Timings affect on AFRs?

[modulation]

I'm just starting to really learn how to tune, and I've read conflicting information on timings affect on AFR. Some stuff I've read says raising timing shouldn't have any affect on the AFR of a given cell.

Other things say that raising timing, will lean out the cell where timing is being raised for one and/or two reasons. 1. fuel is being burned more quickly and 2. to a point more timing is more power, and drawing more air in to the engine because of that power.

What is the right answer?

[Preston]

Number 2 is wrong....at any given load, an engine can only pull so much air, it's a constant other than atmospheric conditions.

[bachig24u]

Ignition Timing seems to affects exhaust oxygen levels, because it directly affects the combustion process. The mixture is actually the same, what is changing is the Oxygen Sensors ability to read anything other than oxygen.

more timing = more power. Not quite
Correct timing = more power. Yes

The correct timing for the conditions (rpm, boost, combustion chamber, intake temps, gas mixtures etc) will optimize combustion producing more torque for a given piston speed.

too much timing advance will reduce piston speed, too little timing advance will result in less torque.

Air isn't drawn into a piston engine. It is pushed by Current Atmospheric Pressure (aka. natural boost).

As Preston mentioned, The air in the combustion chamber at any given time (rpm) is limited to the time that the intake valve is opened, although I can't see it being a constant as variables change as rpm increases, Intake Air temp and pressure change. eg. camshaft profiles adds an even greater variable as valve timing overlap is a dynamic also.

Naturally Pressures above Atmosphere (boost for example) will increase the air Pushed in to the engine during this process.

some tuners comment that what you have stated because I think, generally speaking mixtures closer to the stoichiometric ratio for a given fuel give best burn. Timing can be less advanced giving similar results to a richer mixture with more advance. ..... .. . .

But.... When trying to achieve more horsepower than the average street cruiser the combustion temperatures can become extremely high to the point that pistons melt, rings score the cylinder bores, exhaust manifolds & blocks & heads warp or the cooling system can not cope, never mind torching the tips off your iridium race sparkies.

[Preston]

I meant constant as in say 200cfm max. You won't get more than that by changing timing or fueling or anything else. Of course it's continously variable but it won't change based on afr or timing.

[modulation]

Number 2 is wrong....at any given load, an engine can only pull so much air, it's a constant other than atmospheric conditions.

It says nothing about load. If timing isn't optimal in a cell (too retarded), then wouldn't making timing optimal in that cell produce more power and therefore suck in more air?
Obviously this would cause the engine to reference a difference load value then it did before after it referenced the cell that had been changed?

[modulation]

Ignition Timing seems to affects exhaust oxygen levels, because it directly affects the combustion process. The mixture is actually the same, what is changing is the Oxygen Sensors ability to read anything other than oxygen.

Air isn't drawn into a piston engine. It is pushed by Current Atmospheric Pressure (aka. natural boost).

Can you go into your first statement in more detail? I know your 100% correct, but the more detail the better..

Your second statement confuses me. Are you saying that a engine isn't basically an air pump? That the cylinder moving down during the intake stroke doesn't "suck" in air?
Are you saying that technically, the intake stroke is causing a lower atmospheric pressure in the cylinder then is outside in the intake manifold, and therefore air is going from the higher pressure zone to the lower pressure zone? Does timing not affect the amount of air being drawn/blown/whatever you want to call it in the engine?

[Preston]

Number 2 is wrong....at any given load, an engine can only pull so much air, it's a constant other than atmospheric conditions.

It says nothing about load. If timing isn't optimal in a cell (too retarded), then wouldn't making timing optimal in that cell produce more power and therefore suck in more air?
Obviously this would cause the engine to reference a difference load value then it did before after it referenced the cell that had been changed?

The timing will not affect air drawn in...the engine makes more power from proper timing by more effciently burning the fuel increasing cylinder pressure and forcing the piston down with more force. It can't cant magically change the engine's efficiency to make it pull more air.

I think you are referencing max power. Assume at perfect tune, an engine makes 200hp pulling 200cfm of air. Poor timing/fueling will reduce that power from maximum, but the engine will still pull 200cfm of air, it just isn't burning fuel as efficiently thus reducing power.

Without fuel and spark, an engine will still pull a given amount of air per stroke, once again, the only variable of how much will be atmospheric condtions (air temp, air density, etc).

[modulation]

The timing will not affect air drawn in...the engine makes more power from proper timing by more effciently burning the fuel increasing cylinder pressure and forcing the piston down with more force.

NA engines aren't at 100% VE all of the time (not even most of the time.)
Peak power is peak VE, so that means that there must be differing amounts of air drawn into the engine during the rpm band.
VE is the ability of the engine to draw in enough air to fill that cylinders volume up with air.

If the cylinder is being forced down with more force wouldn't there be another cylinder in the intake stroke at the same time?

Wouldn't optimizing timing from a "badd" tune cause it to run closer to max VE?
And wouldn't that mean it has to be drawing in more air?

If I adjust timing, that changes MAF voltages, so how would it not affect how much air is being drawn in?

[modulation]

The timing will not affect air drawn in...

Also what about the timing stabilization table, or the fact that when you adjust your timing manually at idle your rpm changes?

What would cause the idle rpm to change when adjusting timing unless timing affects the amount of air the engine cylinders draw in?

