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PICKMUP · 05-17-2007, 11:43 PM

All HEIs are not created equal. Most drop in, set up pretty close, and run good...but some of these have just way too much dist advance built in (vac & mech combined) and have to be "fixed" to make them compatible with our heavy trucks.
This is one of those. I don't have separate vac & mech figures on this one yet, but total is over 50.
My 68 GM serv manual (396) calls for mech: 28 @ 4200rpm (only 15 @ 2000) and vac: 15 @ 15"+ with 6 initial setting.
My 79 GM serv manual (454) call for even less with mech: 20 @ 4200 (only 14 @ 2800) and vac: 10 @ 13"+ with 4 initial setting.
Sorry, I don't have a '71 manual close, but it looks like GM set these heavy trucks with BB engines to run around 30 degrees total advance for running in mid-range cruising rpms at part throttle. That seems to agree with what has worked best in the past.
So, until I can get an adj vac adv pot or limit the mech adv to shorten up the curve...I will have to leave the vac adj off. Not best, but the pinging will burn a hole in the pistons over time...and it drives me crazy.

05-17-2007, 11:43 PM	#34
PICKMUP "Trucks with Class" Join Date: Jul 2000 Location: OLYMPIA, WA, USA Posts: 8,168	Re: vacumm advance question All HEIs are not created equal. Most drop in, set up pretty close, and run good...but some of these have just way too much dist advance built in (vac & mech combined) and have to be "fixed" to make them compatible with our heavy trucks. This is one of those. I don't have separate vac & mech figures on this one yet, but total is over 50. My 68 GM serv manual (396) calls for mech: 28 @ 4200rpm (only 15 @ 2000) and vac: 15 @ 15"+ with 6 initial setting. My 79 GM serv manual (454) call for even less with mech: 20 @ 4200 (only 14 @ 2800) and vac: 10 @ 13"+ with 4 initial setting. Sorry, I don't have a '71 manual close, but it looks like GM set these heavy trucks with BB engines to run around 30 degrees total advance for running in mid-range cruising rpms at part throttle. That seems to agree with what has worked best in the past. So, until I can get an adj vac adv pot or limit the mech adv to shorten up the curve...I will have to leave the vac adj off. Not best, but the pinging will burn a hole in the pistons over time...and it drives me crazy.