Timing questions

This will be a long post to try and help me understand timing, just to let you know if you are in a hurry. I understand the basics of engine theory but there is something bugging me about timing (advance and retard). Please bear with me as I will try to be as thorough as possible. This is what I understand (I think :p ):

1. Timing is controlled by the ECU based on feedback from various engine sensors (mostly based on knock).

2. Knock is caused by pre-ignition of the fuel during the combustion stroke of the motor (multiple flame fronts).

3. More knock = retarded timing.....less knock = advanced timing

What I need clarification on:

1. Advanced timing is firing the spark plug x degrees after TDC? (which equals more HP)

2. Retarded timing is firing the spark plug x degrees before TDC? (which equals less HP)

3. Why would firing the spark plug after TDC (i.e. while the piston is already in the "power stroke" give more power?) Shouldn't, for maximum power, the spark plug be fired exactly at TDC?

Here's where I may have a hard time explaining my next question:

Ok (please don't laugh if I screw this up) :rolleyes:

The piston is connected to a rod which is in turn connected to the crank. The crank has large lobes where the rod is connected. At TDC the rod should be exactly vertical? Before TDC the rod is, let's say for example, the left side of the crank and after TDC the rod is on the right side (let's say the crank turns clockwise). If the plug is fired before TDC (retarded timing) the rod has not traveled to the other side, i.e. past vertical (or right side in this example). Wouldn't the force of the explosion try and push the rod in the wrong direction i.e. left side, thereby try and turn the crank in the wrong direction (fighting the clockwise motion)??

2. Knock isn't just pre-ignition or spark knock... it just means a noise that's not normal. The knock sensor is just a microphone, and that's why you can get 'phantom knock' from a noisy valvetrain which retards timing, contrary to what is actually helping.

You have timing advance backwards, but it's a very common mistake. ADVANCED timing is when the spark fires X degrees BEFORE the piston reaches TDC. It works out to be something about the flame front of the burning gas (gas burns, not explodes right?) reaching the piston as it gets to TDC because it takes some time for the flame to get there. Timing retard is the opposite, and it should be clear why you get less power. You never really thought about why people on the swingsets get pushed right at 'TDC', did you?

Anyway, you were on the right track. You want the most push right at TDC, but timing advance allows time for the flame to get there. It also helps in other ways like allowing more time for the mixture to burn. (Retarded timing can cause the mixture to keep burning as it exits the motor. You now know what Kevin spent a lot of time explaining to people about EGT's. If the mixture is still burning as it comes through the exhaust, it's definately going to heat up your EGT probe, and your gauge is going to rise. The information means you have retarded timing, from knock). Lastly, the reason knock retards timing is partly dependent on the kind of knock.
Mostly, 'knock' that DSM guys talk about means something in the combustion chamber is igniting your mixture before the spark plug fires. If you have an oncoming flame front meeting a piston too often (high knock count), your internals are toast. Retarding the timing reduces combustion temperature (and power) so that your mixture isn't igniting until spark fires. Segway into octane anyone? Higher octane gas has a higher ignition temperature, so a high compression ratio, or high compression due to boost, isn't going to ignite it. We remember that compression makes heat, right? Muscle cars used the higher octane they had back in the day, because they have high compression.

Attempting your example, I think you are correct, but the crank doesn't exactly have 'lobes' like a camshaft does. It doesn't push the rod/piston up like a lobe, but you get the picture. Crank just transfers horizontal to rotational energy. Again, the spark plug firing, and the flame front reaching the piston are not at the same time. You are correct though, if you have flame before TDC, you are knocking.

Ok....So if timing retard is the plug firing x degrees after TDC doesn't that give the fuel/air mixture more time to pre-ignite. I know that pre-ignition occurs when the gas is heated too much so much compression that you create multiple flame fronts. So if the plug is fired after TDC, i.e. the fuel/air mixture has passed the point of maximum compression, shouldn't that increase the possibility of knock?? :dunno:

You guys got it all wrong. No, I'm just kidding. I've heard people talking about timing for so long and i've never understood it. And I was way to lazy to read about it. But what spyder said about it pushing the crank the wrong way if it "exploded" before TDC. It seems like that would be true. Is this what contributes to the loss of power? Or is that completely wrong? Hey, one more question. What do the Cam gears have to do with timing? That might be a stupid question but i just dont get it.

Hey, one more question. What do the Cam gears have to do with timing? That might be a stupid question but i just dont get it.


Timing controls when the cam lobes strike opening the intake valves and exhaust valves. If your cam gears skip a tooth on the belt the timing of the valves will be late... uh yeah late...had to think 4 sec.. That is a bad thing...If I remember correctly if its late the fuel air mix could be pushed back into the intake mani a bit and the exhaust can't be pushed out fully when needed to... Please correct me if i'm wrong..anyone..

