Thread: Why I tune to 12.0:1 A/F ratio (0.82 lambda) and why you should too. (all welcome)

Any reason you like richer outside of turbo efficiency map? I would figure it is better to go leaner in those spots?

When you're out of the efficient areas, intake temps rise quickly. The extra fuel is the easiest (obviously not best) way to cool things down a bit and slow the combustion.
A better way to deal with the situation is to retard ignition with high intake temps, and add only a little bit of fuel if you have to.
The best way is to avoid this situation altogether by using a correctly sized turbo and intercooler setup for your goals.

Maybe I need this explained in a different way. True, that the explosion is stronger with 12.0 than 11.0 but we also know that increased timing creates a stronger 'downstroke' hit on the piston. I'm still not understanding why leaner and less timing is better.

If I'm understanding right it's because AFR has a greater impact on power than timing, so it's better to lower timing and increase AFR than increase timing and lower AFR.

Why does leaner A/F have a greater impact on power vs more timing and richer A/F? This is the part I don't get.

Common sense tells me that the time you choose to do combustion is much less important than the actual mixture you use in it (power wise).

I'll explain it 100 different ways until it makes sense. =]

First, you're off on the wrong path a little bit. Yes, the explosion is more energetic (basically because of the burn rate) with leaner mixtures (up until a point, but we're not talking about that extreme). However, increased timing does NOT create a stronger "downstroke hit" on the piston. That is why you're having a hard time with this. Let that go for a second and read on.



You have two, pretty much independent things going on. You have the speed (and thus energy) of the burn, and you have the timing of that burn.

Speed of the burn:
Obviously, the faster burn is preferable because of the added energy. There are many ways to make the burn faster. Higher static and dynamic compression causes faster burns, which is why higher compression ratios and forced induction gives more power. Higher intake temps cause faster burns, but is problematic for other reasons. Air/Fuel ratios close to 0.82 lambda (12.0:1 AFR) cause faster burns, with speed falling off dramatically the richer you go.
Recall the graph from the first post for that:



Timing of the burn:
This is the bit you're missing. There is an exact time when it is best for the flame and pressure front to hit the piston for maximum mechanical conversion of energy. That time is 20 degrees after top-dead-center. This means when timing the ignition, you're pretty much always trying to time the burn to hit the piston at 20 degress ATDC. If you hit the piston early, you lose some energy and if you hit the piston too late you lose energy.

When you are thinking that advancing timing gives more power, this is simply because you're getting the flame front to hit the piston closer to that magical 20 degrees. That is it. No more. No less. If you go too advanced (assuming no detonation) you will obviously start losing power. I hope that makes sense.

So, we should talk about your example now.


With 12.0:1 AFR the flame front will move quickly. For this example, let's assume it moves from the top of the cylinder to the piston in 33 degrees of crank rotation at a given RPM. That means you ignite the mix, and then 33 degrees later the flame front hits the piston, and the piston is at 20 degrees after TDC. Awesome! You have a perfectly timed combustion. And you get the maximum amount of power from the mixture because the burn rate was high.

With 11.0:1 AFR the flame front will move a bit slower. For this example, let's assume it moves from the top of the cylinder to the piston in 35 degrees of crank rotation at a given RPM. That means you ignite the mix, and then 35 degrees later the flame front hits the piston, and the piston is at 20 degrees after TDC. Great! You have perfectly timed the combustion. You couldn't have timed it any better. However, you get less power from the combustion than the example above, because the burn rate is lower for this mixture.


I'm ignoring avoiding detonation for this example. Let me know if you want me to do an example with detonation avoidance. @gomba I've updated this post.

My shit likes 11.5 afr regardless of timing... maybe the higher compression

If that's true, your wideband sensor is more than likely off in calibration. This stuff is not negotiable. It is fact.

Idk man I dont think so b ut I guess its possible..

I always look for knock when I log in nismotronic, I know its only good to like 4800rpm but it still you ca view the voltage all the way up

I remember vadim and others finding the Same thing tho..