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The BMW B58 — A Modern Legend The BMW B58 is arguably the most important turbocharged inline-six of its generation. Found in everything from the Toyota GR Supra to the BMW M340i, M240i, X3 M40i, and Z4 M40i, the B58 has earned a reputation as BMW's best engine in decades. With a single twin-scroll turbocharger, an air-to-water intercooler, and a robust bottom end, the B58 responds exceptionally well to tuning — gains of 80 to 120 horsepower on a Stage 1 flash are typical. But like any mass-produced engine, the B58 has its vulnerabilities. Knowing them before you tune is...