Quick Q about timing vs afr
Apr 6, 2007, 04:33 AM
#2
Yes, Timing will affect your AFR's, the more time you give the combustion mixture to burn, the resulting exhaust gases will be leaner.
However this doesn't necessarily have any correlation with "Available timing" as its a balance. Ignition timing is just that, how much time before Top Dead Center the spark plug is fired. There are several factors that determine how much time before top dead center you can fire the ignition.
1) Quality of Gas
2) How much fuel
3) Engine speed, and Load
4) Cylinder Pressure
5) Quality of your ignition (plugs, spark, etc)
In general, the expanding/burning gases start working against the moving piston as it approaches Top Dead Center, More fuel, Higher Octane Gas, Water Vapor, all will slow this rate of burn, hence why good gas or anything that increases "Octane Number" will allow you to run more timing. This doesn't necessarily mean more power, but generally if there is enough fuel to burn, it will keep producing expanding gases which will produce more power.
There are limits, too much timing and the expanding gases begin to work against the upstroke, you are then recompressing this expanding gas, you lose power and generate even more heat, even worse, knock and damage (this is the WORST kind of knock)
Just keep in mind that richer air fuel ratios slow combustion, and the fuel droplets absorb heat, which reduces detonation, this is the "other" kind of knock, where you get ignition due to hot spots, this is different than too much timing but nonetheless potentially as destructive as you have no control over when it happens and therefore can happen very early in the compression stroke.
Its not a very complex concept, but it is a complex relationship.
Understanding AFR's, Timing, EGT's and how engines work in general gives you a huge amount of insight into what makes power. Which although many of you guys understand adding timing makes power, many of you guys don't understand HOW or WHY it works that way.
Ignition timing is also different than Valve timing, since those events are also synchronized between the engine's piston stroke, and even eachother, and the relationship of valve events and optimum ignition timing because of different cams makes a difference and shifts the power band accordingly too.
However this doesn't necessarily have any correlation with "Available timing" as its a balance. Ignition timing is just that, how much time before Top Dead Center the spark plug is fired. There are several factors that determine how much time before top dead center you can fire the ignition.
1) Quality of Gas
2) How much fuel
3) Engine speed, and Load
4) Cylinder Pressure
5) Quality of your ignition (plugs, spark, etc)
In general, the expanding/burning gases start working against the moving piston as it approaches Top Dead Center, More fuel, Higher Octane Gas, Water Vapor, all will slow this rate of burn, hence why good gas or anything that increases "Octane Number" will allow you to run more timing. This doesn't necessarily mean more power, but generally if there is enough fuel to burn, it will keep producing expanding gases which will produce more power.
There are limits, too much timing and the expanding gases begin to work against the upstroke, you are then recompressing this expanding gas, you lose power and generate even more heat, even worse, knock and damage (this is the WORST kind of knock)
Just keep in mind that richer air fuel ratios slow combustion, and the fuel droplets absorb heat, which reduces detonation, this is the "other" kind of knock, where you get ignition due to hot spots, this is different than too much timing but nonetheless potentially as destructive as you have no control over when it happens and therefore can happen very early in the compression stroke.
Its not a very complex concept, but it is a complex relationship.
Understanding AFR's, Timing, EGT's and how engines work in general gives you a huge amount of insight into what makes power. Which although many of you guys understand adding timing makes power, many of you guys don't understand HOW or WHY it works that way.
Ignition timing is also different than Valve timing, since those events are also synchronized between the engine's piston stroke, and even eachother, and the relationship of valve events and optimum ignition timing because of different cams makes a difference and shifts the power band accordingly too.
Apr 6, 2007, 05:53 AM
#3
beyond the very astute post above, in a nut, lean mix burns very fast, add pressure and it burns quicker still.
most tuners start with adjusting AFR and define timing from that position. With AFR defined and the timing adjusted, they look at the impact a timing change may have made and readjust AFR to suit.
In the end the best Timing, no matter what the number, is the Timing that provides peak pressure in the cylinder AROUND 15 degrees after tdc. without VERY expensive instruments the best we can do (without a roller) is Time to best power before knock.
I want to point out that best Timing may not be most Timing. you are looking for a point in time for PP but you cloud the reactants and thus the speed of burn with pressure, type gas, injectants, spark quality, heat and quantity of each.
quick question answer... yes, but my point is, at some point you begin to add fuel to slow the burn so you can add timing. what good is that?
most tuners start with adjusting AFR and define timing from that position. With AFR defined and the timing adjusted, they look at the impact a timing change may have made and readjust AFR to suit.
In the end the best Timing, no matter what the number, is the Timing that provides peak pressure in the cylinder AROUND 15 degrees after tdc. without VERY expensive instruments the best we can do (without a roller) is Time to best power before knock.
I want to point out that best Timing may not be most Timing. you are looking for a point in time for PP but you cloud the reactants and thus the speed of burn with pressure, type gas, injectants, spark quality, heat and quantity of each.
quick question answer... yes, but my point is, at some point you begin to add fuel to slow the burn so you can add timing. what good is that?
Last edited by nothere; Apr 6, 2007 at 06:08 AM.
Apr 6, 2007, 02:40 PM
#4
If you don't have the time or energy even patients (did I spell that wright?) then you can make good power (and it will feel good too) with added timing and a rich (low 11's) burn. Hell, the car was engineered that way with stock tune since it can control knock quickly or advance for power easily just by timing control in responses to altitude temperature, but mostly fuel quality. Thats why it's so rich from factory - saves engine when your girlfriend fills tank with 87octane.
But from my personal experiences with my car I have found that if a good quality burn with high pressure of combustion fuel (i.e, methanol inj, racegas, pump, but not 87) is used to control detonation, the car seems to respond positively and more consistantly to lean A:F with easy timing (3* peak tq and 18* redline). Just a different approach for some I guess.
But from my personal experiences with my car I have found that if a good quality burn with high pressure of combustion fuel (i.e, methanol inj, racegas, pump, but not 87) is used to control detonation, the car seems to respond positively and more consistantly to lean A:F with easy timing (3* peak tq and 18* redline). Just a different approach for some I guess.
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