BMW E24 M635CSi vs E28 M5: dial M for more

By Martin Buckley

| 7 Nov 2022

Classic & Sports Car – Dial M for more: BMW E24 M635CSi vs E28 M5

The E24 M635CSi and E28 M5 – both brawny, built-to-order products of BMW’s Motorsport GmbH arm – are prime candidates for ‘peak BMW’ status, in my book anyway.

In a world that is becoming ever more sanitised, these cars look, smell and feel like the real deal.

BMW could have come up with a pair of freakish hot rods as a way of promoting the talents of its Motorsport division, but that would not have been its style.

The coupé-bodied E24 M635CSi (right) and four-door E28 M5 share BMW’s race-derived M88 straight-six engine

On the other hand, to call the M5 and M635CSi subtle tends to underplay their capabilities.

Perhaps they are more than just ‘peak BMW’, but somehow represent the height of achievement in the wider realm of analogue, rear-drive, usable performance cars.

Now fast alighting on their 40th birthdays, neither car was a young design even in the early 1980s. Weight-paring and aerodynamics were not exactly at the top of their designers’ priorities, either.

‘M’ badges suggest this is no ordinary BMW on the otherwise subtle M635CSi

But behind those shark-like masks lurks the key to the charm of this M-car duo – and, arguably, something no truly great BMW should be without: a straight-six engine.

From a company that always took more than average pride in even its most humble power units, a semi-productionised version of its Paul Rosche-designed competition ‘six’ – battle-hardened in the works CSLs since 1974 as the M49 – was always going to be special.

With shimmed bucket-tappets and single-row chain-drive for its double overhead camshafts, the 24-valve, 3.5-litre M88/3 was a hand-assembled, civilianised version of the engine found in the mid-engined M1, the car developed by the 180-strong Motorsport division (with a little help from Lamborghini) to take on Porsche in Group 5 racing.

Following its acclaimed launch at the Frankfurt International Motor Show in 1983, the M635CSi arrived in the UK with right-hand-drive in ’85

The dry-sump M1 had Kugelfischer butterfly-control mechanical fuel injection; in the wet-sumped M635CSi, the spark plugs, fuel injection and timing were electronically managed by Bosch for an even meatier 286bhp at 6500rpm, along with a more road-friendly torque curve.

There were huge margins of strength here, given that turbocharged versions of the same engine gave up to 900bhp in the Group 5 M1s.

With a slightly bigger bore and a shorter stroke than the single-cam 635CSi lump it was based on, the M88 was said to be good for 8500rpm, but was limited to 6900rpm.

Both BMWs use a Getrag ’box with fifth on a dogleg

With less restrictive exhausts, 10.5:1 forged pistons and a beefed-up 12-counterweight crankshaft, this was probably the most exciting straight-six ever fitted to a production car.

Getrag’s strengthened, close-ratio 280/5 five-speed ’box was obligatory in both these M-cars, along with thicker, vented front discs, ABS and a 25% limited-slip diff.

In the not very slippery 6 Series coupé (0.39Cd), the top speed was a whisker shy of 160mph.

The rear seats are best suited for occasional use in the BMW E24 M635CSi

Shod with wider Michelin TRX metric tyres on BBS split-rim alloys, and riding 10mm lower on shorter, stiffer springs, the M635CSi was launched to wide acclaim at the Frankfurt International Motor Show in 1983, stickered at DM89,000 – with velour trim.

Initially, at least, there were no plans for a right-hand-drive version, but that soon changed.

When well-optioned right-hookers did eventually arrive in the UK, in 1985 – complete with air conditioning, a sunroof and full leather – they were priced some £7000 higher than a standard, non-M 635CSi and £10k stronger than a Jaguar XJ-S HE V12.

Both the E28 M5 (left) and E24 M635CSi emit a classic BMW ‘six’ tone, with a bassy hint of motorsport breeding

Apart from ‘M’ badges, quietly flared arches and a deeper front spoiler, the M635CSi didn’t look much different from any other 6 Series: but that was the point.

More importantly, Bavarian loyalists – those with the deepest pockets and the heaviest right feet – could now buy a car that was able to square up to the fastest Porsche and Ferrari ‘supercar’ opposition.

Having turned the slightly flabby 635CSi into a worthy replacement for the much-missed E9 3.0 CSL, BMW’s attentions turned to the unlikely looking E28 5 Series and the M5 was launched at the Amsterdam International Motor Show in February 1985.

By then, 25 cars had already been delivered to customers, and these limited-edition models were sold directly out of Motorsport GmbH, a separate commercial entity to the main BMW operation.

