Vacuum advance question

[FyreDaug]

If I hooked up 2 full vacuum sources to both vacuum advance ports on the distributer would it advance it more?

Reason being I like the high advancement for low end power but it is hurting top end (I have the end of the dist cut off, so technically I could just keep turning the dist when its not tight to get high advancement). So if I could retard it a little more and add some vacuum to advance it more, technically at high end no vacuum it should retard it more than it is now if I set the timing the same with both advances running.

Aside question: If Im detonating with very very high advancement would higher octane help, or is the piston just too close to BDC for it to do anything? :uh:

[A20A1]

Yes it would advance more. Higher octane should help.

[FyreDaug]

Why would it help though? Maybe I need a read about gasoline. Why does it detonate at high advancement basically?

[A20A1]

Higher octane resists detonation,

As for advancing the distributor body, I've notticed the same lack of power in the top end, so I leave it at or near stock.

[FyreDaug]

Yeah I tried that too, but it doesnt do good in the low end.

Theoretically (this is where I'm not too good with motors) if it was advancing it because of vacuum (throttle plate not 100% open). If I did open it right up at 2000 rpm, does my advancement go out the window or does it stay until higher rpms?

I know higher octane resists detonation, but why does it detonate with such high advancement?

And lastly when I have constant vacuum going to the advance port on the dist, can the dist be advanced more by putting more vacuum to the port? (IE, not having a split on a vacuum line would result in higher vacuum pressure on the dizzy?)

[A20A1]

You could have early-ignition... thats not detonation.
Too early ignition timing resists the movement of the pistons upwards.
Same as if you were to get Pre-Ignition.

[FyreDaug]

well it wouldnt be early ignition if it was advanced right?

I dont even have a timing light so I do all my timing by ear, and I'm usually dead on. But just for reference if vacuum advance was disabled and it was fully advanced, what degree is it? What about all the way retarded?

And what does each vacuum port on the vac advance add assuming enging is all full vacuum? I noticed the secondary one (the one that should only be hooked up when its cold only) changed it less than the primary one. So is it safe to assume 6 degrees and 2 degrees?

EDIT: By the way, I retarded it ~8 degrees and hooked up the secondary advance and power seems to be distributed all throughout, didnt notice any power decrease in low end, though it will "bog" a little if you punch it rather than increasing the throttle.

[FyreDaug]

Alright me and ian3769 are sitting across the room arguing about how ignition timing works my point being that the slower the engine is moving the more advancement you need, and the higher it is, the less advanced.

Ian is saying its the other way around.

Now unless Im wrong, timing is measured AFTER the piston reaches TDC, so 2 degrees past TDC means its slightly on its way down before the gas is ignited. 30 degrees meaning its probably close to half way down the cylinder before its ignited. Is that correct? Ian is saying that ignition timing is done before it reached TDC. Maybe I'll get him to respond too, hes on the other computer.

[Ian3769]

what i'm saying is when the piston is going inside the engine, you want the spark to happen just slightly before top dead center, so that the air fuel mixture is combusting exactly when the piston reaches it's apex, therefore it does the work at the appropriate time. thus advance the spark to get this effect. I don't know if octane booster is going to help signifigantly though, but then i've been wrong about some of Dallas's other ideas that seem to work okay(dual ignition for example).

edit: (entire car for example)

[joebeets]

If your timing is too far advanced, combustion will be complete roughly at TC and this will cause knocking due to the high pressure and temperature. If it's too late, you'll never develop optimum pressure in the cylinder and never develop optimum power. The stock setup advances and retards automatically in response to load and engine rpm. I don't think you should mess with it especially with all the considerations involving the catalyst, emissions, warmup, and so on.

[A20A1]

Advancing is early ignition... the spark if formed earlier in the stroke.

[FyreDaug]

Hmm so apparantly I had my thoughts mixxed up.

[joebeets]

It's easy. You want to keep the beginning and end points of combustion roughly symmetrical about TC. At fixed load, you need to advance the spark as RPMs go up, to give the mixture enough time to burn. At constant RPM, higher loads cause combustion temperatures to increase, which will cause knocking unless the spark is retarded.