B5908 - Fuel Mixture Spark Correction table question

[BlackGMC]

I have a question about this table. Does this table operate all the time or just in PE?

The row values on this table are in GMEq. Do they translate like the following? Attached is my stock 4.8 table, I added the AFR column.



If so, does it mean given a certain AFR and RPM this is how much the PCM will add/substract Timing?

Does a Negative add timing and a Postive substract?

[SOMhaveit]

You've done your calc from EQ to AFR incorrectly. You should have divided 14.63 by the EQ ratio, so an EQ of 1.45 would actually be about an AFR of 10.09. So I think your table would start adding timing at commanded AFR of 12.72.

[BlackGMC]

Ok, so is this right?



So I am assuming that this table works with B3618. Given a certain Commanded AFR and RPM it will determine how much timing to add? Ok so does a positive add timing?

[SOMhaveit]

That looks more like it to me. And it makes sense, doesn't it, that you would add timing when you go rich.

[BlackGMC]

Yeah it makes more sense now that the AFR are correct.. Thanks for your help.

[SOMhaveit]

From one tuner-in-training another, you're welcome.

[2002_z28_six_speed]

That looks more like it to me. And it makes sense, doesn't it, that you would add timing when you go rich.

Richer burns faster.
Leaner burns slower.

So, ...

[SOMhaveit]

Richer burns faster.
Leaner burns slower.

So, ...

I'm sure this guy knows more than I do. Take a look.

Application Note: You CAN be too Rich
By Klaus Allmendinger, VP of Engineering, Innovate Motorsports
Many people with turbochargers believe that they need to run at very rich mixtures. The theory is that the excess fuel cools the intake charge and therefore reduces the probability of knock. It does work in reducing knock, but not because of charge cooling. The following little article shows why.

First let’s look at the science. Specific heat is the amount of energy required to raise 1 kg of material by one degree K (Kelvin, same as Celsius but with 0 point at absolute zero). Different materials have different specific heats. The energy is measured in kJ or kilojoules:

Air ~ 1 kJ/( kg * deg K)
Gasoline 2.02 kJ/( kg * deg K)
Water 4.18 kJ/( kg * deg K)
Ethanol 2.43 kJ/( kg * deg K)
Methanol 2.51 kJ/( kg * deg K)

Fuel and other liquids also have what's called latent heat. This is the heat energy required to vaporize 1 kg of the liquid. The fuel in an internal combustion engine has to be vaporized and mixed thoroughly with the incoming air to produce power. Liquid gasoline does not burn. The energy to vaporize the fuel comes partially from the incoming air, cooling it. The latent heat energy required is actually much larger than the specific heat. That the energy comes from the incoming air can be easily seen on older carbureted cars, where frost can actually form on the intake manifold from the cooling of the charge.

The latent heat values of different liquids are shown here:

Gasoline 350 kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg

Most engines produce maximum power (with optimized ignition timing) at an air-fuel-ratio between 12 and 13. Let's assume the optimum is in the middle at 12.5. This means that for every kg of air, 0.08 kg of fuel is mixed in and vaporized. The vaporization of the fuel extracts 28 kJ of energy from the air charge. If the mixture has an air-fuel-ratio of 11 instead, the vaporization extracts 31.8 kJ instead. A difference of 3.8 kJ. Because air has a specific heat of about 1 kJ/kg*deg K, the air charge is only 3.8 C (or K) degrees cooler for the rich mixture compared to the optimum power mixture. This small difference has very little effect on knock or power output.

If instead of the richer mixture about 10% (by mass) of water would be injected in the intake charge (0.008 kg Water/kg air), the high latent heat of the water would cool the charge by 18 degrees, about 4 times the cooling effect of the richer mixture. The added fuel for the rich mixture can't burn because there is just not enough oxygen available. So it does not matter if fuel or water is added.

So where does the knock suppression of richer mixtures come from?

If the mixture gets ignited by the spark, a flame front spreads out from the spark plug. This burning mixture increases the pressure and temperature in the cylinder. At some time in the process the pressures and temperatures peak. The speed of the flame front is dependent on mixture density and AFR. A richer or leaner AFR than about 12-13 AFR burns slower. A denser mixture burns faster.
So with a turbo under boost the mixture density raises and results in a faster burning mixture. The closer the peak pressure is to TDC, the higher that peak pressure is, resulting in a high knock probability. Also there is less leverage on the crankshaft for the pressure to produce torque, and, therefore, less power.

Richening up the mixture results in a slower burn, moving the pressure peak later where there is more leverage, hence more torque. Also the pressure peak is lower at a later crank angle and the knock probability is reduced. The same effect can be achieved with an optimum power mixture and more ignition retard.

Optimum mix with “later” ignition can produce more power because more energy is released from the combustion of gasoline. Here’s why: When hydrocarbons like gasoline combust, the burn process actually happens in multiple stages. First the gasoline molecules are broken up into hydrogen and carbon. The hydrogen combines with oxygen from the air to form H2O (water) and the carbon molecules form CO. This process happens very fast at the front edge of the flame front. The second stage converts CO to CO2. This process is relatively slow and requires water molecules (from the first stage) for completion. If there is no more oxygen available (most of it consumed in the first stage), the second stage can't happen. But about 2/3 of the energy released from the burning of the carbon is released in the second stage. Therefore a richer mixture releases less energy, lowering peak pressures and temperatures, and produces less power. A secondary side effect is of course also a lowering of knock probability. It's like closing the throttle a little. A typical engine does not knock when running on part throttle because less energy and therefore lower pressures and temperatures are in the cylinder.

This is why running overly-rich mixtures can not only increase fuel consumption, but also cost power.

[cmitchell17]

So if a richer mixture burns slower why does the factory add timing at a richer mixture?

My truck tune does but a LS1 tune just has a spark table with no AFR correction table?

[Telco]

When you add timing, what you are actually doing is moving timing further back from TDC. An initial timing setting of 10 degrees means the spark is taking place 10 degrees before the piston hits top dead center, otherwise called advanced 10 degrees. When you add timing, you move the ignition event even further in advance of top dead center.

A slower burning fuel, be it due to octane or richness, takes longer to burn, but you still want that full explosive force felt at the same position AFTER top dead center. There is one point after TDC that you get the most power from (not sure what that point is but I'll guess 15-20 degrees after TDC) and the idea is to adjust timing so that the full force is felt at that point. It takes time to go from the initial spark to the fuel air mix being at maximum pressure, which is also why you advance timing with RPM. It always takes the same fuel the same amount of time to burn, but at a higher RPM the engine rotates further around in that amount of time. Octane level (or a mixture richness) will increase or decrease that burn rate, and timing must be adjusted to compensate for both the burn rate and engine speed.

If full force happens too soon, you get piston knock because the force will actually try to force the piston to go backwards if it happens before TDC, and the force will hammer down more directly on the bearings, both of which is what causes engine damage with severe pinging. If the full force happens too late, then you've lost power because the full force is already trying to fill an ever expanding area, and will never be able to build full pressure.

So, advancing the timing is a bit of a misnomer, unless you understand what the basis for advance is. Hope this helps.