Now it's the turn of the engine. According to the good people at Birds, the 'N55' 3-litre turbocharged six-pot can be wound up to almost 400hp with just a remap. The wick can be turned up even further, of course, but that causes fairly significant issues. "We did push one of these engines beyond 400hp," says company founder Kevin Bird, "but we found that the ignition timing was being retarded at maximum torque rpm in the higher gears. If you kept the throttle pinned for more than ten seconds you could also see that the throttle was beginning to close, ultimately triggering limp mode. We determined that the detonation protection was being triggered, and clearly the thermodynamic and physical limits of the engine were being exceeded at the level of boost required for 400hp plus. To get more power we would have to either open up the engine, or provide additional external equipment to increase the detonation limit of the engine." In other words, you would have to spend an awful lot of money, which really isn't the point of this project.
What it doesn't feel is over-engined. It isn't as though we've shoehorned an unnecessarily powerful motor into an inappropriately small car. Instead, the engine now feels very well matched to the rest of the - much improved - chassis and drivetrain, which is exactly how I like it. I'm sure there'll be some people out there who want 500hp or more from their M135is and I've no doubt it can be done if you're willing to open your wallet wide enough, but on this occasion we've set out to build a cohesive and well rounded car, not an explosively fast one.
What I haven't mentioned yet is that the M135i is now more powerful than the 370hp M2, as well as a whole lot torquier. I haven't had a chance to run a set of acceleration figures yet, but all signs point to this car now being faster than the pugnacious little M2. The Birds remap, by the way, costs £2,249.
And that was supposed to be the end of our M135i project. But then, a few days ago, an email dropped in from Birds asking if I would fancy upgrading the car's brakes early in the new year. Well, it'd be daft not to, wouldn't it? The standard brakes have been just fine on the road, but I know from past experience that they aren't really up to sustained circuit work. The M135i will be around for a little longer than expected, then. Soon, it should stop just as well as it goes.
Car: BMW M125i
Run by: Dan Prosser
Bought: July 2017
Mileage: 36,100 total, 2,800 this month
Purchase price: £17,500
Last month at a glance: Remap boosts power to almost 400hp