The Z4 (and others BMW Straight 6 applications) use a ram air intake system. There's a drain hole in the air box that can supposedly be detrimental to maintaining maximum possible air pressure into the engine as it effectively 'leaks' some of that air through a drain hole.
Funny things that I've seen on our dyno at work:
1) 17bhp gained by correctly inflating the tyres on the driven wheels
2) 15bhp gained by removing ill fitting carpet mats
3) rear tyres on a 964 Turbo delaminating
4) 9bhp lost over two days as one was cold, the other was warm.
5) Cosworth owners claiming at least 400bhp and walking away with a graph showing less than 270bhp.
Lol ! Love the Cossie quote ! Seen many a " big bhp" claim over the years. Normally 380-400 bhp with chip/ intercooler and injector change only. They struggle to hit 300 . Not to mention all the companies claims that come nowhere near. You certainly know when you're In a 400 bhp plus car. Dyno dynamics rollers have a good reputation from a well known lad in the Ford scene with a mk1 escort that does 12.7 qtrs terminal speed of 110ish on road tyres, of being the most accurate out there. His figures are less than others quoted from rollers, but he then blitzes them on the strip.
