On paper, the BMW M235i sounds like a brilliant everyday performance motor. For the enthusiast, it ticks many boxes: fast, rear-wheel-drive, choice of manual or automatic gearboxes, small enough to be wieldy, big enough for four and a fair bit of luggage, and now available used for as little as £20,000.
The trouble is, cars don’t ride on paper. On roads, the generally very good M235i is hamstrung by not having the standard limited-slip differential that arguably should be fitted on any RWD sports coupé, and by its poorly damped suspension on any roads that aren't as smooth as a marble floor, which is pretty much all of them.
Buckingham-based tuning company Birds claims to have unlocked the M235i’s potential with its B2 conversion. Birds has a long track record in nicely-modded BMWs. For this M235i they asked ex-racing driver James Weaver and experienced chassis engineer Pete Weston to play with the upgraded Bilstein suspension. The B-Series Sports Suspension kit is built to their Birds-exclusive specifications and costs £1554.23, including installation and warranty, but excluding VAT.
A £2248 Birds engine management upgrade takes the turbo six’s power up to 385bhp, and a £1694 Quaife unit plugs the limited-slip differential gap. Throw in a set of Goodyear Eagle F1 tyres and front-axle spacers and you’re up to a VAT-included total of £7,238 for the conversion. Then you can start considering other options like uprated anti-roll bars and Alcon brakes, and – for manual cars – a short-shift kit and clutch pedal weight adjustment kit.
Does the B2 conversion work? From a handling and all-round performance perspective, totally. The suspension’s ability to sort out poor roads and its ride quality are both greatly enhanced. The standard car’s vertical pogo-ing has been replaced by an air of calm composure. Even without the optional axle spacers, which apparently improve things even more, the steering has a new precision about it.
That Quaife differential really brings the best out of the M235i. The standard car’s open diff frustrates by smoking an unloaded tyre or zapping the car into surprise oversteer when you stamp on the throttle. The Quaife adds much more playfulness, letting you slide the BMW like a classic rally Escort or hold it in neat little corner-exit drifts. You can get a dealer-fit M Performance LSD from BMW, but at £2500 it’s a lot dearer than Birds’ one.
The software upgrade is the only engine mod, but it works a treat. Now there’s serious pull from just 2000rpm and extreme mid-range urgency en route to the fast-arriving limiter. It’s a properly quick car, although you wouldn’t realise it from the slightly colourless exhaust note.
If you’re the owner of a M235i, either new or used, and want it to be both fast and fun on a daily basis, we strongly recommend the Birds B2 upgrades. You can get Birds to bolt it all on in one shot, or do it gradually piece by piece. In that case the best order would be chassis first, then diff, then engine.
As a by the way, it’s worth noting that you can fit the entire kit to the M135i hot hatch, and everything bar the engine upgrade to the M140i or M240i.
Birds BMW M235i
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
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