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The ESS tune is spot on, as long as you don't modify beyond OEM U.S. or Euro headers. ESS will not tune for stepped headers, so I had to seek an alternate dyno tuning source, which was expensive. ESS said stepped headers will not work on the Z4M (they do, as both my cars have stepped headers). However, if you intend to run OEM headers, ESS has their tune down perfectly.
Overall I'm happy with my ESS car. It feels like it wants to pull to 9k rpm. Power keeps climbing until you slam into the rev limiter.
The VF bracket is better designed for more boost, with an extra pulley to increase belt wrap on the supercharger pulley. I've had no slippage running standard boost levels on the ESS kit however... the bracket is only an issue if you want to run more boost. If you run the ESS kit within spec, you shouldn't have issues with slip.
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