Timing Question

[james singleton]

I am a little confused about timing and the affects by electronic ignition systems. I am sure that I am missing something here, but maybe someone will be so kind as to explain something to me. Here goes: I have read in my manual that "IDEALLY" the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC on the compression stroke (forcing the piston down), but because it takes a fraction of a second for the spark plug to ignite the mixture, the spark plug must fire a little before the piston reaches TDC. Therefore the timing spec'd out for a 65' - 200c.i.d. engine (with a manual trans.) is listed at 6 * BTDC! Now here is my question: Since electronic ignition makes for a better spark than the orginal points & condenser set-up, it would seem to me that adjusting the timing closer to TDC, say 2 or 4 degrees BTDC, would make more sense than 10 to 14 degrees BTDC like everyone seems to advocate when changing over to an electronic ignition? Since the spark is supposedly better with an electronic system, it would seem to me that the spark could be delivered a little closer to TDC!? By the way, I just installed a Pertronix l just recently and have it set currently @ 11 degrees BTDC. Do I have advance and retard somehow backward here? I know there is something that I am missing here. Could someone please explain. Thanks, Jim

 

[JackFish]

I have read in my manual that "IDEALLY" the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC on the compression stroke (forcing the piston down), but because it takes a fraction of a second for the spark plug to ignite the mixture, the spark plug must fire a little before the piston reaches TDC.

I know there is something that I am missing here; I must have something ass backwards here. Could someone please explain. Thanks, Jim

Gas doesn't explode, it burns slow enough that the plug must fire before TDC in order for the gas to be ignited optimally.
6° BTDC is the old stock spec, most of us are running 10°-12°.

 

[CoupeBoy]

Have you looked at?
How Automobile Ignition Systems Work
And the reason you want to fire BTDC is so that the flame starts burning early to ensure all the fuel is burnt before the piston reaches BDC. Anything burning on the way up is costing you power.

I'd be willing to bet that the reason you can use more advance is that with the old points/low voltage coil it cannot inject enough juice to ingite the fuel properly. But with more precise control and more volts..

-ron

 

[ludwig]

The explosion/burn takes the exact same amount of time to go off whether at idle or at 5500 RPM but the time between TDCs shortens as the engine goes faster. Thus the advance makes a spark earlier relative to the cylinder position so that the constant max explosion is always just past TDC. The spark has to come earlier as the engine revs (advance) or later as it decelerates for the constant explosion/burn time.

 

[james singleton]

I understand the concept that the "burn"/explosion (the manual uses the word explosion, so I guess I can also) needs to happen just before reaching TDC "because it takes a fraction of a second for the spark plug to ignite the mixture. And I understand that you want this "burn"/explosion to force the piston down on the combustion stroke, but yet not exert force on the top of the piston when it is on it's way up on the compression stroke. The question I have has to do with electronic ignition which is a better, more efficient spark, and how this better spark relates to the timing being set at 10*- 14* instead of 6* BTDC. How does this additional advancing of the timing (14 versus 6) benefit the burn/explosion???? Is it the fact that the spark takes place when the piston has not traveled as far on the up-stroke (compression stroke) and therefore there is less "compressed" gas&air in the cylinder and this causes a slower burn, or something like that?? I am trying to figure out the 14*BTDC versus the 6*BTDC!???? I am not questioning the additional advance with Electronic Ign; I am just trying to understand it. Thanks, Guys

 

[66 Fastback]

Keep in mind that the 6* advance is only one component of the advance. It is set with the distributor vacuum line disconnected and plugged. Connecting the vacuum line will typically advance timing even more at idle. At idle and low engine speed, the volumetric efficiency is lower and your incoming fuel charge is diluted with unexpelled exhaust from the previous cycle. This diluted charge takes even more ignition advance to get an adequate ignition and burn. As was mentioned, there is not an explosion but rather a burn that generates pressure. You want the peak pressure to occur as the piston is past TDC. If you set the timing to 2*, you have now retarded the timing or initiated the burn later. Therefore the peak pressure will occur later on the down stroke. This will cause a decrease in power that is caused by lower peak pressures.

Many folks will argue that an electronic or CD ignition offers no advantage over a properly tuned and maintained points type ignition. While that may be true, my experience is that the electronic and CD ignition is more forgiving.
Doug