The days of you unscrewing a couple of spark plugs to track your engine's air/fuel ratios are long gone. Today you've got access to all sorts of fancy electronics, of which almost every single one of them'll do a better job than your ability to eyeball and interpret whatever that brownish-shade of soot on that spark plug's porcelain tip means.
We're talking about electronic wideband air/fuel meters, devices that at one time remained exclusive to professional tuners and racers with expendable budgets and that, typically, didn't travel far from the dynos they were paired with, yet today are affordable, portable, and a whole lot more accurate.
THE OXYGEN SENSOR
Every air/fuel meter relies on an oxygen sensor positioned within the engine's exhaust stream to send the data you care about to the screen or gauge you're looking at. If you're smart, you'll use that information to make all sorts of meaningful changes within your ECU that'll allow you to do things like extract more power, operate more efficiently, or just keep things from blowing up. The process isn't a whole lot different than the job your car's existing oxygen sensor(s) already does where, instead of you staring at a bunch of numbers, your car's on-board computer interprets them to determine how much fuel ought to be injected.
NARROWBAND VS WIDEBAND
In either case, there are two kinds of oxygen sensors you need to know about: older narrowband sensors that are slow and are only able to monitor a limited range of activity, and newer wideband sensors that are faster and are able to interpret a whole lot more data.
Narrowband sensors are only accurate when lingering around the stoichiometric range—also known as lambda 1—which, chemically speaking, is the point where just enough air's been added to burn almost every bit of fuel. As far as your engine goes, when running gasoline, it'll always be 14.7:1, or, 14.7 parts air for every single part of fuel. Ratios less than 14.7:1 indicate a rich condition where too much fuel's been added; anything larger means things are getting lean. The problem, however, is that it isn't uncommon for high-performance engines to dip into the 11:1 range under heavy load, for example, at which point that narrowband sensor just became useless. It'll tell you that things are getting rich, but it won't accurately tell you by how much.
Wideband oxygen sensors can interpret a much broader spectrum, which means all of a sudden you're aware of air/fuel ratios that are significantly richer or leaner than 14.7:1. If you're in the market for an air/fuel meter, you'd be a fool to consider anything other than a wideband.
WHY AIR/FUEL RATIOS MATTER
The amount of air and fuel your cylinders have got to play with is what'll ultimately determine how much power your engine will make, how efficient it'll be, and how long it'll hold together. There are all sorts of justifications for rich conditions that go beyond the scope of this article, but an excessively low ratio will never be a good thing and can rob an engine of horsepower or even affect how well its piston rings seal. Moderately lean conditions have their place as well and can positively affect tailpipe emissions, for example, but higher ratios for extended periods of time will almost always result in chaos.
THE WIDEBAND'S JOB
Wideband air/fuel meters—also called wideband AFR controllers or UEGOs (Universal Exhaust Gas Oxygen)—have one job: to continuously monitor how much air and fuel has been burnt and give you that information as quickly as possible. Despite many wideband air/fuel meters using the same Bosch 4.9 LSU oxygen sensor—an industry standard—some are able to interpret and relay that information back to you a whole lot faster than others. The controllers themselves and their circuitry both affect how fast and how accurate any given meter might be. And speed matters more than you might think. A high-compression, force-fed engine running on the ragged edge of lean can't afford the split-second it'll take an inferior meter to get you the information you need to know to add more fuel or to let off that throttle. Here's where pistons melt and exhaust valves burn to a crisp. And we haven't even talked about the consequences of an inaccurate meter.
Every air/fuel meter's got to wait for the exhaust stream to reach its oxygen sensor before anything can be interpreted. That waiting period is a component of a phenomenon called deadtime, which describes the delay between air/fuel ratio changes and the meter actually being able to tell you what they are. There's nothing you can do about the time it takes the exhaust stream to reach the sensor other than place it as close as possible to the turbine or exhaust collector, but you can partially decrease deadtime with fancy electronics. At least that's what the people at AEM say their controller's capable of, going on to explain the dangers of longer deadtimes and how, when viewing the data while tuning, longer waiting periods can lead to air/fuel ratios that appear to occur later than they actually have. In other words, you just might be adding or taking away fuel at the wrong times.
AEM Electronics says it's got the fastest wideband air/fuel meter around that's also one of the most accurate, and to find out whether or not they do, we've pitted the company's X-Series Wideband UEGO Air/Fuel Sensor Controller Gauge against a couple of its competitors, both which shall remain nameless to protect anybody from getting embarrassed.
Variables are the enemy of almost any comparison like this so the people at AEM got rid of them. For starters, all three meters were tested on the same vehicle at the same time, in this case an '05 Mitsubishi Evo VIII that, for testing purposes, was good for just south of 400 whp. All three meters' oxygen sensors were plumbed into the exhaust downpipe one inch apart from one another, roughly 18 inches from the turbine. In the interest of fairness, AEM's engineers recommended positioning their sensor farthest downstream, which which would give the advantage to its competitors, albeit only marginally. Installing all three meters in the same car at the same time also means any voltage or grounding discrepancies are eliminated and that all three meters can be tested and logged simultaneously. And finally, each meter was connected directly to the Evo's AEM Infinity ECU and recorded at 1,000 samples per second; all that means to you is that every base was covered for the most accurate test. You couldn't get rid of any more variables if you tried.
AEM says its X-Series wideband controller is the fastest around (and an independent test against 17 other wideband controllers already confirmed this), so naturally each meter's response time was logged under various loads and throttle-sweeping conditions to see if all of this was true. Overlaying each meter's results using the Infinity's data-logging feature allowed us to see how each meter's data compared to one another; although our testing equipment didn't give us the ability to confirm whether or not AEM's meter was the most accurate, it did reveal obvious discrepancies with at least one other meter, which was the only one of the four (if we're including the Infinity's on-board wideband sensor) that deviated from the same general graph. The results can be seen in the accompanying screenshots, but to summarize, AEM wasn't kidding. In terms of speed, it delivered data faster than the rest, which might be the only thing keeping you from an engine with a rod poking out of its block.