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

DDownpipeDan

@downpipe_dan

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The X4 M40i shares the B58 engine with the M340i, M240i, and Z4 M40i. Stock output is 382hp and 500Nm. But does it tune the same as the sedan?

I had my X4 M40i tuned with BM3 stage 2 (catless downpipe + tune on 98 octane). Dyno results:

  • Stock: 348whp / 472Nm at the wheels
  • Stage 2: 428whp / 612Nm at the wheels

That is an 80whp gain from a downpipe and tune. The B58 is an incredible engine for tuning — the stock internals handle stage 2 power without any issues.

The X4 is heavier than the M340i by about 100kg, so it is slightly slower in a straight line. But the higher seating position and xDrive traction make it feel very planted and confidence-inspiring.

The catless downpipe does produce a check engine light. I am running a BM3 map with the CEL coded out. No issues with emissions testing in my country.

For anyone considering an X4 M40i — the B58 tuning potential makes it a seriously fast SUV for relatively little money.

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  1. #1
    BBM3 Brian

    @bm3_brian

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    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.

  2. #2
    IIvan

    @intercooler_ivan

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    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.

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