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Old 02-27-2006, 04:38 PM
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curtis73 curtis73 is offline
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Don't worry about how much initial you have. If it takes 36* to make it idle properly, then you need 36*. Actual timing at idle is of minimal importance. You have to take into account several things that happen after idle, but its not a big deal.

Mine for instance is locked at 36* No curve except from the vac advance. Of course, my example is a BBC with 8.6:1 compression. BBCs need a good bit of lead and my low compression needs even more. I could have gotten away with as little as 22* and still idle, but 36 still let me crank it to start and gave me another 2 inches of vacuum and a smoother idle.

I suggest finding how much initial your combo will take. Advance it a couple degrees and try to start it. Keep going until it kicks the starter back. Come back down a couple degrees and that setting is where it will let you start and idle. Make sure you do all that hot so it won't cause trouble later. You can guess that your combo will probably like about 36-38 total, so you can make a curve between what you discovered at idle and the total advance. Lets say for instance you start kicking back at 28* initial. You want 38* total. You then just use a combo of bushings that gives you a total of 5* in the dizzy or 10 on the crank. Then pick your springs to set where it comes in.

Then you can use ported vacuum to bring more timing in under part load. That you can set with an adjustable can and just loading it down under various conditions and backing off the canister to keep detonation away.

There are certain situations with heftier cams, larger chambers, or certain combos where the kickback at startup doesn't occur until well after 38*. Its rare since it often occurs with very low cranking pressures like might be had with a mismatch of low compression and too much cam, but I don't think that will be the case here.

I assume you have an auto tranny. If so, the task is pretty easy. If you have a manual, chances are you'll have to broaden your total curve. If you just set it (like the example above) to your optimal 28-38* curve, 28 might be too much when you floor in third at 1500 rpms. If that's the case, you'll have to back it down (for example) to maybe 24* initial and then broaden your curve to 14* of sweep. You'll have to open up the throttle just a touch to compensate, or drill some holes in the butterflies to keep the ported vacuum hole covered at idle.

In short, the answer to your original question: Best initial timing setting is as much as you need, so long as it doesn't make too much at part throttle or cause annoying starter kickback. In theory, the more initial the better on a non-smog car. The more initial you have, the less throttle opening you need which improves vacuum and throttle response while providing a smoother idle. On a smog car it might make NOx above acceptable for idle tests.

Idle timing settings are only important as a means of setting a benchmark for the rest of the curve. When Chevy made a 1978 350, they had a curve that was set for X degrees of advance. The way to set it was simply by putting it at 12 BTDC, but that number has very little to do with what it needs at idle, its just a reference. Setting it to 12 BTDC simply means that it will provide the engine with 36 total by 3200 rpms. Since you're building your own curve, initial timing is not a benchmark for you. You can find your own optimum idle setting and build your curve from that.
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