Question about air/fuel

t-ray

First, in naturally aspirated form, the car runs too rich. There's a bit more power left on the table just by leaning the car out a bit. I've seen HR's run afr's in the 11's in the heat of the summer, bone stock. I know my wife's car does that(07 g35 with the hr). The factory ecu probably does that to manage heat - fuel cools the combustion chamber.

And because fuel keeps internal temps down, it can help prevent premature detonation. And that's the biggest fear when running forced induction - compressed air is much hotter than air at ambient pressure. If the combustion chamber becomes too hot, the air/fuel mixture in the chamber can combust prematurely - which can cause all kinds of nasty things.

zZSportZz

I believe my car was at 11.2 which sparked the conversation (might have even been 11.6).

370Z Purist

Isn't ideal AFR around 14.1 : 1 by mass?
That sounds kinda lean...
Or maybe it was 12.1 : 1...

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theDreamer

Per wiki:
Any mixture less than 14.7 to 1 is considered to be a rich mixture, any more than 14.7 to 1 is a lean mixture

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Modshack

During Closed loop operation (partial throttle, most of the time) the MAF and O2 sensors will attempt to keep the car at stochiometric which is 14.7:1 by cycling back and forth from lean to rich corrections. When you go to open loop (WOT as in a Dyno run) the ECU defaults to Fuel maps programmed into it. From many dyno reports I've seen here the car does run rich. Semtech posted one with a few mods and it was still at 11.8:1. My Scangauge indicates a pretty steady rich correction number of -7 to -9% with just an exhaust which confirms this. This is why this car makes such good power improvements as you add performance parts and it leans out some. Max power in a N/A engine is made at around 12.5:1 A/F though that will vary from car to car. The ECU also seems to have a pretty wide adaptation range which is why many mods can be done without re-tuning. This is a good article on A/F ratios:

Autospeed article on A/F ratios

Rich mixtures generally cost you in Power output, but provide some Safety factors in that the charge will be cooler and the chance of detonation lessened. Leaning out a car to a certain point makes more power. Go too lean though and you face high heat and detonation issues.

On my old Corvette that I had tuning software for, you could easily see a 5% HP improvement just by adjusting it's Factory rich tuning to a better, yet still safe, A/F.

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