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This isn't a general question regarding air/fuel ratios. I want to know why Subaru specifically insists on making their cars run ridiculously rich under boost. For instance, my 2007 Forester XT has an AFR of 10.5 under full boost (hard acceleration). ANd it's not just mine, all turbo Subaru's run very rich. My previous car rarely went below 11.5. As far as I was aware, the best AFR for heavy loads is around 11.5 to 12.5. 10.5 just seems a bit dangerous (Yes, there is such a thing as too rich). I have a small-ish TD04 turbo on it and it doesn't boost more than 0.8bar/11.5PSI and quickly tapers off to 0.6bar/9PSI so there's not much heat.

I should probably point out that under normal driving conditions, the AFR hovers around a pretty healthy 14.7.

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  • Might just be your car. I'm on my 4th Subaru - this one (2.5l Forester STi, 400bhp) sits at 11.6. Previous one (2.0l Impreza PPP, 330bhp) was 12.1. Can't remember the others.
    – Rory Alsop
    Commented Mar 13, 2015 at 13:38
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    Neither of those are stock. The STI Forester made around 300BHP stock. Don't know about the stock Impreza, but they never made one that put out more than 300BHP either, except as a special (tuned) edition. And those AFR figures are about what I would expect from a tuned EJ engine.
    – Captain Kenpachi
    Commented Mar 13, 2015 at 13:42
  • It's probably worth mentioning that our best fuel is only 95RON (90/91 Octane in American speak). Which probably answers my question. Richer is better at lower octane.
    – Captain Kenpachi
    Commented Mar 19, 2015 at 10:54
  • Whereabouts are you Juann? Here in Scotland I can at least get 100RON in some areas, and 98 almost everywhere, or at least boost my octane rating as needed. This does mean I have major problems if I take my car to those places that only have 95...
    – Rory Alsop
    Commented Mar 19, 2015 at 11:16
  • South Africa, mate. We have 95 at the coast, and 93 inland.
    – Captain Kenpachi
    Commented Mar 19, 2015 at 11:31

2 Answers 2

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Actually running rich is less dangerous than running lean. I don't know specifically about Subaru, but in general, running lean can cause detonation more easily. And yes, while 14.7:1 is the stoichiometric ratio deemed as perfect for an air fuel mixture, running rich has the big advantage of reducing the chances of pre-ignition or pinging. In a turbo car, this could be considered death rattle because it can destroy the engine quickly. I found this entry on the interwebz which sums things up pretty well in concern of boosted engines:

Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

Let's take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process.

The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.

There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.

Mind you, while we say 14.7:1 is the perfect air fuel ratio for a complete burn of the mixture, most vehicle manufacturers run their tunes a bit rich in the first place straight out of the box. This holds true whether the vehicle uses forced induction or is naturally aspirated. Because of this, I was a bit surprised you said your car runs at 14.7:1 when not under load as it doesn't seem right ... that is unless you've had some tuning done on it.

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  • I get all that. Which is why I pointed out that my previous car (also a turbocharged 4 cylinder) never went under 11.5. In fact, it ran at 1.5bar max boost under load at that AFR without issue. Granted it was tuned, but the max boost was twice stock, so there seems to have been lots of safety room. If you hook up the Torque App or another OBD logger, you'll see that when your car maintains a constant speed on a level road, it will run close to 14.7. It only drops under load.
    – Captain Kenpachi
    Commented Mar 13, 2015 at 10:30
  • Also, the AFR under boost was tuned RICHER than stock, when it wouldn't go below ~12. So there's quite a difference between my previous car and my Forester. In fact, the difference is 1.5 units. I want to know what Subaru's motivation is for this.
    – Captain Kenpachi
    Commented Mar 13, 2015 at 10:33
  • My assumption is that they know lots of kids will just install a bigger turbo and/or 3" custom exhaust system, and they want to build in a huge error margin to protect their reputation for reliability. But I have no evidence for that.
    – Captain Kenpachi
    Commented Mar 13, 2015 at 10:35
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    It has nothing to do with kids putting larger turbos on the cars, because unless a kid is completely stupid, you wouldn't run a larger turbo on a car without retuning. The fuel map needs to be completely reworked when you change any portion of the turbo boost profile (change turbo to larger, change boost range with a different BOV or respring, changing waste gate profile, etc.) Engine destruction would be eminent if you don't and engine function will suffer, but I figure you know that already.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Mar 13, 2015 at 10:38
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    Running too rich can mess with your lubrication, which is really bad. It can also kill your cats and 02 sensors. 10.5 Isn't dangerous (yet), but it's much lower than what e.g. the Germans or the Americans think is adequate.
    – Captain Kenpachi
    Commented Mar 13, 2015 at 11:22
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So after LOTS of research, it turns out that my Subaru Forester XT is running at 10.5 AFR at WOT because Subaru's don't like our 95RON fuel very much and it's a safety precaution to add an additional margin for error to prevent knocking if I ever tried filling up at a dodgy fuel pump in rural Africa.

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