Well, ignition timing isn't valve timing... your timing is either right or wrong.

Compression won't ignite the mixture. No matter what your timing is, compression isn't going to cause knock unless you have a major problem.

But what if you put like 60 octane in it or something. Or if you turn up the boost way too much. Dont those cause knock?

that would be the major problem, yes.

Remember how a diesel works. Super low octane. It doesnt even need plugs to ignite the mixture. The laws of adiabatic compression state that as fluids are compressed they go up in heat. The Ideal Gas Law states this as well, expressed as

PV=nRT

I have done some pretty extensive writeup on knock, I think on is on my site. Ah yes, here, from the tech pages:

http://www.posracing.net/Knock.html

The main thing to remember about timing is afvancing is firing the plug earlier, retard is firing it later. You always start the combustion event before the piston reaches TDC, but peak cylinder pressure occurs well after TDC cause that piston is moving very quickly. At idle, you light the spark 5-10 degree before TDC. The faster the engine is rotating, the earlier you need to light the plug, of course. The time to burn up to peak cylinder pressure doesnt change, but piston speed does. So timing advances as rpm goes up. Around 6000 rpm it peaks at 17 degrees according to stock 2g timing maps. Timing will also be more advances with less load (the ECU uses airflow/rev to determine what load is). This is why using devices like the AFC to compensate for larger injectors by showing the ECU a lower airflow signal will increase timing, sometimes it can be dangerous.

The thing to remember is that all this messing with timing BS is simply to get peak cylinder pressure to occur when the piston is just starting to go back down. The first 10-15 degrees of crank rotation past TDC, the piston still really isnt going down, the throw on the crank is still kinda moving "sideways." So you need peak cylinder pressure to happen just after that.

Do it too soon, and the piston just doesnt have enough leverage to turn the crank and continue down. The pressure will keep rising with no where to go, and knock is likely. Do it too late, and you have wasted precious time where the piston is moving down. Using the swingset example, the swing is already at the bottom position when you push it. Kinda pointless. The idea is to advance timing as far as you can without doing damage. There is the possible to run too much timing and not get knock though! And this is how I lost my last motor. Only 4 degrees knock retard, but it took apart my number one piston. ;)

The best time to reach high pressure changes with flame front propagation, piston speed (rpm included), dwell time, air/fuel ratio, etc. So, this ideal time to reach high cylinder pressure is a moving target, and thats what all the fuss is about. :)

Its hard to keep this topic simple, but I hope this helps.

Might as well get into some ECU stuff while its on the top of my mind... I'll use the 2g ECU since thats what I know best, but the same thing applies for a 1g, the numbers might change a touch though.

The ECU uses load (based on airflow per rev) and rpm to come up with a timing advance figure to aim for. Once you get over 2.1 grams per rev of airflow, you move to the highest load map the ECU has. I think anything 16g and larger can do this. TIming will rise with RPM up to about 6000 where it peaks.

The knock sensor is always looking for a certain type of noise. The ECU will come up with a baseline for what is "normal" and anything that is more intense is considered knock, and the longer it goes on the more the ECU wants to call it knock. Timing will be pulled by some amount based on the severity and length of the noise event. The ECU also keeps long term records of this known as the "octane" value. Think of it as a long term fuel trim for knock.

So timing will be whatever the load/rpm map says it should be, minus knock if there is any. But there are other things that will affect timing. If Intake Temps are over 84 degrees F, the ECU will pull one more degree of timing. Same thing is intake temps go below about 30 degrees! If coolant temps go over 206 degrees, you lose one degree more, and over somewhere around 220 it takes two.

So lets say we are on a 20g and over 2.1 g/rev, at 6000 rpm, intake temps at 90 degrees, and coolant at 210 degrees. Whats total timing now? About 15. 17 minus one for intake temps over 85, and minus one for coolant temps over 206. And this is not theory, its fact. Its simple math. You can see it in any DSMlink datalog :)

Now, when using something like an AFC (VPC, GCC, emanage, etc) that lies to the ECU about airflow, things get tricky. Since airflow plays a huge part in timing advance, messing with it will change timing of course. WIth larger injectors we end up with lower airflow signal, and therefore higher timing (ECU assume lower load on the motor, so more timing should be safe. You'll get ~35 degrees cruisng on the highway for example, very low airflow). SOmeone running 750s on an AFC can see timing advance in the high 20s, if it doesnt knock. Which it almost always does. And without DSMlink, you cant see knock on a 2g. So the ECU could be shooting for 30 degrees timine, knock retard of 10 degrees, equals 20 degrees shown on the logger. Looks normal, and people will assume they have zero knock :D Very tricky for the 2Ger without DSMlink....