I don't know a thing about tuning. Can someone explain what timing does and how advancing it and retarding it effects how your engine runs? What do our cars run from the factory in terms of degrees of timing? What's optimal?

 

Greetings from a fellow texan, and gig em aggies.

timing is basically just that. Timing. If you look at 1 cylinder, the timing will be synchronization between intake valves, exhaust valves, piston position, fuel injection, and ignition spark. However, for common purposes, timing is meant to mean spark timing. Basically, spark timing is measured in degrees (of crankshaft rotation). As your crankshaft rotates, the piston gets close to, reaches, and then retracts from its top dead center TDC position. Most engines have the timing for spark set at an optimum position for the piston, which usually is near, but not on, top dead center. By adjusting the timing, you can ignite your fuel air mixture at a point when the combustion chamber is slightly smaller (piston closer to TDC). this means that the higher pressure at point of ignition will create more energy to force piston back down, and drive the engine. Car's timeing is set to reduce detonation, or pre-mature ignition of the fuel air mixture without use of spark. The heat and pressure alone of hte combustion chamber can be enough to start the gas burning, which means your power stroke occurrs out of synch with the other tasks mentioned (valves, etc), as well as being out of synch with the other cylinders. This puts serious stress on teh engine. Timing can be advanced or retarded, depending on how the engine has been modified, in some cases, its needed. Advancing or retarding too much will result in engine damage. There are others here who are better at it than I am, they can answer in more specific details about where most cars stock ignition points are.

 

Figured I would jump in here.

The timing that we usually discuss here in the computer section is ignition timing, or the point at which your PCM (car computer) tells the spark plug to spark.

The whole point of timing is to ignite the air/gas mixture in the combustion chamber at such a time so that the gasses in the cylinder expand and apply pressure atop the piston at ~10 deg crankshaft rotation after the piston has reached the top of the bore and begun traveling downward.

Why 10 deg after the piston has begun to travel down the stroke?
It is like pushing a kid on a swing. If you push to early, you will jar the kid (damage your engine), if you push too late your not getting the most out of your push. You have to apply your hands at just the right point/time to get the most out of your push.

What are advance and retard?
Well, the difference between pushing a kid on a swingset, and the piston is that the piston is moving at thousands of feet per second.

The piston is traveling so quickly in the bore that the air/fuel mixture is actually ignited while the piston is traveling up the bore or before Top Dead Center (when the piston is at the top of the bore). The amount of time/crank angle that the spark is applied before the piston reaches Top Dead Center is called ADVANCE.
If everything works properly, the spark will spark before the piston reaches TDC, there will be a momentary delay, the gasses will ignite, the gasses will expand at such a rate that they act upon the piston just as the piston has passed TDC and began to move down the bore (10 deg after TDC).

Typicall timing for the LS1 Engine (some variances ie Z06)
1998-2000: Part-throttle ~30 to 45. WOT 28
2001-2002: Part-Throttle in the 30's. WOT from 26 to 19

Good Luck,
Kevin

 

Since you live in Texas you just wait till Feb. 1. Get on I-10 eastbound. Drive thru Houston to Baytown. Take FM 565 to Houston Raceway Park. Walk into the track and ask that question to anyone of the 10,000 people you see in the pits. You should go home as a timing expert.

 

Thanks for the info. I was interested in this post as well. The only thing I don't understand is why there is more advance at part throttle than WOT. Is the piston not moving slower at part throttle, and therefore require more time to reach the 10 degrees after TDC, so less advance would be necessary? I don't have a very analytical mind, so bear with me. Thanks in advance (no pun intended).

 

So it's lower timing that hurts the engine? I thought it was the other way around. You want to advance timing to right before you get detonation right?

 

Timing is not only a function of piston speed, but a function of how much air is in the cylinder.

At part-throttle driving there is very little air in the cylinder. Most of us drive around at 20% throttle. If you look at the Throttle body in this condition, you will see that it is barely open, and there is very little air entering the cylinders.
The less air that is combusted in the cylinders the longer it takes for the pressure expansion to ACT on the piston (simply because there isn't that much air expanding). This is why you have to ignite a part-throttle combustion much earlier, like 35 degree's before Top Dead Center.

At WOT there is alot of air entering the engine and at combustion there is alot more expansion that takes place simply because there is more air. The increase in expansion rates results in the piston getting ACTED upon very quickly. As a result you have to wait before you ignite the mixture otherwise the combustion will ding the piston while it is still on its way up or at TDC.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ZYouL8R:
<strong>So it's lower timing that hurts the engine? I thought it was the other way around. You want to advance timing to right before you get detonation right?</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Too much Advance is what hurts the engine.

If you recall, Advance is how much we ignite the mixture BEFORE the piston reaches Top Dead Center. If you run too much advance, you aren't giving the piston enough time to reach TDC and travel back down the stoke before the combustion front smacks into it.

Less advance means that we are letting the piston get closer to TDC on its way up before we ignite.