What's the correct Air/Fuel Ratio for an M20B25? [includes dyno chart]

[Earendil]

I'll throw in the words "Stock" to the title description as well, as well as "max power at 5000 RPM", since I know that partial throttle at 2000 rpm will have a different ratio.

I'm reading on the internet that the best mixture at high RPM is about 12.8, but I'm having a hard time finding that number repeatedly enough for me to take it as fact.

I just did a a couple dyno runs on my car and I am very happy with the results. However the richest the car ever gets is 13, and stays upwards of 13.75 for the rest of the pull. Thoughts on what information I may glean from this?

[digger]

there is no magic number but 13:1 give or take a little is fine.

Many stock m20b25's I've seen are also a little leaner than ideal at the top end idk if this is an inherent thing or just old age as you dont see many bone stock low mileage engines around. That is a graph of torque not power. The power is about 157whp which is healthy

[ak-]

What digger said - hovering around 13:1 WOT is suitable for an NA car, though yours runs a little lean after 5.75 RPM for my taste.

Was this on 91 - 93 octane?

[TobyB]

13:1 give or take a little is fine.

That's what I've found...

t

[Earendil]

What digger said - hovering around 13:1 WOT is suitable for an NA car, though yours runs a little lean after 5.75 RPM for my taste.

Was this on 91 - 93 octane?

91 octane with a Marc D chip. There is also no CAT, but that only effects potential power, and not A/F right?

I believe one of the reasons that Pro-3 cars have an adjustable FPR is so that they don't go so lean (and hot) as they approach redline. So I don't think it's unusual. I just sort of assumed that the car would get leaner and leaner until it was "lean" at redline. Instead, what I see is that I'm lean as I approach 5k, am below 13:1 for a couple tens of RPM, and then run lean again.

For record, here is both the Torque and Power graphs that actually show the peak values in numbers, so no guessing at graph or calculation required.

http://i.imgur.com/dE4V7IJ.jpg

http://i.imgur.com/bBfrknn.jpg

[nomansland92]

My Ix with miller chip always ran around 13.5 and now that it's a 2.8 it ran around 13.9, so I haven't beat on it until I get the ms2 up and running

[TobyB]

My stock '90 325i Pro3 car with an open 2.5" exhaust stays between 13.2 and 12.8 right up
to the stock 6500 rev limiter. In fact, it trends a bit rich over about 5200.
But it's all stock, including the intake tract.

It's not fast, either.

t