Building a high HP 383. About 1/2 track and 1/2 street, but not a daily driver. Trying to choose a distributor (going with CD), and trying to decide if I should just go with Mech advance only....from what I've read, it sounds like the vacuum advance is only going to come in at idle, and very low rpm cruising. Don't expect to have much vacuum. Any experience on this issue to share?
Timing - Mech Only Dist vs Adding Vacuum Advance
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There are a host of vacaum cans available that come in at all sorts of vac levels, down to 8", or maybe even 6", plus adjustable ones.
You have vacaum in the manifold virtually all the time, except WOT and maybe a lil less, so a vac can will add advance particularly in any steady state cruising.
While a radical com may have little manifold vac at idle, at a steady 2-3k cruise, there is plenty, 18" or more.
On more radical motors, it doesn't help as much as on a milder combo, heck, at >[email protected] .050, many just lock out the advance and set the initial to 36 and call it done.
It will probaly help response, economy, and driveability at part throttle cruise. May or may not help at idle depending on your idle vacaum, may even hurt with a radical cam and automatic trans (use ported vacx then)
you can always unhook it, and it don't cost any more. It's pretty darned hard to put one on a dizzy that's not made for one tho.
You have vacaum in the manifold virtually all the time, except WOT and maybe a lil less, so a vac can will add advance particularly in any steady state cruising.
While a radical com may have little manifold vac at idle, at a steady 2-3k cruise, there is plenty, 18" or more.
On more radical motors, it doesn't help as much as on a milder combo, heck, at >[email protected] .050, many just lock out the advance and set the initial to 36 and call it done.
It will probaly help response, economy, and driveability at part throttle cruise. May or may not help at idle depending on your idle vacaum, may even hurt with a radical cam and automatic trans (use ported vacx then)
you can always unhook it, and it don't cost any more. It's pretty darned hard to put one on a dizzy that's not made for one tho.
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601 Posts
accel makes a dist with vacumn advance and is adjustable. also springs in dist you can change to get mech advance out of it.
I've run distributors without mechanical advance in vehicles with 2500 stall or higher converters. But I never got rid of the vacuum advance, it helps most anywhere but full throttle. I did have an ignition cutout switch on the dash, 32* initial timing can be rough on hot starts.
With a bit of cam, the engine likes the extra advance at idle and off idle.
Good Luck!
With a bit of cam, the engine likes the extra advance at idle and off idle.
Good Luck!
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There's no down side to running the VA. You engine will run cooler, have more off idle snap, and you'll get better mileage. For the drags, the VA doesn't come in to play at WOT.
If the VA jacks with your idle run it to ported vacuum on the carb,..for instance if you have a can that starts to pull at 8hg and your idle vacuum is 7-10 the timing will flutter giving you inconsistent idle speeds. You can find a ported vacuum source using your vacuum guage.
Otherwise, run manifold vacuum, for a cooler running engine in stop and go traffic.
Good luck.
If the VA jacks with your idle run it to ported vacuum on the carb,..for instance if you have a can that starts to pull at 8hg and your idle vacuum is 7-10 the timing will flutter giving you inconsistent idle speeds. You can find a ported vacuum source using your vacuum guage.
Otherwise, run manifold vacuum, for a cooler running engine in stop and go traffic.
Good luck.
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