Power Stroke

Suck, Squish, Bang, Blow Part Six:  Ignition Timing Basics

By Mike Kojima

Ignition Timing is expressed as the number of degrees of crankshaft rotation in advance of Top Dead Center or TDC, when the sparkplug is fired.  This kicks off the explosion of air and gasoline that pushes the piston down to drive the crank.  Since even explosions take a few milliseconds to develop, the engine's ECU usually commands the sparkplug to fire a few degrees of crank rotation before TDC.  This way the explosion will be fully underway by the time the piston reaches TDC and will be able to push the piston down with the greatest effect. 

igntion advance
Until a few years ago, you turned the distributor in the opposite direction of rotation to advance the timing.  Then it was turn the crank angle sensor.  In the past few years, new cars are much harder to advance the timing.  If your car was made before the year 2002 or so you are probably in luck and can adjust your timing!

If the sparkplug fired after TDC the piston would already be going back down the bore and the explosion would not be able to give the piston the biggest push.  Firing the spark retarded, or after TDC can also cause the engine to overheat by increasing the amount of time that the burning mixture can impinge upon the engine's internal parts as the piston travels downward.  It can also dump a lot of heat out the exhaust valve, possibly damaging it and other exhaust components.

Timing light
To adjust your timing you use a timing light which is a strobe light synced with the firing of the #1 cylinder.  This freezes the crank pulley motion at the timing pointer where there is a scale of where your ignition is firing.

The number of degrees that the plug fires before TDC is called Advance.  Generally the more advanced the spark is the bigger of a head start the combustion event can get before the piston reaches TDC.  This allows for more pressure to be built up and a more powerful push to be put on the piston on its way back down the bore.  It is possible to advance the timing too much and cause the engine to lose power because the explosion of a too far advanced spark will occur too early and actually hamper the pistons rise to TDC.

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Wednesday, January 26, 2011 6:18 AM
Sorry, I should have made those links. Admin can delete my first post if they want.

I thought these two videos would be appropriate:

Recording of combustion event using an endoscope:

Recording the pressure of a combustion event:

In the first video you can see lots of stuff happening, including the spark event happening before top dead center as discussed in the article.
In the second video you can see the boundary layer, flame front, ect.
Dusty Duster
Dusty Dusterlink
Wednesday, January 26, 2011 10:09 AM
Hmmm, I haven't checked my ignition timing in a while. I should break out the timing light.
Wednesday, January 26, 2011 1:15 PM
As usual, kickass article. Two thumbs up!
Der Bruce
Der Brucelink
Thursday, January 27, 2011 7:06 AM
Mike, excellent explanations!
Since you alluded to it for the future, I was wondering if you had read through the book "How to tune and modify engine management systems"?

I'm about half way through it but wondering if I'm going to have a good enough base from this book or any book to start with some simple tuning techniques and then build up from there. Maybe that's the direction you're taking us in, as hinted by the end of this article?

Thanks for all the help, Mike!
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