I've never really had an in-depth description of what timing is (other than how much certain cars with certain setups like, to back it down when you're getting detonation, and so forth). If anyone could shed some light on this or point me in the direction for an in-depth description, and also possibly explain what dictates timing in our cars, I'd appreciate it.

 

I would love to see this aswell.
I'm fighting off some KR right now, and not sure where to start.

 

Timing is the amount of degrees before top dead center of the compression stroke that the spark plug will fire off the sprak. Here is a part of a good article from howstuffworks.

When the fuel/air mixture in the cylinder burns, the temperature rises and the fuel is converted to exhaust gas. This transformation causes the pressure in the cylinder to increase dramatically and forces the piston down.

In order to get the most torque and power from the engine, the goal is to maximize the pressure in the cylinder during the power stroke. Maximizing pressure will also produce the best engine efficiency, which translates directly into better mileage. The timing of the spark is critical to success.

There is a small delay from the time of the spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder reaches its maximum. If the spark occurs right when the piston reaches the top of the compression stroke, the piston will have already moved down part of the way into its power stroke before the gases in the cylinder have reached their highest pressures.

To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke, so by the time the piston starts down into its power stroke the pressures are high enough to start producing useful work.

http://auto.howstuffworks.com/ignition-system.htm

So all the timing numbers does is tell the PCM how far before the piston reaches TDC it will fire off the spark.

 

How much KR are you seeing?

 

Thanks for the info, Brad. But why does adding more timing (igniting the spark earlier) sometimes cause detonation? I would think it would be the opposite, as the piston would be compressing the air more before the spark had a change to light, thus making it more susceptible to spontaneous combustion due to stagnant heat.

 

Because if it lights to early the explosion will occur while the piston is still moving upward. The downward forcing explosion combined with the upward moving piston causes the piston to be put under tremendous pressure from the opposing forces, so what the piston does is it shakes around violently in the cylinder. That is what detonation is and it can destroy an engine.

Remember that there is a slight delay in when the spark is sent by the PCM and when the spark jumps the gap in the combustion chamber. That delay is the reason for the PCM sending the spark out early like at 39 degrees or 28 degrees at WOT.

 

No, I realize all that and what exactly detonation is, but why does it do it when the timing is too far advanced rather than too far retarded?

 

Because when it is too early the 2 forces fight each other, when it is retarded the spark will occur at TDC or later, in which case the explosion force will not fight the piston but rather aid it in its already downward motion on the power stroke. If you retard timing too much you will just see a drop in power rather than damage.

 

I understand the advance a bit more now, but am I wrong in thinking that retarding the timing allows for more compression of the mixture making it more vulnerable to being spontaneously combusted or is that just totally wrong?

 

Yes you are wrong in thinking that. The advance is there because of the delay between when the PCm sends the signal and when the sprark actually jumps the gap. So in theory when you have lets say 30 degrees of advance, the spark still jumps the gap 30 degrees later, which is TDC, which allows for full compression. Now if you retard to 20 degrees under those same operating conditions that spark will jump the gap after TDC which results in power loss. Either way there is full compression.

 

Gotcha. Now for more questions...

I understand how too much compression causes detonation (which I believe is was what I was alluding to in my most recent question), but how does a certain head's chamber make it less/more prone to detonation? For instance, some people with X heads can run 30 degrees of timing while someone with the same setup and Y heads can only run 25 degrees. Assuming the chambers are both the same size, only the design is different, say, from manufacturer to manufacturer.

 

I'm getting somewhere in the range of 4* but that number is typical mostly when I get on it from a higher gear, leading me to believe its Burst Knock correction from the computer.

I've adjusted the spark table reducing advance up to 8* in trouble spots and it would still remain in the same areas. I only spent a little time on it though as I've been working to get my VE as close as possible and then move on to Knock issues. I also just got some new injectors in so ill have to get them worked out first.

Thanks for that article, that was excellent.
Pretty close to the understanding I had of spark timing and advance and retard. But it gave more info as far as how pressure affects it. Which was nice.

