Question about air/fuel
 

When I got my car dynoed over the weekend, the guy running the dyno could not believe how rich our cars run from the factory. He just kept shaking his head after every pull, and it become quite a topic of discussion.

Some other guys standing around mentioned this was a good thing, because it will allow a lot of wiggle room for tunes and what-not...and that tuners will have a field day with this car. Can some car geek (i say that with love) explain to me why?

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.

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

If our car runs to rich, would this be why stillens exhaust or intake cause such massive gains?

I would assume better air flow combined with already rich enviroment would equal things out?

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

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

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.

Wow, that's very rich. Which is definitely safe for the engine but not good for performance. That does work out for us though as we can make a lot more power.

Well fact is, the more breathing mods you add, the more the airflow meaning in terms of tuning, the leaner your car will become (more air for the same fuel => Leaner mixture). So if your car is rich from the factory, then leaning her out will get it closer to the optimum A/F ratio, making her mod-friendly

Thanks for the input. Makes more sense now.