91 or 93 on a Stage 1 EA888: what the octane actually buys

The most common question before a booking is whether to run 91 or 93. The most common wrong belief is that the higher number makes more power. It does not, on its own. It makes it safer to ask for more.

What octane is

Octane is a fuel's resistance to igniting from pressure and heat alone, before the spark. A turbocharged engine under boost squeezes a hot, dense charge; the higher the boost and the more the timing is advanced, the closer that charge gets to lighting itself. When it does, that is knock, and the ECU pulls timing to stop it. A higher octane moves the line where knock starts.

So octane is a margin, not a power source. If the calibration does not use the margin, 93 and 91 drive identically. If it does, 93 lets the file hold a little more timing and a little more boost before the ECU has to intervene.

What the EA888 Gen3 does with it

Volkswagen calibrates the Gen3 for 91 in North America and the engine makes its best power close to that on the factory map. The simulator on this site models the same thing: on the EA888 preset, the safe timing ceiling on 91 sits about a degree above the optimum, so better fuel buys margin rather than a big number. In a Stage 1 file, 93 is worth a few horsepower over 91 and, more usefully, a wider gap between the timing the car runs and the timing at which it would knock on a hot day.

Why the file is written for one fuel

A Stage 1 file has a knock strategy built in, but its baseline timing and boost targets are chosen for one octane. The service asks which fuel you will actually run, and then a second question: will you always run it? If the honest answer is "mostly", the file is written for the lower grade. That costs perhaps five horsepower and removes the tank of fuel that would otherwise find out how good the knock control is.

What happens on the wrong tank

Run 87 in a car tuned for 93 and the ECU will hear knock, pull timing hard, and the car will feel flat and possibly stumble under load. The engine will usually survive because the factory knock control is good, but "usually" is doing work in that sentence, and the tune is now a stress test instead of a calibration. Run 93 in a car tuned for 91 and nothing bad happens: the ECU keeps its 91 targets and the extra octane sits there unused.

That asymmetry is the whole reason the service asks. Rich costs power; lean costs engines. The same is true of octane: too much costs money, too little costs margin.

The practical answer

Pick the fuel you can get at every station you use, tell the form that fuel, and run it. If that is 93, the file will use it. If it is 91, the file is written for 91 and nothing is lost that you would notice on the road. The simulator below has a fuel selector; move it and watch what the knock gauge does at the same timing.