What is a safe NA AFR?
 

What is the leanest and richest AFR that is typically considered "safe"? I have done some searching on the web and I've seen people talk about the ideal AFR settings with a tune, and people warning of leaning out a car. But at what point is the AFR considered too lean that it hurts the engine? Anything above 13 under full load or 14.7 at idle or is there another general AFR point that I am missing? From what I have seen, ideal power is 12.5 (rich) to 13.5 (lean) but under normal cruising and idle, you want to be around 14.7. Would idle or cruising at 15, 16, or 17 be alright? Or is where the AFR falls under load what is important?
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From the other thread. And your graph shows "ideal" down to 11.5 or so. Adding fuel adds safety. A too lean AFR under load is what's going to kill your motor. Going too lean will cost you power since you'll knock, and the ECU will pull timing out.

I was just confused because my understanding and the graph say that leaner is higher. And 11.5 seems on the rich side. At what point does knock usually start to become a concern in the vq's?

On the rich side compared to what? I haven't seen any normal injection cars with high compression running leaner than 14.7 at idle. Direct injection is a whole different animal. 11.5 or so at WOT fits with the way I understand the way you want to tune a NA car- dial in AFR for peak power, then add a touch of fuel back in to allow for temperature swings, fuel variations, etc. Also, fuel works to cool down the intake charge to some extent. You can also run a little closer to the edge with 93 vs 91 octane, so it also depends where you are.

Alright. That makes a lot of sense. Thanks.

11.5 on an N/A car at wide open throttle is too rich, especially on a VQ motor. It will kill power and too rich of mixture can knock just as much as too lean. Most people will find the "sweet spot" around 12.3 - 12.9 at wide open throttle, then you adjust timing accordingly for the best power output. At idle and normal cruise you want to be around 14.7 for best fuel economy. Anything leaner, you may have idle and backfiring / exhaust popping issues.

Staples is more along the lines of what I had heard in other forums. Any other tuners out there that can verify this?

Yes. I aim for 12.7~12.9 @WOT

In general: It depends on load (a value calculated based on RPM, air mass and the volumetric efficiency of the engine), compression ratio of pistons, port vs direct injection, and octane (as well as kind of fuel). Other factors that play a role are ignition and valve timing and transient factors such as air temperature and air pressure. Then there are things to consider such as the temps of exhaust gas and potential for blowback.

For our motor, running everything stock, running a stock tune, and using at least 91 AKI, somewhere between 13.0 and 12.5 is probably around ideal approaching redline, with the leaner value being more forgiving with colder climates, 93 AKI, or higher altitudes (i.e., lower air pressure). Skewing toward the richer value is probably best if running more ignition timing or only minimum recommended octane.

Most engines are tuned to run at or leaner than stoichiometric at idle or light load, which will burn all of the fuel (for petrol, stoich is 14.7:1), optimizing fuel consumption and minimizing harmful exhaust emissions, running incrementally richer as load increases towards redline (afterwhich, the engine is found to dip below peak torque). Peak torque is commonly found to be made (on port injection engines) somewhere around 13.2:1 AFR.

Under high load on our motor, OEM tune plays it uber-safe and has it running rather rich (11.45, going by target AFR) to quench potential hot spots and probably to compensate for an unintentionally sluggish knock sensor response should an idiot consumer fill the tank with any octane petrol below 91 AKI...

On that note, there's a thread on octane requirements every 6 months or so by someone who insists on running low octane fuel in their high performance sports car... :rolleyes:

I only put in shell 93 and in MI it rarely gets above 90. It is usually 60-80 for the high in the summer. So it sounds like I have that going for me. Thanks for the info everyone.