Help me understand timing, retard, advance

[NickF509]

There are always posts on retarding timing when knock or advancing timing when under WOT etc. Because of this i was wondering what the heck this is actually doing. There are a couple of good sites on ignition timing but i'm not sure I understand fully what is happening.

From my understanding timing is basically the concept of igniting the fuel air mixture at exactly the right time to make the most efficient use of the fuel. Igniting the mixture at the timing of 0 would be saying that the spark happens exactly when the piston is compressed to the fullest extent. However from what I have read this is not desirable for power because at 0 the piston is already starting into the downward motion of the powerstroke and is not truly compressed to the greatest extent anymore.

So advancing timing would be to say that the spark would fire earlier into the compression stroke. It would make sense that timing would need to advance to a higher degree the faster the car goes because the piston is moving quicker so the spark would need to happen earlier to try and get the piston at the very best compression point. If timing never advanced and the spark always happened at 0 but the piston moved faster then it would really be firing far into the power stroke and have very little compression.

So retarding timing is forcing the spark to fire into the powerstroke, reducing compression and causing less power.

Am i correct?

 

[Blk93tsi]

Its when spark at the spark plug ignites fuel and air in the cylinders near the end of the comp stroke. Simply put, retarding slows down the Combustion. Um like it chases the piston down the stroke but combustion dies cause its to slow, visversa advancing.

 

[talonDSMerr]

That's right!

The one thing that you need to keep in mind is "flame propogation," which is how quickly and efficiently the flame in the combustion chamber travels from the spark to the cylinder walls. This takes a small amount of time and is the reason why advancing timing creates more power. Advancing timing before TDC allows the combustion event to occur closer and closer to when the piston is at TDC (which corresponds to maximum brake torque). You are spot on when you describe how retarding timing prevents knock.

 

[MY1GDSM]

Ignition timing - Wikipedia, the free encyclopedia<-- Read this

What your trying to achieve is or Maximum Bake Torque "MBT" or as some call it Minimum Best timing that's what you tune for...

The best timing for maximum power in every case is right at TDC well, slightly before due to the reality of a finite combustion time. as early or late combustion from TDC both rob realized mechanical power. However, knock and emissions keep you from realizing the maximum power in every case.

Hope this will help explain better,

MBT = Maximum Brake Torque timing. Knock limit aside, at a given speed and flow rate, it is the spark timing that gives maximum torque

Basically, if combustion starts too soon, the gas pushes back on the piston as it is coming up in the compression stroke, creating negative work which reduces torque. If the spark is too late, peak cylinder pressure is reduced, and hence, so is expansion work. So there is an optimum spark for maximum torque. If one were to draw out a curve of torque vs advance, (starting out below MBT), as spark is advanced torque will increase until MBT timing is reached, after which point further advance will cause torque to go back down. The top of the curve is somewhat flat, so the percentage increase in tq drops off as spark advance approaches MBT.


Here's a cool video that let's you See what's going on

A look inside

 

[NickF509]

Thank you everyone for the help. That video is pretty cool as well.

One more quick question, during the intake cycle what stops air/fuel from leaking back up the intake valve?

 

[rts91tsi]

Basically just the valve overlap and when fuel injector delivers fuel or injector timing.

During the initial inlet valve opening, the exhaust valve remains slightly open while the exhaust cycle is completed. This valve sequence is referred to as “valve overlap.” During the overlap period the inlet valve remains at high pressure until the exhaust valve closes, creating a momentary backflow into the inlet port.

Backflow is more prevalent under low manifold pressure (vacuum) conditions because of the significantly lower pressure in the manifold, compared to the positive pressure created by the end of the exhaust stroke. As throttle is opened, backflow decreases because the differential of pressure is reduced and the higher-velocity inlet air mitigates the backflow to a large extent.

Once the exhaust valve closes, airflow at the inlet port reverses direction and increases in velocity and flow as the inlet valve opens and the piston travels down the bore. It is at this point that fuel should be injected into the combustion chamber.

 

[syndr0me]

Wow... it's threads like this that make a forum very intuitive and make for a great self learning tool.

 

[1983gsp]

Mean best timing is like hitting a home run each and every time the ball is thrown at you. Maximum power achieved every time. Advancing the timing too much is like swinging too early. Retarding is like swinging to late.

Octane is relative to how fast the ball is thrown. 40mph fast ball is like 87octane. You can't advance the timing or you will swing early or knock so you retard the timing to make maximum power, known as mean best timing. A 98mph fastball is like 116octane. You have to advance the timing or swing earlier to make maximum power.

Sorry for the over simplification, it works on my 13yo son....

 

[2slow4now]

Mean best timing is like hitting a home run each and every time the ball is thrown at you. Maximum power achieved every time. Advancing the timing too much is like swinging too early. Retarding is like swinging to late.

Octane is relative to how fast the ball is thrown. 40mph fast ball is like 87octane. You can't advance the timing or you will swing early or knock so you retard the timing to make maximum power, known as mean best timing. A 98mph fastball is like 116octane. You have to advance the timing or swing earlier to make maximum power.

Sorry for the over simplification, it works on my 13yo son....

HA HA I'll use that on my son! That will probably actually make sense to him. It took awhile to explain how I was timing my car when he watched. I couldn't get it in his head that when I moved the cas it was just moving the time that the spark occurred and not actually moving the cam.

Lots of good info here and I'll give him the youtube link as well. Thanks