or if it need more. Or if that much initial advance causes any problems like hard starts when hot, pinging at the hit of the throttle, surging under light throttle cruise, or starter kick-back. But usually there will be no such problems w/18º initial advance.

If you have a performance cam that has more duration/overlap than stock, the initial needs to be higher than 10º in many cases. And using 18º-20º and more can be necessary when using cams w/more radical cam timing events. Often getting the right amount of mechanical advance from an OEM distributor will mean the advance mechanism will need to be modified, often to reduce the amount of advance. Does your distributor has adjustment bushings that set the mechanical advance amount?

Total timing is dependant on a number of things to do w/the engine, including (not all-inclusive):

• CR
• cylinder head material (aluminum or cast iron)
• cam timing
• cylinder head combustion chamber design
• piston design
• quench distance/effectiveness
• carb jetting
• fuel quality
• operating temperature/cooling system efficiency, etc.

As well as:

• vehicle weight
• final drive ratio
• intended usage
•ambient conditions- temperature, humidity, barometric pressure/elevation

Quote:
Next is the vacuum on the dist and the adjustment inside it using the Allen wrench. I believe the more the adjustment is backed out the quicker the advance starts coming in or maybe just the opposite. Question on this ... Again what performance difference is there between coming in early or late and does this depend on the power curve of the cam?
On the vacuum advance cans I have experience with, all the way CW will allow the advance to start the soonest, i.e. takes less vacuum to initiate advance and less to reach the max. The exact setting varies from engine to engine so there's no "right" setting. It takes trial and retrial to get the tip-in point where you need it to be. That said, 9 in/Hg is as good a starting place for the tip-in as any.

If your vacuum advance can is giving too much advance you can use a Crane #99619-1 vacuum advance limiter plate. Crane also has an adjustable vacuum advance can kit- p/n 99600-1 that has the limiter plate, springs and an adjustable vacuum advance can.

Quote:
Lastly the "all in" rpm and the curve it takes to get there. I'm all in around 2500-2800. My dist is brand new so it's not worn but it has the stock springs. Changing the springs out will change the curve.
Question here .... this changes when the "all in" is maxed out and the time it takes to get to that point correct? And if anything soaked in with my reading when I hear pinging I have it advancing to quickly. Now at this point would the Allen adjustment on when the advance kicks in make a difference ?
Changing springs changes when the mechanical advance starts, the rate of the mechanical advance as well as the RPM where the max is reached.

If you have pinging, there is too much timing. If the total timing is not too high for all the conditions I outlined above, then the rate of the mechanical advance can be slowed down by using stronger springs.

Changing the vacuum advance tip-in point- if the vacuum advance is adjusted correctly- will not have an effect on detonation IF you are accelerating enough that the vacuum has dropped low enough to eliminate (or nearly so) the vacuum advance from the timing curve. It's the "adjusted correctly" part that you are going to be searching for, as you change the tip-in point.

You may have a slight "transient" ping even w/the vacuum advance adjusted correctly when you hit the throttle from a light throttle cruise condition because it takes a second for it to drop out- but this can mostly be ignored as long as it's VERY brief. If you are pinging when you are gradually adding throttle, THEN you'd raise the tip-in point.

Quote:
I'm just trying to figure out how these different functions work and if any over ride the next so I can experiment and see what works best for me with my app. What throws a wrench in everything is I have seen time and time again that vacuum advance only effects mpg with highway driving
Using a vacuum advance has advantages other than economy. It depends on the situation, but a vacuum advance can (if hooked to a manifold vacuum source) help smooth the idle, the engine can run cooler at idle, the vacuum will be higher so the throttle blades will not need to be opened too far to have a good idle.

You will want to experiment w/both ported and manifold vacuum for the vacuum advance, though. There's no set rule for all apps, but don't be too surprised if you end up using manifold vacuum.

More info on vacuum advance tuning:
http://www.corvette-restoration.com/...c_Adv_Spec.pdf

Last edited by cobalt327; 10-07-2011 at 10:59 AM.
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