We have bought a used BMW M135i to see how far we can improve this rear-wheel-drive hot hatch

Why we're running it: To discover how much we can enhance this used BMW M135i with a little help from our friends at Birds

Month 1 - Month 2 - Month 3 - Month 4 - Specs

Life with a used BMW M135i: Month 5

A duel with an M2 is on the horizon - 28th February 2018

Right now, you should be reading all about the M135i’s lovely new brakes. Unfortunately, the supplier was unable to deliver them before the end of our loan of the car, so the standard brakes will be staying put. No matter, actually, because we didn’t set out to build a track car and I’ve never had an issue with the factory stoppers on the road.

Still, there is some progress: the world’s most determined key scratch has finally been repaired. Whichever cheery soul it was who vandalised the car started at the very front and keyed the paintwork all along the right- hand side, damaging all three panels in the process. They then turned around and scraped their way back to the front again, just to be certain.

I took the car to Mustoes, a bodyshop near Chepstow, where they did a proper job of the repair. The whole side of the car was masked off, sanded down and resprayed. The car looks as good as new now and the repair cost a very reasonable £360.

As I type, the M135i will be going back to BMW tuning specialists Birds for good within a couple of weeks, but not before we’ve put it up against an M2. It’ll be an intriguing comparison. Since its engine remap, the M135i has more power than the M2, so it will probably be quicker in a straight line. Everywhere else, though? We’ll have to wait and see.

Finally, I’ve been reading about BMW’s plans for the 1 Series. Reports suggest the third generation will switch from rear- to front-wheel drive next year, with high-performance models set to use four-wheel drive. I get why BMW would want to move away from a rear-drive layout for the 1 Series. The overwhelming majority of hatchback buyers would never appreciate the benefit of a rear-driven chassis, or even notice the difference. What they certainly would notice and appreciate, however, is the extra cabin space such a change in layout would bring. Rear leg room has been a problem for the 1 Series since day one.

But BMW has spent many decades telling us rear-wheel drive is best. And all of a sudden, it isn’t. A pity, too, that the really quick models will almost certainly drop the current six- cylinder turbo engine in favour of a four-pot. (Try making six cylinders fit in an east-west installation...)

If these reports are accurate, the next high-performance 1 Series will share the same mechanical layout as the Audi S3, Mercedes-AMG A45 and VW Golf R: four-wheel drive with a four-cylinder turbo motor. Who knows, it may well become a better car for it. But that loss of variety in the hot hatch sector will be lamentable.

Mileage: 38,467

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Life with a used BMW M135i: Month 4

Turning the wick up on our used BMW M135i – 24 January 2018

The plan for our M135i project car was always to get the suspension sorted first to make it ride and handle the way we wanted it to.

Next, we would fit a Quaife limited-slip differential, (LSD) and only then would we go anywhere near the engine.

That is exactly how it has worked out so far, but with the engine now having been remapped to liberate a good wedge of extra power and torque, I felt somewhat sorry that the project was close to coming to an end.

Which is precisely why I’ve twisted the arms of the good people at Birds – the tuning company we’ve been working with on this project – to let me keep the car a little longer and make an upgrade not in the original plan. At some point in the next few weeks, then, I’ll be telling you about the M135i’s lovely new brakes.

But that’s for then and this is now. With just a remap, Birds is able to wind the N55 3.0-litre turbo engine up to almost 400bhp. In a car the size of a 1 Series, that really is a huge amount.

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Making the M135i a barrel of laughs – 08 November 2017

The M135i project car has been riding on its trick Bilstein dampers for a couple of months now, so I’ve had plenty of opportunity to compare the new set-up with the junked standard suspension.

The ride quality has improved and body control is also better now, but it was a quick blast along a lumpy, uneven Cotswolds B-road that really highlighted how much better the uprated springs and dampers are.

