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Old 10-20-2003, 03:33 PM
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HEI/vac advance/mech advance

I had been having trouble with my timing jumping around and running a bit rougher than usuall, I took the truck to a speed shop for a run on the dyno and a dyno tune. They dissconected the vac advance and plugged the carb telling me that my timing was way too advanced and I could basically melt my pistons as it was, they suggested a new HEI and dialed everything in.

Is it okay to drive all the time with the mechanical advance only hooked up? Whats the difference between the mech and vac advance.

I know, lame basic question but distributors are a new thing for me.

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Old 10-20-2003, 06:40 PM
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The vac advance gives you better throttle response at lower rpm's and helps improve gas milage, keep the vac advance on a street car.
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Old 10-20-2003, 06:45 PM
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I wouldn`t buy a new HEI, I`d rebuild the one I had, rebuilding a HEI is a breeze. mechanical advance is under the rotor, it`s rate of advance is controlled by spring tension, weights, and centrifical force from accelaration, that`s why advance kits sold in parts stores have different springs and weights so you can dial in the rate of advance you need. vacuum advance is just that, it works of a vacuum signal off idle. your timing should be 12 degree`s before top dead center and 38 degree`s total all in by 3000 RPM`s, if detonation is heard, retard the timing 2 degree`s at a time until it stops and it`s set. set the spark timing with the vacuum advance removed and plugged. it`s okay to drive it without it, in most causes if a big cam and other mods are used, you don`t need a vacuum advance, or no where near as much advance, that`s why adjustable vacuum advance canisters are made.
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Old 10-20-2003, 10:51 PM
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Vacuum advance will make it run smoother at part-throttle & get better gas mileage.
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Old 10-21-2003, 08:43 AM
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thanks guys. the truck is actually running better now with the vac off for some reason. my cam specs are 306 duration and 550 lift so I pretty much have very little vac at all, timing is 36-38 total.
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