I also thought that at higher RPM's, spark would need to be advanced as much as possible as the time between TDC and power stroke became less and less and Air/Fuel can only burn so fast.
I'm still unclear as to how advancing too much can cause damage, but I havn't read the rest of this thread yet.

Thanks for the article brad.

DJ

 

Nice article.

 

I think you had it right in the first description you gave him, but here it seems the wording is a little off. My understanding is it's not the delay in the time from when the PCM commands the spark until the spark plug fires. Rather the time from when the PCM commands the spark, the plug fires (which is virtually instantaneously), and the time the mixture takes to burn. Electricity moves a lot faster than the burn rate of the mixture in the cylinders. This is also connected to the octane fuel used. Lower octane fuel burns differently than higher octane, which affects how much advance can be run. If you look at a cylinder cut-away, imagine the piston coming up...compressing the mixture...and when the spark plug fires x number of degress before TDC, the mixture burns down towards the piston in all possible directions (like the ripples seen when a stone is dropped in a pond). Of course, the head and cylinder walls prevent a 360* burn and the path of least resistance for the force created is to push the piston down. It's the timing of the burn rate that determines when the plug needs to fire - not so much the delay in when the PCM commands it and when it actually happens.

 

Yeah, Im sorry bout that. Yes the mixture does take time to burn after plug is fired as well, that is the main factor. Also keep in mind that a faster moving piston(higher rpm's) will affect timing because there is less time to burn the mixture before it reaches TDC. This is where fuel octane comes into play as well, higher octane takes longer to burn, allowing for more aggressive timing.

 

Spark plug placement has something to do with it too as well as the fact that different heads of the same chamber size still have different flow characteristics.

 

Ever play baseball...have a game of catch? When the ball is thrown to you, you have 3 options to catch it. You can obsorb the catch, keep your glove still or stab forward at it. This is very similar to advance. The force of the ball is like the mixture burning in the cylinder. The perfect timing advance is like holding your glove still. It's not too painful, but if the ball is moving fast enough...it has enough force to make your palm move back (and sting a little). Now, assume the ball is traveling just as fast and you stab your glove forward at it. You might be looking at a fractured bone or two in your palm. This is like having the timing too far advanced. Your hand moving forward is like the piston still traveling up in the cylinder, the ball is like the mixture burning too early, and the fractured bones in your palm are the result (aka your pistons, rods, bearings, etc.). The extra force is a bit more than your hand was designed for. Assuming you obsorbed the ball, it would be just the opposite - no pain because the force of the ball hitting your palm came at a time when it was less effective. This is like having too little advance, which will also make less power in a motor. That at least describes the mechanics. When you incorporate cylinder pressures (max seen at peak tq), that effects the burn rate. Since the mixture burns better with more pressure, you typically see less advance around peak pressures (aka peak tq). To get the most out of your motor, you ideally want to find the point where you can run the most timing and fuel while making the most power. Just because you see knock retard doesn't mean the motor can't take more timing. You first need to increase the amount of fuel delivered and check to see (on a dyno) if it makes a little more power and the KR goes away. There's a specific term for this, but I can't remember it off the top of my head. I must be losing it...

 

Good analogy SS. Good point to make that a little knock retard does not mean that you need to reuce timing always. Fuel can be added to slow down the burn rate of the mixture, which will alow the mixture to burn slightly later than the leaner mixture, which in turn will cause combustion to occur at TDC rather than earlier in the compression stroke.

 

Excellent analogy. Thanks!
Do we want to throw cylinder temperature vs. RPM into the mix and see how that effects timing/knock aswell?

I think heat plays a big part in knock/preignition, so I'm guessing you have to find a happy medium between enough advance in the upper RPM's to get the spark at just the right time, while avoiding preignition from the higher temps due to higher revs?

On the right track?

Taking it further, is there some sort of formula to calc the "perfect" timing per Temp Vs. RPM, or is it an "aim - shoot - record - adjust" type learning curve?

 

There are so many variables to deal with that it is almost impossible to calculate perfect timing. Most people just increase it until they see knock and then go back down till it stops.