Friday, January 30, 2009


I have a few more topics I want to cover, but if you have any suggestions or burning questions you'd like to see my take on, please do comment and let me know! It's no good if I'm not getting at the issues and areas that you are interested in.



Wednesday, January 7, 2009

Air Fuel Ratio - Basics

Now that the holidays are past and our year end accounting is nearly done I have a little time to get another post out for you.

We covered timing before, but as I said previously AFR (Air Fuel Ratio) matters a lot as to how the vehicle will perform. First a simple background on AFR. When it comes to standard gasoline stoichemetric (meaning chemically balanced for both sides of the equation for a chemical reaction, in this case combustion) is 14.7 parts of air to 1 part of fuel or 14.7:1. ANYTHING higher than this (say 15.2:1) is considered a lean mixture, and anything lower is considered a rich mixture. A lot of times people will say when a turbo car is running 13:1 in boost that is running to lean, but what they really mean is that the car is not running as rich as they think it should, because even at 13:1 the air fuel mixture is definitely a rich mixture.

So that all said, what do various AFR mixtures get you. Well obviously the leaner you go the less fuel is being used, but that does NOT mean the vehicle will use less fuel. In fact, as you go leaner than about 15.5 you typically lose enough in power output from the leaner mixture that the additional throttle input and resultant use of fuel to maintain that mixture uses more fuel than if you had simply run a mixture closer to 14.7. So in other words, don’t think that if you managed to safely tune your car to 16.8:1 that you are getting better gas mileage as this is often not the case! Additionally, when you go to mixtures leaner than 14.7 two major things happen: combustion occurs at a higher temp thus heating your cylinders and valves more significantly, and second some of the “bad” pollutant gases increase significantly leading to a more polluting or problematic emissions footprint from the car. This is why most manufacturers design cars to run at 14.7 AFR. In fact in the vast majority of vehicles out there, the only oxygen sensors in the vehicle are all what is referred to as a narrowband sensor which can ONLY tell you if you are rich or lean relative to 14.7 and provides essentially no useful information otherwise. Sorry folks on a budget, but you can not ever tune a vehicle accurately to some other AFR value with a narrowband sensor, you need to drop the money and get a wideband! At 14.7 AFR emissions are typically the most balanced and allows catalytic converters to do their job.

The fact that manufacturers tune most street vehicles to 14.7 AFR is exactly why chipping a vehicle can sometimes get you more horsepower with fuel only changes. 14.7 is the magic number for naturally aspirated vehicles trying to make emissions. However, that number changes whenever you are in boost for a forced induction vehicle, or if you are trying to make power on a naturally aspirated vehicle. The values are hotly contested, and as I alluded to in my last post on timing these values change quite a bit when you work to balance out your timing values with your AFR. But to speak in general rules as guidelines is relatively simple. Gasoline mixtures in MOST modern combustion chamber designs will provide peak power output at AFR’s between 12.3 and 12.8. The value is often narrowed down to be 12.5-12.6 but as I said these values are hotly contested. I personally consider 12.3 to 12.5 to be optimal for turbocharged vehicles and 12.6 to 12.9 to be optimal for naturally aspirated vehicles. So when I’m tuning an NA car I tune wide open throttle AFR’s to 12.8 to 13.0 and then maximize the timing. NA cars are much less sensitive for timing and air fuel balance so this typically works well and is quick and easy. Turbo cars it is much more complicated. Turbo cars I usually tune to about 11.8 and work out the timing. If I can advance the timing fully without detonation issues then I work to see if more can be gotten from leaning out the AFR. This rarely is the case so I settle in near that AFR range almost always.

There are lots of other purposes and uses for AFR though that can yield interesting improvements. Running very rich mixtures can sometimes help with turbo lag and help cool off the valves. The type of injector being used and it’s placement and the atomization of the fuel can also change what AFR is effective in a given vehicle. Sometimes, you need to use rich mixtures to help warm up a car or stabilize an idle, and sometimes a lean mixture will help with rough running on a vehicle. There are a host of options, but we’ll leave that for another post.

Thanks for reading and happy motoring!