With its B2 conversion, Buckinghamshire-based tuning company Birds reckons it has realised the car’s potential. The company has been modifying BMWs for decades, but for its M235i conversion, it employed former racing driver James Weaver and experienced chassis engineer Pete Weston for the first time to tune the upgraded Bilstein suspension. The B-Series Sports Suspension kit (£1554.23, including installation and warranty, but not VAT) is built to their specifications exclusively for Birds. There’s even a label on the damper unit to show as much.
Birds has also increased power from the turbocharged six-cylinder engine to 385bhp with an engine management upgrade (£2248.91), as well as fitting a Quaife limited-slip differential (£1694.25). The full B2 conversion – which also includes four Goodyear Eagle F1 tyres and spacers for the front axle – costs £7,238, including VAT.
Additionally, Birds also offers uprated anti-roll bars, reducing the car’s tendency to understeer at the limit, a short-shift kit and clutch pedal weight adjustment for manual cars, and heavily upgraded Alcon brakes.
What's it like?
Without exaggeration, the B2 conversion is transformative. There’s more composure in the ride quality and the uprated suspension is much better equipped to deal with uneven roads. The car actually smothers and absorbs the road surface rather than lolloping about all over it, giving you real confidence. Body control is also much improved, with the car remaining calm and poised rather than pogoing up and down. Even the steering feels more precise (according to Birds, the spacers, which weren’t fitted here, improve that even further).
It could be argued the Quaife differential does even more to bring the best out of the M235i (BMW does offer a dealer-fit M Performance unit, but at £2500 it’s much more expensive). The standard open differential can be frustrating, either causing the unloaded tyre to light up hopelessly or tripping the car into sudden oversteer when you stand on the power. You’re never quite sure which it’s going to be.
The uprated differential, though, gives you so much control over the rear axle that you can play with it endlessly. The car will slide like Ari Vatanen’s Mk2 Ford Escort if you want it to, but it’s actually more satisfying to hold the little coupé in modest, neat and perfectly balanced drifts away from tighter corners; the kind of slide that still pings you along the next straight with real impetus.
Untouched apart from the software upgrade, the engine pulls really hard from as little as 2000rpm and feels incredibly urgent in the mid-range before spinning out to the limiter with real ferocity. This is an impressively quick car; it’s just a shame that the soundtrack is a little bland.
Should I buy one?
If you own an M235i and want to unleash its potential, the Birds upgrades are a no-brainer. If you want a fast and fun coupé that can be used daily, we’d even recommend buying a used M235i just to have the Birds B2 conversion fitted. The kit can be bolted on in one hit, or over time to suit a budget (best to have the chassis upgrades fitted first, then the differential and then the engine mods).
Incidentally, the whole kit can be fitted to the M135i hot hatch, too, while everything but the engine software upgrade can be applied to the newer M140i and M240i.
Birds BMW M235i
Location Chobham Common, UK; On sale Now; Price see text; Price as tested see text; Engine 6cyls in line, 2998cc, turbocharged petrol; Power 385bhp at 6500rpm; Torque 391lb ft at 3250rpm; Gearbox 8-spd automatic; Kerbweight 1530kg; 0-60mph 4.3sec estimated; Top speed 155mph