The BMW breathed so brilliantly with the bumpy road surface that I actually got to wondering how many cars I’ve driven that were better damped than it. There are a few, I decided, but not many.

With the chassis sorted, the next round of upgrades concerned the transmission. No sense in uploading the grenade engine map until the limited-slip differential was in place, after all.

It’s curious how reluctant many manufacturers are to fit limited-slip differentials.

They say they increase understeer tendencies, but I suspect it’s more to do with cost. The M135i never had a factory-fit LSD option (a dealer-fit locker was available), which seems odd given the power the car develops, as well as its billing as a proper driving machine.

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Along with the cost, it might well be that BMW sees this as one of the key differentiators between the semi-skimmed M Performance models and its full-fat M cars.

Anyway, what it means for us is we can make the car a good deal more fun for relatively little outlay. The Quaife differential costs £2033 at Birds – the BMW specialist we’ve teamed up with for this project – and takes a couple of hours to fit.

I’d been looking forward to this upgrade. Without an LSD, the M135i is a bit like an Italian pasta dish without parmesan: good, but short of the finishing touch that makes it brilliant. Snap the throttle wide open mid-corner with an open differential and the inside rear wheel spins up hopelessly. It’s messy and annoying. On track, it’s more frustrating still.

In overly simplistic terms, an LSD splits the torque between both wheels, rather than allowing drive to follow the path of least resistance. This means you get much better acceleration away from a corner and better traction in low-grip conditions too.

And if you want to slide the car, you can. In fact, with the LSD fitted, the M135i is wonderfully controllable, the rear end breaking away in a completely fluid and predictable way. It’s brilliant.

And I’m not talking smoky, showy powerslides here but those cheeky little drifts that make you giggle to yourself.

RWD and LSD really do go hand in hand.

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Mileage: 32,150

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Life with a used BMW M135i: Month 2

Getting to grips with the M135i’s suspension – 13 September 201​7

Perhaps the single biggest upgrade we’ll make to our used M135i is also the first – the suspension.

Out goes BMW’s Electronic Damper Control equipment in favour of a set of Eibach springs with passive Bilstein dampers, both tuned specifically for the M135i.

Birds, the specialist we’re working with on this project, recruited former racing driver James Weaver and experienced race engineer Peter Weston to tune the new suspension.

Between them, Weaver and Weston chose appropriate off-the-shelf springs for the car while also specifying their own damper curves to Bilstein, which built the damper kits accordingly.

The result is that the Birds suspension upgrade for the M135i is proprietary. You can’t get it anywhere else. The new suspension took half a day to fit, costing £1554.23 with installation, but not VAT (which takes the total to £1865.08).

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“I drove the standard car and it was a bit of an eye-opener to say the least,” says Weaver. “The ride quality was pretty poor. The problem is that in Germany new cars have to be able to do 124mph, five-up and with a fully loaded boot, normally on smooth roads.”

Consequently, as Weston explains, modern cars are desperately over-damped most of the time, when they only have one or two occupants. They are also often not optimised for the UK’s broken, bumpy road network.

It means that most cars, the M135i included, offer plenty of room for improvement, which certainly tallies with my own experience of the car.

Weaver and Weston test drove a standard M135i using a loop of about four miles – mostly made up of narrow, bumpy lanes – near Birds’ Buckinghamshire headquarters.

They found that the ride quality was poor, mostly because there was far too much rebound in the standard set-up, and the car also lacked grip, traction and steering precision.

“The damping I’ve incorporated into the shocks is what we’ve learned over the years with James in a racing environment, with a high-performance car on a bumpy circuit like Sebring,” says Weston. “That experience taught us an awful lot about how to support a car on the springs and yet have damping that allows you to go over bumps. It blows the theory that for a car to be good over bumps you need soft springs – that isn’t always the case.”

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The BMW M135i was launched in 2012, making this 62-plate example one of the early cars. The M135i was so well received by the press and car-buying public that you couldn’t read through more than 15 internet forum posts without somebody telling you to get one. 

