Can someone help explain the relation between timing and compression ratio.
People spend big bucks in new high or low comp pistons.

But for example if you wanted to raise compression in your V8 motor, why bother spending thousands on new pistons when you could just advance the hell out of your timing?

I mean the timing can be advanced so much that the motor starts pinging, so obviously its making a big difference on the compression?

There is no relationship between static compression ratio and ignition timing or cam timing.

It doesn't matter when you fire your spark you'll only ever compress your mixture by the same amount without changing the size of your combustion chamber.

Yes, you can get an increase in power by advancing your ignition timing, but only to a point, and past that point you'll lose power.

The relationship between knock and CR or timing is a different matter. Compressed gas heats up, more heat means a higher chance of spontaneous ignition and hence knock. I'm not certain on the relationship with ignition advance, but presumably because you've ignited the charge earlier it's hotter already by the time it's compressed, so knock is more likely.

I thought when you increase the timing, the spark happens later so the fuel has more time to compress.

Same goes if you decrease timing the spark happens earlier so fuel has less time to compress so then the fuel isnt compressed as much.

But are you saying it doesnt matter when the spark happens, the fuel will still be compressed the same amount? I guess thats true because the crank and piston still have to move through there set path and that can only be changed with a physical change in size of these things.

But lets saying timing is advanced, does that mean that the spark can happen while the fuel is still compressing? so the piston will be trying to compress the fuel while the fuel is burning/expanding and trying to push down on the piston, isnt this counter productive?
Wouldnt the best time to fire the spark always be when the piston has completly compressed the fuel?

I thought when you increase the timing, the spark happens later so the fuel has more time to compress.

No, when you advance the spark timing you make the spark occur earlier, before top dead centre. When you retard ignition timing, you place the ignition closer to or even after top dead centre.


But are you saying it doesnt matter when the spark happens, the fuel will still be compressed the same amount? I guess thats true because the crank and piston still have to move through there set path and that can only be changed with a physical change in size of these things.

Yep, that's what I meant.. you can increase power by altering the ignition timing (and thereby fine-tuning at what point ATDC the mixture explosion becomes most extreme), or by increasing the static compression ratio (and thereby increasing the potential energy in the charge). Keep in mind that the tuning of ignition timing is just that, a tuning.. if it's spot on already then there is nothing to gain.. go too far advanced or retarded and you miss the sweet spot. But compression ratio changes are absolute, up to the limits of the fuel to resist detonation.



But lets saying timing is advanced, does that mean that the spark can happen while the fuel is still compressing? so the piston will be trying to compress the fuel while the fuel is burning/expanding and trying to push down on the piston, isnt this counter productive?
Wouldnt the best time to fire the spark always be when the piston has completly compressed the fuel?

Yep, advanced compression means you're igniting the fuel/air mixture before the piston reaches the top of it's compression stroke, but the mixture takes a while to burn fully and you need to time it right so that maximum cylinder pressure happens at the right time during the subsequent power stroke. Advance it too much and max pressure happens too soon (or even before!) TDC and you're right, you're resisting the compression or not complementing the downward motion of the piston, too late and you've missed the boat.

Apparently the optimum point for maximum cylinder pressure is 20 degrees ATDC, but the amount of ignition advance required probably depends on engine speed, and fuel octane (note: higher octane means fuel is more resistant to combustion, so you can give it more advance)

okai all makes sense now, thanks for the help :D

okai all makes sense now, thanks for the help :D

Dont forget that as the fuel burns the gasses expand building up the pressure in the cylinder, which is different to simply the compression ratio. Pinging/knocking occurs when the pressure in the cylinder overcomes the momentum of the piston before top dead centre, forcing the gasses to escape past the rings (blowby). Causes huge damage to rings and pistons very quickly.