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Old 01-20-2011, 04:41 PM
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ignition curve

can you have the advance to come in too early in a street driven car w/ hei? say if you have an advance curve that has 12 degrees initial at 800rpm and advances to a total of 34 degrees total at 1600 rpm, is that too early? what problems could arise?

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Old 01-20-2011, 06:01 PM
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The problem of detonation can occur, and you can`t always hear it. But if it`s occuring and doing it`s dirty work it`s only a matter of time before stuff starts coming unglued. Usually at idle, with a 9.5:1 comp ratio, I shoot for 12 degrees initial, with 12 more Vac advance so the total at idle is 24 degrees. This is due to lean mixtures which burn slower so more advance is needed. Idle and off idle mixtures are lean so more timing in those situations are okay.
However, 34 sounds like alot by 1600 RPM. But, it is recommended to have the timing all in by 3000 RPM. But it`s been so long since I`ve looked at a sbc timing curve with a timing light I couldn`t say for certain if it`s too much. If it runs okay and your positive you don`t hear detonation then I`d say it`s okay.
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Old 01-20-2011, 06:59 PM
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Theres an old saying like "as much advance as fast as possible without detonation". Every engine is unique. On a bone stock 350 it might be a bit too fast but it doesnt sound totally unreasonable to me.

If you dont hear knock and dont feel sluggish you might be ok. However, more advance doesnt always equal more power....which is why theres a "curve" to give timing at the proper rpm.
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Old 01-20-2011, 07:26 PM
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Depends on the engine. The 36 degrees total is giving the rod angulartiy maxiumum advantage over the crank to make kenetic torque at WOT. This 36 degrees advance brings the piston rod 6 degrees past TDC power stroke. This geometry happens past half of 5250rpm or 2725rpm - on, after 5250 the engine will naturally retard . Before 2725 less than 36 degrees will be favorable for maxiumum power.

Last edited by spinn; 01-20-2011 at 07:42 PM.
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Old 01-20-2011, 08:21 PM
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Im kind of confused now, i was going to change the springs so that the curve would take longer before fully advanced say around 2800rpm. I had 2 light springs installed before, now i installed a heavy spring and a medium spring just to see what would happen, the curve is EXACTLY the same. What could be the problem? do the weights have anything to do with it?
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Old 01-20-2011, 08:31 PM
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Vacuum advance unplugged? Yes.
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Old 01-20-2011, 08:42 PM
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yes thats with vacuum advance unplugged. im getting 22 degrees of advance, but its coming in extrememly early, right off idle. what can i do to stop it advancing so early? I just bought the mr. gasket recurve kit for $10 and i feel like they just raped my wallet.
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Old 01-21-2011, 09:20 AM
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The timing curve often is unstable and inconsistent when using the lightest springs from an advance curve kit. if you want a fast curve use 1 light and 1 medium tension spring.
Automatic application cars tend to do better with more base timing and a more
slower curve peaking around 3000 rpm.

The just right amount and rate of vacuum advance has to be found by trial and error drive testing.
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Old 01-21-2011, 09:31 AM
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Quote:
Originally Posted by stroker444
yes thats with vacuum advance unplugged. im getting 22 degrees of advance, but its coming in extrememly early, right off idle. what can i do to stop it advancing so early? I just bought the mr. gasket recurve kit for $10 and i feel like they just raped my wallet.
If you have swapped out the original weights for the weights in the kit, you may not EVER get the curve right. Swap the original weights and cam back in and use a pair of medium springs from the kit to start with, and go from there.

Now, if using the original weights and cam, using heavier springs doesn't cause the advance to start later and peak later, you will need to either change to different OEM weights and cam or take some weight off the OEM weights you now have.
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Old 01-21-2011, 01:16 PM
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Quote:
Originally Posted by cobalt327
If you have swapped out the original weights for the weights in the kit, you may not EVER get the curve right. Swap the original weights and cam back in and use a pair of medium springs from the kit to start with, and go from there.

Now, if using the original weights and cam, using heavier springs doesn't cause the advance to start later and peak later, you will need to either change to different OEM weights and cam or take some weight off the OEM weights you now have.
is there a guide on how to do this? I still have the weights that my distributor cam with, i only installed a medium and a heavy spring from the kit.
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Old 01-21-2011, 03:20 PM
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The problem you will have the idle will be fluctuating. The idle will drift up and down.
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Old 01-21-2011, 04:41 PM
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Quote:
Originally Posted by stroker444
is there a guide on how to do this? I still have the weights that my distributor cam with, i only installed a medium and a heavy spring from the kit.
You can a good idea on how to work on an HEI here:

http://www.rustynutscarclub.com/HEI.htm

Typically 22 deg of mech advance within 800 rpm is really too quick but if you are not experiencing knock/ping then so be it for now, but it has me wondering why thats for sure

What is your total timing at when you add vacuum advance?
TJ
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