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Old 07-18-2007, 08:11 AM
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Setting initial timing by manifold vacuum

I was just cruising around the board looking at some old Hemi threads when I came upon on a thread posted a few years back about setting initial engine timing by using manifold vacuum as a reference.
In tuning my engine, I noticed that my manifold vacuum still increased with more advance, even after what I thought was the correct timing for my setup. I was running 12 deg. initial and getting between 15-16 inches of vacuum, which isn't bad, but as I gave it more advance, my vacuum steadily increased to a maximum of 18 inches at idle...Timing was now up to 18 deg. initial, which is more than usually advised by many Hemi guys, but the engine didn't seem to mind that much advance at all.
Just wondering who else has set initial advance by manifold vacuum and their experiences.
I understand that I'll have to take a little total timing off of the other side, by re-curving the distributor, but that's not a problem.
I can't do any drivability reports yet since the engine is on a run-in stand, but so far, so good.

Steve

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Old 07-18-2007, 08:28 AM
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That's how I set all my engines. Once you mess with cams, compression, ignition, etc. there aren't any 'factory specs' to go by. Manifold vacuum doesn't lie. Set initial timing @ max vacuum. Then drive and if it pings, back off a degree or so and re-drive. Keep this up 'til the thing doesn't ping at any driving condition and your timing will be perfectly set.
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Old 07-18-2007, 09:02 AM
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You need to remember that if you dial in 6 degrees of advance at idle then ALL the timing curve is going to be advanced that far. So if you dialed in 39 total, with the base at 12, then now your total is 45.

Vacuum advance hooked to manifold will do exactly the same thing, except keep the base mechanical curve intake for total timing under wide open throttle conditions.
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Old 07-18-2007, 11:30 AM
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You will find that initial timing requirements are dependant on camshaft
design. Long duration cams will need more advance at idle because of
the slow burn caused by increased exhaust gasses in the fresh charge.

For best idle quality, add initial and limit centrifgul to compensate.
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Old 07-18-2007, 01:35 PM
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Thanks to all who replied. Willys36---yours was the original post from a few years back that I read. Thanks for the confirmation.

Steve
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Old 07-18-2007, 01:53 PM
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Hi Steve,
Here's a link to pictures of some of the Local Hemi cars running

Cool Race Pics
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