Edit.
Nevermind. I thought alot about this, and realized that just the increased cylinder pressure would cause the rpm's to increase, and doesn't actually change the VE of the engine.
This part has nothing to do with air.

[DVS JEZ]

The timing will not affect air drawn in...the engine makes more power from proper timing by more effciently burning the fuel increasing cylinder pressure and forcing the piston down with more force.

NA engines aren't at 100% VE all of the time (not even most of the time.)
Peak power is peak VE, so that means that there must be differing amounts of air drawn into the engine during the rpm band.
VE is the ability of the engine to draw in enough air to fill that cylinders volume up with air.

If the cylinder is being forced down with more force wouldn't there be another cylinder in the intake stroke at the same time?

Wouldn't optimizing timing from a "badd" tune cause it to run closer to max VE?
And wouldn't that mean it has to be drawing in more air?

If I adjust timing, that changes MAF voltages, so how would it not affect how much air is being drawn in?

Peak torque is peak VE. Cylinder filling has alot to do with camshaft size and timing.

[Torque]

To add my 10 cents:

NA can very well operate at over 100% VE.
(That's why the intake camshaft stays open after BDC)
And air is either either being sucked or pushed (or both) in.

[skyline_stu]

So are we talking about idle, transition or full power ??? An important point to know.
Changing the timing can directly change AFR using Oxygen (wide or narrow band) sampling mixture evaluation devices. Oxygen sampling devices NEVER give the full story !!!

Sorry, Peston your wrong. Bachig24u your spot on with your comments.

Simple experiment to prove it on the dyno with a NA vehicle (A turbo has too many factors).
The fuel curve and timing mixtures should be the same from your start RPM to finish (say 12.5afr and 20 deg timing). We're trying to simulate a locked distributor and constant flow mechanical injection !!!
Do a dyno run, log your RPM, AFR, MAF and IGN advance. Retard the timing 5'. Do another run. You should see the retarded run produce a RICHER afr with less power either side of peak VE due to the drop in combustion efficientcy. Look at your tractive effort graph. If you have 5 gas sampling available, do a static load every 1000rpm until your reading stabilize and see the changes retard add in.

This is a confusing and sometimes difficult to explain phenomenon !! Internal Combustion is a VERY complicated process. I'll try and explain this in an easy to understand form, rather than sound like an engineering thermodynamics 101 lesson.

A few pointers before hand. Engines, Temperatures, pressures and operating conditions are assumed IDENTICAL in all cases.

Let's talk about NA PISTON engines. We need to think about the BIG combustion cycle here.

If the ignition timing is retarded, combustion happens later in the engine cycle. Overall cylinder pressures are down, not only at peak but all across the range. There is a point at which the cylinder volume increases but the pressure drops rapidly (@ near 1/2 stroke point). Improper combustion effectivly wastes the fuel's 'contained' energy. There is more wasted heat sent through the head, piston and exhaust port/valve from the crank lower angular velocity vs time (rate of acceleration). Exhaust pulse energy is lower due to the drop in combustion efficientcy. Returning scavenge waves also contain less energy, hence overlap scavenging is less efficient. Engine breathing is affected, sometimes dramatically. Hence, a rich and retarded motor is not any fun, gets hot, wastes fuel and causes frustration.

If you were to look at a PV diagram (pressure vs volume) you'd see a drop in peak pressure and a lot steeper overall pressure drop. Here is where is starts to get complex. Depending on the type of exhaust gas analysis equipment, AFR (based on 02 readings) might RISE or DECREASE (often marginally richer), depending on engine design and characteristics. We need to be checking 5 gasses - CO, CO2, O2, NOX and HC numbers for the full picture of combustion effectiveness. (Note: I'd LOVE to see water vapour %'s too!, make it a 6 gas!)

Correct timing should produce a more gentle drop in overall pressure above 1/2 stroke point (flatter line), with the peak pressure just after TDC being higher. More push on the piston at the MORE EFFECTIVE MECHANICAL LEVERAGE POINT (8-15'ATDC) means more cylinder pressure transfered to the crank. There will be a more effective returned scavenge wave, hence better breathing at overlap. This will also contribute to better power after peak VE point from better breathing and less charge dilution from internal EGR. Less charge dilution REDUCES timing requirements. Correct 'End of Injection point' can increase power and reduce fuel consumption.

Over advanced timing creates excessive pressures before/around TDC, potentially leading to detonation, and possibly pre-ignition from the disruption of the boundry layer (pre ignition is FAR, FAR worse than detonation!!!). Cylinder pressures and temperatures can rapidly exceed mechanical component limits. Many late model engines loose power before knock is audible. MEASURED OXYGEN readings can again become rich or lean (often LEAN), but the true determinator is still 5 gas sampling !

Differing compression ratios change this too (A turbo motor is really a variable compression engine!)

Forced induction PISTON engines.
Supercharged engines don't have to contend with a turbine and the heat retained in the chamber/valves, and often run slightly higher advance and leaner mixtures.
Turbo's on the other hand are a whole different kettle of fish due to the turbine and it's inherrant back pressure!!
Almost all forced induction engines are limited by the knock point of the fuel used, with richer AFR's attempting to cool the chamber/valves/manifolds/turbo.

BTW, a very rich mixture can cause or induce detonation!!

High CO levels often are fuel issues - O2 levels are affected.
High HC numbers generally are spark or flame travel issues (if there's no spark, there's no CO!), but camshafts often influence this too. O2 levels are influenced as well.

Stu