Flying Closer To The Sun: How Does ECU Tuning Work?

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Gotta get a tune. Get that tune dialed in! You need to tune it! It’s not ready yet, the tune’s off. If you’ve walked through a car show parking lot, you’ve heard the word “tune” tossed around like it’s going out of style. But what does it mean? And how does it work?

Back in the days of ignition points, carbs, and spark plugs that might last an oil change, a tune-up was a big deal. If you didn’t maintain and replace those parts of your car, it would run poorly. Or maybe not at all. Watch any of the “we fix a muscle car that’s been parked in a barn, but we do it in a weekend with zero planning or preparation” shows, and you’ll see that it’s always something like a wrong carb jet, a bad ignition point, or a fouled spark plug that ruins the glory. Or with those engines, it could just be a Tuesday. It didn’t take much.

On a modern car, spark plugs work well until long after they’ve become a part of the cylinder head. Fuel injectors with good gas will last pretty much forever. And distributor points are when the guy at the parts store shows you with a finger that your car has ignition coils mounted directly to the plug. So why is “tune” so popular now?

That’s because a tune today isn’t replacing those wear items. It’s modifying your car. Tuners, Fast and Furious-style, add performance parts that make your car go faster. Hopefully. Be careful with that manifold.

Once the Mad Scientist has ripped apart the block and replaced the piston rings you fried, you need to make some small changes. In the bits and the bytes. The car knows you’re getting more air from that sweet turbo, but once you go too far past stock, the car doesn’t know what to do. Enter the tune.

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Today a tune is just modifications to the computer. And with modern cars that can mean anything from changing how long your lights stay on after you lock the doors to telling the car how to handle 22 pounds of boost from your seven-second Vette Cart.

Aside from the changes to courtesy features, the tune changes how your car handles air, fuel, and spark. It can make more power, or it can turn your car into an undrivable mess.

Tuners come in many forms. From back in the 1980s, when you had to solder a chip onto your car’s ECU, to today’s plug into your OBDII port and go devices. The only difference is the complexity of the computer and how hard it is to change the settings.

The basics of a tune are fuel and spark. On a turbocharged car, add boost to that equation.

Modern cars are very carefully calibrated, with years of development and millions of test miles. They set things up conservatively because they need the car to live in every condition from -40 to 140 with the AC running and they need it to do it for the length of the warranty. That means it’s programmed to leave some power on the table. It’s also programmed to make sure the car has acceptable emissions.

So air-fuel ratios are pushed a little more toward efficiency than power. And spark advance (how early in the cycle the spark plug fires) is set to listen to the knock sensor in case you bought 87-ish octane from the sketchy place around the block instead of the 91 you’re supposed to use. That’s why cars run so well for so long, even if you don’t treat them so well.

Tuning shaves some of that margin of safety and sells it for power. Like those ridges on the side of the quarter in your pocket. People would shave the metal. Until they went to far and got caught.

The Golden Air Fuel Ratio

How does it do that? It starts with the air fuel ratio table. The table is just that: a big spreadsheet that’s full of desired air fuel ratios for every RPM, air pressure, and throttle opening. Cruise, deceleration, idle, and, of course, wide-open throttle.

Change the AFR and you might make more power. Change it too much, and you might blow up your engine. The only way to set it for sure is to get data from the O2 sensors and set the values based on that and the dyno figures. Keep tweaking every box in the table until everything is right. Then, adjust those until it’s drivable in every condition.

Tuned cars will run richer than stock, usually, because the programmers want to make sure that there isn’t any oxygen left over after combustion. The best way to do that is to add just a bit more fuel than the engine can burn. It’s not clean (it’s why diesels roll coal when modified poorly) but it makes power.

Advancing The Spark

The other big change is spark advance. The spark plug doesn’t fire exactly when the piston gets to the top of the cylinder. That’s too late. The explosion would be chasing the piston down instead of pushing it. So the spark happens slightly before.

Advancing the spark–that is, making it happen even more before the piston gets to the top–makes more power. Until you go too far. Then the explosion hits the piston on the way up and things go boom. Nobody said this was easy.

The amount of advance you want depends on RPM, throttle opening, and boost settings. IT takes work to dial it in. Running the edge means more power, but if you get a bad tank of gas, that low octane can mean the explosion happens too quickly. And boom goes the engine.

All Hail The Boost

The last one is an easy one. Booooooost. Basically, you turn that up (while adding to the AFR tables) until the turbo starts to stall or get too hot. Turbos have a window where they work best. Push them harder and they just blow hot air. Pushing it closer to the edge (and you might notice a theme here) makes more power. Until you go too far and it stops.

How much power can you make this way? Depends on the car. Tuners claim 8-12 peak hp for a car like the Subaru BR-Z. More importantly, though, it allows for 12-14 hp more under the curve. And a higher redline.

Add a turbo like on the Audi S4 and tuners offer nearly 70 hp more. And if you’ve made any other changes, like to the intake and exhaust, then an aftermarket tune can make an even bigger difference. They can also remove emissions compliance tricks, like how modern cars hold the same RPM for what seems like an eternity after you push in the clutch.

So an aftermarket computer or chiptune works by sacrificing some of the manufacturer’s margin of reliability for speed. You might never need that margin, and enjoy a lifetime of extra power. Or you might fly too close to the sun and end up at the dealer trying to explain that there’s no way your car was modified when the engine exploded.

But they can tell. They can always tell.

Oh, and one last thing. A tune can remove the speed limiter. Closed course, do not attempt, and all that. Just make sure if you’re headed for broke that you’ve picked tires that can handle the speed.

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