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vacuum advance question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by caryhenry, Jan 23, 2003.

  1. caryhenry

    caryhenry Registered Member

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    Ok guys, I'm new to the world of carbs, and I keep hearing about vacuum advance. My question is this: What is vacuum advance and what exactly does it do? I do know that mine is not hooked up on my 85, and it doesn't seem to make a difference. any info would be very helpful
     
  2. muscle

    muscle 1/2 ton status

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    As your rpm increases so does your vacuum. This increased vacuum is used to advance the timingon your distributor. Think of skeet shooting. The faster the target is moving from side to side, the more you have to lead it with your shot. The same principle applies to the speed of the piston. The faster it is moving in the cylinder, the faster and sooner you need to detonate the fuel/air mixture because the piston will be at the optimum location much sooner when detonation occurs.

    At idle your engine may be timed to be approx. 12 degrees BTC. At 3000 rpmthe timing may advance to 30 degrees BTC. Without advancing the timing at higher rpm your performance and fuel economy will be greatly diminished.

    Im sure there is a better explanation out there tho. /forums/images/graemlins/cool.gif
     
  3. BorregoK5

    BorregoK5 1/2 ton status

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    That was a good explaination of mechanical advance but you left out the vacuum advance. As your RPM's increase you need to advance (or lead as you say, I like that skeet description) the spark ahead of the piston reaching top dead center. While the piston is travelling faster as the rpms rise, the speed at which the fuel ignites remains the same, so you have to in effect pre-ignite the chamber so that the piston barely crests the top of its stroke to take full advantage of the burn. This is accomplished with mechanical advance. Your engine pulls air into itself as a result of this burn process through other cylenders. The carb or throttle body restricts how much air can be drawn in and at idle creates a vacuum in the intake manifold. When you open the throttle up and allow air into the engine the vacuum drops until the engine can catch up and pull more air than the carb or throttle body will allow, whcih creates a vacuum agian. This just so happens to be a good reference to how much load the engine is under since a low vacuum would mean the engine cant draw air in as fast as its allowed to do so. By attatching an advance to the vacuum signal, you can lead the spark even further under no load which results in better fuel economy and retard the timing under load to get that spark closer to happening right near top dead center and take advantage of a full stroke of power.
     
  4. muscle

    muscle 1/2 ton status

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    Like I said,I'm sure there was a better explanation out there. /forums/images/graemlins/thumb.gif

    I still like my duck huntin, skeet shootin tale as much tho...
     
  5. caryhenry

    caryhenry Registered Member

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    what should the vacuum advance canister connect to?
     
  6. R72K5

    R72K5 Banned

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    technically in most cases it needs ot be connected to ported connection to take advantage of higher rpm vacuum source to help increase advance that is needed then, actuation/pull of the vacuum advance diapragm rod that rotates the plate in the dizzy
     
  7. caryhenry

    caryhenry Registered Member

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    where is the ported vacuum on an 85 303 with stock M4M carb?
     
  8. Blue85

    Blue85 Troll Premium Member

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    In the stock application, the vacuum advance is tied to the engine vacuum through a thermal ported switch, so that it is not active until the engine is warmed up. If you still have the emissions label underhood, it will tell you exactly how it should be connected.
     

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