Real-world AFRs at WOT...

[Enthalpy]

I meant the map deep inside the OEM ecu. Obviously, it was not optimized, though the new miatea have knock sensors and are likely better. My fuel pressure was like, 30 psi in boost with the AFPR on . After seeing that, using my friend to hold a gauge while I drove, I instantly realized what the problem was, lol. It did lean-misfire once, now that I think of it, but normally the car would just kind of... stop accelerating when you stomped on it.

Outwardly identical engines, even from the factory, can have different calibrations when fully optimized. This fact has amazed me for some time. OEMs know this as well. Tollerances produce different calibrations, but since they can't possibly tune each engine, they have to be intelligent about how they manage tollerances and mapping.

The AFR at which the engine misfires at is HEAVILY dependant on the position in the map the engine is at. Simply misfiring isn't in itself proof of lean operation.

[Pierre]

Hello

Pierre -
I suppose their might be specific circumstances where the AFR is doing some things in one specific engine it isn't doing in other engines like changing the way the air flows into the engine.

I don't understand your answer. I don't think that AFR change the way air flows into the engine. But I think that AFR lets you know if there is enough fuel injected for the air you have.

My post was meant to outline the fact that AFR is a thermal management device and not a method for gaining power. The results of this little AFR study is obviously heavily dependant on setting the timing dead on.

Again, I don't understand. You say that most engine with the same timing will output the same power with AFR in the notmisfiring zone?

Pierre

edit :
Ok, I read several time this post, and I am a little bit confused, it seems that my believes about afr where not so correct....
Do you have an URL about you study from 1930 ?

[Enthalpy]

I wish. I have only seen a hard copy of it at the library (the old way of doing research ;) ).

I'm sort of crappy at explaining myself, I'm sorry about that.

If you don't touch the ignition timing, then yes, AFR will have a big impact. A fully tuned engine sets the ignition timing after the AFR is spot on to achieve MBT ignition timing, or the timing that puts the peak cylinder pressure at a point in the engine's cycle where it's most mechanically advantageous.

If you do readjust your ignition timing after varying the AFR, you will find the power to be very similar.

Please realize it's only for that AFR window, and only for NA engines. Boosted engines often require AFRs that are outside that window and therefore don't apply.

[Pierre]

I wish. I have only seen a hard copy of it at the library (the old way of doing research ;) ).

I'm sort of crappy at explaining myself, I'm sorry about that.

If you don't touch the ignition timing, then yes, AFR will have a big impact.

That was my case, MS is only controlling fuel. Maybe it was the reason.

If you do readjust your ignition timing after varying the AFR, you will find the power to be very similar.

Please realize it's only for that AFR window, and only for NA engines. Boosted engines often require AFRs that are outside that window and therefore don't apply.

Ok for this. We are only talking about NA.

A fully tuned engine sets the ignition timing after the AFR is spot on to achieve MBT ignition timing, or the timing that puts the peak cylinder pressure at a point in the engine's cycle where it's most mechanically advantageous.

Let me re-formulate (is that an english word?) that. You mean that you choose an AFR, for other reason that maximum power (to cool the engine, to save gas...) then you tune ignition to have the maximum power ?

I always thought that if you want maximum power, you have to find the AFR that gaves you the max power, and most of the time, it's around 12,5.

Pierre

[Enthalpy]

Let me re-formulate (is that an english word?) that. You mean that you choose an AFR, for other reason that maximum power (to cool the engine, to save gas...) then you tune ignition to have the maximum power ?

I always thought that if you want maximum power, you have to find the AFR that gaves you the max power, and most of the time, it's around 12,5

You mostly have it.

You are absolutely correct except in the cases where:
-Additional fuel will supress knock and allow more timing and power safely.
-Additional fuel will cool dangerously-high EGTs (Exhaust Gas Temperatures).

[natesully]

I always thought that if you want maximum power, you have to find the AFR that gaves you the max power, and most of the time, it's around 12,5.

Pierre

No necessarily. You might get max power, and insanely high EGTs that melt stuff at around 12.5:1, for example- so you would need to go richer, and sacrifice power to save your engine. For most NA engines, 12.5:1 is a really good guess. If one had super-high compression or something, or perhaps was air-cooled, you may need a richer mixture.

Also- timing comes before fuel, because advancing timing results in lower EGTs, meaning you can run leaner (but still rich), wich makes more power and saves gas. Too advanced timing results in reduced power, or engine-destroying knock.

[natesully]

I always thought that if you want maximum power, you have to find the AFR that gaves you the max power, and most of the time, it's around 12,5.

Pierre

No necessarily. You might get max power, and insanely high EGTs that melt stuff at around 12.5:1, for example- so you would need to go richer, and sacrifice power to save your engine. For most NA engines, 12.5:1 is a really good guess. If one had super-high compression or something, or perhaps was air-cooled, you may need a richer mixture.

Also- timing comes before fuel, because advancing timing results in lower EGTs, meaning you can run leaner (but still rich), wich makes more power and saves gas. Too advanced timing results in reduced power, or engine-destroying knock.

[eightvalver]

I thought I read somewhere here on msefi.com that for all motor/NA that

13.0:1 at WOT was good for peak torque while
12.5:1 (somwhere around there) was good for peak HP

[Enthalpy]

Also- timing comes before fuel, because advancing timing results in lower EGTs, meaning you can run leaner (but still rich), wich makes more power and saves gas. Too advanced timing results in reduced power, or engine-destroying knock.

While timing and fuel are definitely iterative, I must say the first thing you do is set the fuel with a known-safe timing table, then you visit the timing. As you dial that in, pay attention to the AFR and make sure it hasn't shifted.

If you are trying to figure out what the best AFR is, then it's more iterative.

[Pierre]

Also- timing comes before fuel, because advancing timing results in lower EGTs, meaning you can run leaner (but still rich), wich makes more power and saves gas. Too advanced timing results in reduced power, or engine-destroying knock.

While timing and fuel are definitely iterative, I must say the first thing you do is set the fuel with a known-safe timing table, then you visit the timing. As you dial that in, pay attention to the AFR and make sure it hasn't shifted.

If you are trying to figure out what the best AFR is, then it's more iterative.

Thanks for this thread, I have to read it again that several times, so I understand really what must be done. Hopefully, I am still tuning fuel only....

Pierre, tuning is not as simple as I thought....