G42 Tuning

The M240i is the quiet one. It runs the same B58 as the M340i — the 3.0-liter turbo six covered on the G20 page, here making 382 hp and 369 lb-ft — but wraps it in a smaller, lighter coupe body. The result punches well above its badge: it runs within a few tenths of the far pricier M2, carries more torque down low, and in some real-world comparisons has actually edged the heavier M2 in a straight line. Dollar for mph, it's one of the best values BMW sells — and most people walk right past it.

What makes the G42 genuinely different from its B58 siblings, though, isn't the body — it's how you tune it. The 2021-and-up G42's ECU is locked: there's currently no clean way to read or write it directly the way you can on an older B58. That changes the playbook. Instead of a traditional flash, the proven route on these cars is a piggyback module (the JB4 and its kind), which intercepts and trims the signals to deliver real gains without ever cracking the locked tune. It works, and it's the right tool here — but it's a different conversation than "just flash it," and anyone telling you to flash a locked G42 hasn't kept up.

The power is there in abundance. The stock turbo runs out of breath around 20 psi — call it 600-plus crank horsepower of theoretical ceiling — but the practical wall comes sooner, from fueling: the factory high-pressure pump taps out near 450 whp on pump gas, and reaching the ~540 whp the turbo can actually support takes flex fuel plus port-injection and low-pressure-pump support. For the engine's maintenance items — the oil-filter-housing gasket chief among them — see the G20 page; they carry over unchanged, because it's the same engine.

The M240i's whole pitch is that it does most of what the M2 does for a lot less, and asks only that you tune it the modern, locked-ECU way. Get that part right, and it's the sleeper that quietly keeps pace with cars wearing far more expensive badges.