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roadman65 04-03-2013 06:36 PM

Vacuum advance not needed?
I have been restoring my 84 C 10 and installed a GM 350 HO crate engine. The spec sheet from GM states not to use the vacuum advance on distributor but to keep it plugged at all times. I'm just wondering if any of you experts would comment on this. Seems strange to me. Engine has small hyd roller cam. Thanks.

vinniekq2 04-03-2013 06:58 PM

try it and see if mileage changes?

Custom10 04-03-2013 07:46 PM

The GM instructions do this in part to cover their butts, less advance means less chance of detonation/pre ignition due to over advancing and thus fewer warranty issues.

Learn the advance curves of you distributor, how much mechanical advance does it add, how much vacuum adv does it add. There are hundreds of threads on this. Set them up to suit the engine/truck

For the most part it is advantageous to run vac advance as it can help improve part throttle effiecencies (fuel milage as vinnie states), clean up idle characteristics etc. some motors can benefit others not so much.

roadman65 04-04-2013 05:12 PM

Vacuum advance not needed?
Thanks Custom for reply and Vinnie. I'll hook it up and tune accordingly. I just thought it was might strange.

Custom10 04-04-2013 10:32 PM

I might strange indeed, makes one wonder.

If you do get the vac can hooked up you can measure its total added advance using a timing light. Trick at idle is to get full extent of the canister rod to pull full advance and not start the mechanical advance addition via rpm increase. Also if the engine vac is less than 14 at idle you may need a vacuum pump, most vacuum cans dont pin out until around 12-13 " hg

The point is if you have mech adv and vac advance at idle you cant always count on mech advance being steady one way or the other at lower rpms, making the vac adv determination tricky so just try and keep mechanical adv out of the equation.

Otherwise the skip my morning coffee method is to spin the RPM to 4000+ and take a BTDC timing reading without and with the vacuum advance applied, a vacuum pump is good plan here.

All this needs is a good vacuum gage, dial back light or marked balancer to 60 BTDC and someone to hold the brake :sweat:

454C10 04-05-2013 04:42 AM

read this.

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