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Old 11-03-2008, 09:37 PM
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Timing Curves

Inside a sbc distributor,,,, the springs and bushings.
I'm wandering what are the do's & don'ts?
In my experience in playn w/ timing curves, it seemed like the lighter springs,,,,causing the timing to become fully advanced sooner,,,made the motor run better.
In my MSD pro-billet I can change stop bushings which controlls the amount of total centrifugal advance allowed.
I've heard that the more initial timing (made by turning the dist.) that you can safely run,,, before your motor becomes too hard to turn over,,, the better.
What are the avantages of vac. advance?
I run 15 base + 18 cent. advance for a total of 33 (nitrous motor).

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Old 11-03-2008, 09:51 PM
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Compression and cam overlap has a lot to do with initial advance required. A lower static compression coupled with a 'lumpy' cam will benefit from more initial advance.

Vacuum advance tapped from a ported source (metering block) is a good way to tune IMO. That way the additional advance comes in only at cruise for max economy.

Take a reading of your actual additional vacuum advance timing. 50+ total is common on cruise.

Your initial and centrifugal looks 'good' but we don't know your engine.

The Pro-billet is a very tunable piece.
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Old 11-03-2008, 10:00 PM
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383 sbc
Hyd. roller 294/300 dur.
.540/.562 lift
110 lobe sep.
1.6 rockers
9.8 compression
And the vac. adv is hooked up to the timed port not full time vac. port. on carb.
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Old 11-04-2008, 06:16 AM
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I would advance your curve (15+18) 4 or 5 more degrees. 37 or 38 total.

Make sure the mechanical adv. is rock steady at idle. Idle with initial adv. only. Make sure that your primary throttle blades are within the transfer slot (Holley) and the timed vacuum is zero at idle. Once you tip in the throttle you should notice vacuum at the timed port.

Limit vacuum adv. to 15 degrees, for a total 'all in' cruise adv. of around 53 degrees. Of course read the plugs and listen for detonation.

IMO.. of course..
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Old 11-04-2008, 05:02 PM
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I'm pretty sure 37-38 total advance while spraying nitrous will not be healthy for my motor.
I'd like to know if a faster or slower advance,,,,lighter or heavier springs,,,is better.
I put a timing light on it,,,,w/ vac. adv. plugged off,,,,reved the motor to about 3g and it was at 33. The current springs installed allows max timing at 3g,,,I could put heavier springs in and make it take 4g to 5g to reach full advance.
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Old 11-04-2008, 05:08 PM
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you can probably bennefit from more initial, but lock out some mechanical to keep your total at 33. I'd say you'd be safe with 17 initial. You have relatively low compression with that big cam. you want to keep your total down though for nitrous use. The lighter the spring you can run without detonation is usually better.
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