Five years on, the cheapest, leggiest M135is have lost more than half their value. You can pick up a 70,000-mile car today for around £14,000, which looks like very good value indeed for a modern rear-wheel-drive hatch with 316bhp.

This particular car has covered 30,000 miles and it cost £17,500. The spec is just about spot on: three doors, manual gearbox, black leather.

The M135i cost a fiver under £30,000 when it was new, which meant it wasn’t much more expensive than high-performance Volkswagen Golfs, Renault Sport Méganes and the like. But it was more powerful, it had a more prestigious badge and it was, of course, much more rear-wheel drive than any of them. Little wonder it proved so popular.

Years later, it’s still a compelling package. The turbocharged six-cylinder engine gives it strong straight-line performance, the rear-driven chassis is nicely balanced and, on the much less interesting side, it’s practical and pretty refined.

However, it’s actually somewhat flawed. I won’t say too much about the styling, except that I never much liked the big, triangular headlights, but I love that you can tell just from looking at it where the power is sent: the cabin that’s pulled backwards over the car and the distance between the base of the windscreen and the front axle line give it away.

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The biggest problem, though, is to do with the dynamics. Now, the M135i is very good to drive up to about seven-tenths, but when you really start hustling it, or when the road becomes particularly difficult, it all starts to unravel.

There isn’t the precision in the steering or the tight-fisted body control you need to really fling it around with confidence.

And when you start throwing crests and little jumps and compressions into the mix, the damping feels all at sea. No matter which of the two damper modes you select, the chassis is never able to iron out lumps and bumps in the road surface while also keeping the body locked under tight control.


Working with BMW tuners Birds, we’re going to replace the standard, under-specified suspension with much higher-quality Bilstein components tuned specifically for the M135i, we’re going to fit a Quaife limited-slip differential and we’re going to lift power to somewhere approaching 400bhp. We might yet make a few other modifications, too.

When it’s upgraded to the full Birds B1 specification, it should be quite a car. We tested the company’s M235i demonstrator – the upgrades are interchangeable across hatchback and coupé – and found it to be a vast improvement over the standard car.

What makes us so sure that we, a car magazine, and an independent garage, can build a better car than BMW?

Every component in a mass-produced car is built to a strict unit price. The cars are also designed for a wide cross-section of drivers and for countries on opposite sides of the world. We, however, are able to spend much more on suspension parts than BMW could justify and we also have a very specific idea of how we want the car to feel once it’s finished.

The M135i will be upgraded over time and you can follow our progress in these pages.

Please do get in touch via the usual channels to let us know what you think of our new project car.

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BMW M135i specification

Specs: Used Price £17,500; New Price £29,995

Test Data: Engine 6 cyls, 2979cc, turbocharged petrol; Power 316bhp at 5800rpm; Torque 332lb ft at 1250-5000rpm; Top speed 155mph; 0-62mph 5.1sec; Claimed fuel economy 35.3mpg; Test fuel economy 30.4mpg; CO2 188g/km; Faults None; Expenses Suspension upgrade (£1865), clutch pedal modification (£113), short-shifted kit (£532), limited-slip differential (£2033)

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Join the debate

Add a comment…
ap18 1 December 2017

What short-shift kit was installed and where can it be bought?

What short-shift kit was installed and where can it be bought?

Rich_uk 12 November 2017

The irony FredD

The point xxxx initially raised was the value in buying a new M135i for (circa) £30k and how little the buyer would lose in 5 years later. Maybe you didn’t read the comments properly... 

FredD 13 November 2017


well sometimes you try to prove a point with irony. Noone noticed except you so it may have backfired

FredD 10 November 2017

Used new

In my world the only way you could compare it to a new Focus is if the BMW is used. The insurance and taxes are higher. When the BMW is new the price of running the two cars would be quite different over 3-4 years. I might be wrong though. I have never owned a car in GB