X4 M40i B58 — same engine as M340i, same tuning potential?

Great numbers. The B58 is the same across all models — M340i, X3 M40i, X4 M40i, Z4 M40i, Supra. The tuning potential is identical. The only difference is weight and drivetrain losses.

I am running BM3 stage 2 on my M340i with a VRSF catless downpipe and the numbers are very similar to yours. The B58 stock internals are incredibly strong — people are running 600+ whp on stock internals with upgraded turbos and fueling.

For the X4 specifically, the xDrive system is a benefit for traction but it does add weight and drivetrain losses. On a dyno, expect about 15-20 percent drivetrain loss compared to the crank numbers.

If you are going stage 2 on the B58, I would strongly recommend upgrading the intercooler at the same time. The stock intercooler is adequate for stage 1 but at stage 2 power levels it heat soaks quickly, especially in warm weather. A Wagner or CSF intercooler upgrade makes a significant difference in consistent power delivery.

The B58 intercooler upgrade is a straightforward bolt-on job — about 2 hours of work. It is one of the best bang-for-buck mods on the B58 platform.