[chrisdack]

Quick question, with respect to AFR and turbo charged cars (N54 in particular), why is AFR of 12 cited as desirable. I thought 14 to 15 is generally considered ideal.

Is this in order to de-tune to prevent premature ignition?

I'd appreciate someone more knowledgeable than myself chiming in please.

Thank you
Chris

[Tzu]

Are you confusing Lambda tuning with AFR? Stoich is 14.7:1.

[Brey335i]

Running richer is safer than leaner. Hence 12 is safer than 16 for example

[chrisdack]

Originally Posted by Tzu

Are you confusing Lambda tuning with AFR? Stoich is 14.7:1.

yes, why isn't stoichiometric ratio targetted? or is it?

cheers

[Tzu]

In conjunction with what Brey stated above, running at stoich ratio is great for the environment! Not so great for engines. More of a necessary preventative measure. While there is no "ideal" AFR, there is a range which people here generally use. Some people prefer different points depending on load/rpm, than others. I like a nice mid-low 12's throughout, and that is just my preference for low boost AFR targets.

[Bash]

Originally Posted by Tzu

In conjunction with what Brey stated above, running at stoich ratio is great for the environment! Not so great for engines. More of a necessary preventative measure. While there is no "ideal" AFR, there is a range which people here generally use. Some people prefer different points depending on load/rpm, than others. I like a nice mid-low 12's throughout, and that is just my preference for low boost AFR targets.

i like your sign, maybe AFR dont matter in tunning too who knows..lol

[scottro]

I agree with Tzu, people just have a range they like to see. For example, I use to tune my Camaro to about a 12.5-12.8 range and that made me happy. So now, that is where I like to see the BMW running. But I'm not really sure if you should be tuning different AFR targets for NA vs turbo. I think generally if you want to be a bit safer, run as rich as you can until you start losing power.

[Tzu]

Originally Posted by Bash

i like your sign, maybe AFR dont matter in tunning too who knows..lol

My sign?

Don't get me wrong, AFR definately matters in tuning, and you have to get it right to go any further. I'm saying that you can get it in a range rather than obsessing over a number. I like 12.3, but if I can get it 12.5 midrange down to 12 at redline, I'm good.

[Joshboody]

Originally Posted by Tzu

My sign?

he's referring to your sig

[Joshboody]

If you had a consistent 14.7:1 throughout the cylinder, then you would be good… this creates the most power. Richening the mixture is to make sure there’s enough fuel to mix with o2, preventing lean pockets. On these engines you could probably run 13:1 throughout the revs without issues, but no one’s really tested it I guess. Even BMW hits 12.5:1 past 6000rpm.

Generally with FI you want to be safer with richer mixture due to higher cylinder pressures, heat so detonation is more of an issue.

[Tzu]

Originally Posted by Joshboody

If you had a consistent 14.7:1 throughout the cylinder, then you would be good… this creates the most power. Richening the mixture is to make sure there’s enough fuel to mix with o2, preventing lean pockets. On these engines you could probably run 13:1 throughout the revs without issues, but no one’s really tested it I guess. Even BMW hits 12.5:1 past 6000rpm.

Generally with FI you want to be safer with richer mixture due to higher cylinder pressures, heat so detonation is more of an issue.

I dont suppose anyone has observed cylinder temps or EGT's running that lean? Would be interesting to see for knowledge sake.

[rader1]

Originally Posted by Tzu

I dont suppose anyone has observed cylinder temps or EGT's running that lean? Would be interesting to see for knowledge sake.

+1
That's one thing this community needs, actual testing and data. There's a lot of suppostions but as far as actual testing goes, theres not much.

[chrisdack]

wow. this is all great stuff. I spotted the ratio's on some dyno graphs and it stood out as a little odd to me. Now I get it.

btw just got my AP today and will shortly try it out. this probably all will be hidden from me as average user dipping his toes in this area

[Tzu]

I dont mind putting in an EGT probe into my exhaust runners, its just too expensive for my current scope. Maybe I can justify it one day, but I'm pretty sure Cobb and perhaps other tuners have done it before.

[nitehawk]

Originally Posted by Joshboody

If you had a consistent 14.7:1 throughout the cylinder, then you would be good… this creates the most power. Richening the mixture is to make sure there’s enough fuel to mix with o2, preventing lean pockets. On these engines you could probably run 13:1 throughout the revs without issues, but no one’s really tested it I guess. Even BMW hits 12.5:1 past 6000rpm.

That's what I figured as well. DI engines are less likely to encounter lean pockets, so running a little leaner should not be an issue - running stock. I suppose it becomes more critical once you start targeting high boost levels.

[Zeph]

yeah my stock dyno started up around 16 and worked its way down to around 12 or so by redline. It was still up around 14 @ 4500 rpms. With Cobb it drops to 12 pretty quick and sits there. I wonder if we could have a little more power with 12.5 or 13? I would like to see my tune run 13 until 6000 then drop to 12 by redline. I usually shift around 6200 anyway.

[OG Ryder]

To OP: Basically there are different AFR targets that you aim for depending on driving conditions. From Idle, to boost onset, to lean cruise will all demand a different a range of AFR to run smoothly and happy. When running boost people tend to tune richer to supress detonation and to combat combustion temps, adding a safety margin.

On port injection engines adding a bunch of fuel has additional cooling capabilities through evaporation and droplet formation on walls of intake. On this DI engine most people say it's power output under boost is less sensitive to small changes in AFR range.

Buttttt it all depends what your desired timing, boost press, and egt's are.

[Jeff@TopGearSolutions]

Originally Posted by OG Ryder

To OP: Basically there are different AFR targets that you aim for depending on driving conditions. From Idle, to boost onset, to lean cruise will all demand a different a range of AFR to run smoothly and happy. When running boost people tend to tune richer to supress detonation and to combat combustion temps, adding a safety margin.

On port injection engines adding a bunch of fuel has additional cooling capabilities through evaporation and droplet formation on walls of intake. On this DI engine most people say it's power output under boost is less sensitive to small changes in AFR range.

Buttttt it all depends what your desired timing, boost press, and egt's are.

This ^

There is no cut and dry AFR to run on this car as there are different methods and theories.

I would say generally we like to see 13.0 AFR at on set of boost tapering to about low 12's by top end.

We have seen leaner and richer with no significant benefit in performance or loss of power as this engine really isnt as sensitive to AFR as port fuel injection counterparts.

[chrisdack]

Originally Posted by OG Ryder

To OP: Basically there are different AFR targets that you aim for depending on driving conditions. From Idle, to boost onset, to lean cruise will all demand a different a range of AFR to run smoothly and happy. When running boost people tend to tune richer to supress detonation and to combat combustion temps, adding a safety margin.

On port injection engines adding a bunch of fuel has additional cooling capabilities through evaporation and droplet formation on walls of intake. On this DI engine most people say it's power output under boost is less sensitive to small changes in AFR range.

Buttttt it all depends what your desired timing, boost press, and egt's are.

cheers