The M635CSi runs on split-rim BBS alloy wheels, shod with Michelin rubber

With four full-sized seats and a slightly matronly profile, the M5 was an even more remarkable car than the longer, wider M635CSi.

It also just happened to be the fastest four-door saloon car in the world.

Creating it did not take a great leap of technical imagination, because the 5 and 6 Series were virtually identical under the skin, sharing the latest, lift-off-oversteer-curbing suspension tweaks of the E28.

With more negative camber at the front and more positive camber at the semi-trailing rear to keep the wheels upright under load, the chassis on these slightly nose-heavy cars was as good as BMW knew how in the ’80s.

Crackle-black cam covers and individual throttle bodies top the high-compression M88 straight-sixes in the E24 M635CSi (front) and E28 M5

The anti-roll bars were stiffer at both ends, and it even went to the trouble of relocating the battery to the boot.

Today, the real headline is that the E28 M5 – only built to the tune of 250 units a year – is one of the rarest production BMWs of all, with just 2241 examples built through to ’89; at 5855 cars, the M635CSi is much more numerous.

Both are very rare as right-hookers: 524 M635CSis – including 102 facelift cars – and just 187 M5s.

The BMW E24 M635CSi (closest) and E28 M5 are civilised, smooth machines with knockout punches

David Rapoport is in the enviable position of owning lovely examples of each, although he would probably part with the four-door car for the right offer.

The former stockbroker had an earlier, single-cam, 12-valve M535i in the ’80s, but concedes that it was a rather less special, less focused device that didn’t amount to much more than shoehorning the biggest ‘cooking’ straight-six into a smaller, lighter body.

“I prefer the M6,” he admits when pressed. “It suits my style, my age, and it’s more comfortable with the factory-fitted Recaro seats.”

The BMW E24 M635CSi shares its easy-to-read main instruments and chunky three-spoke sports steering wheel with the E28 M5

Both cars were lockdown purchases, but now reality has kicked in: “I will be 73 next month – so I’m trying to shrink the fleet – although I’ve just bought a low-mileage Porsche targa…”

Before taking the wheel of an M635CSi or an M5, you have to pay your respects to the M88 engine.

It’s a visual treat and a magnificent bonnetful, heavily canted over and with ‘BMW Motorsport’ cast proudly into the purposeful, ribbed cam covers, with individual throttle bodies set into a handsome inlet manifold.

Both the E24 M635CSi and E28 M5 display the clean lines of ’80s BMW design

Driving the two M-cars makes Rapoport’s preference for the late-model M6 easy to understand.

It’s not just that the coupé shape, with its deep glass area, has aged better than the 5 Series.

Something about the proportions and the design of the 635’s brightly lit cabin just make it a nicer place to be than inside the slightly gloomier four-door car.

There are more grand views across its longer, wider bonnet, and the commanding driving position is fractionally more relaxed – although the seat/pedal/steering-wheel relationship in both cars is excellent. Likewise the clean, clear dashboard designs, an area in which BMW led the world at the time.

The Check Control Panel of the E24 M635CSi is very of its time

Both wisely ditched the economy gauge used in lesser versions – fairly meaningless in cars that struggled to muster 20mpg when driven even moderately briskly.

They share clearly calibrated 170mph, 8000rpm main instruments and chunky three-spoke sports steering wheels with ‘M-Technik’ detailing.

As in any coupé, back-seat space in the 635 is subordinate to a sleek roofline, but most grown-ups could tolerate a long trip in the carefully designed, individually tailored rear chairs.

The unmistakable, shark-like front bodywork of the BMW E28 M5 (left) and E24 M635CSi

The M5 is an unapologetic full four-seater, five at a pinch. Prior to the Lotus Carlton and Mercedes-Benz 500E, it’s hard to think of a comparable ’80s saloon rival.

It evidently struck a chord with BMW buyers. While the M635CSi was not directly replaced, the M5 nomenclature – as applied to the E34 5 Series and its successors – went on to become a benchmark badge for fast, luxurious mid-size saloons, and still is today.

Common to both cars is the pushbutton Check Control Panel – seemingly derived from a Casio pocket calculator – and long, deep luggage areas, complete with the familiar BMW tool tray in the underside of the bootlid.

A rubber spoiler and the distinct badge define the M5’s rear

The Recaro seats in the coupé are more figure-hugging yet marginally comfier than the M5’s.

Both have powered controls (four buttons, but less intuitive than the Mercedes equivalent) and adjustable steering columns, so nobody need be uncomfortable in either.

On the move, you sense that the M635 is the heavier car – which it is, by 150lb. Certainly it is a gentler machine than the M5, with a more suave demeanour than the ever-so-slightly hooligan four-door saloon, a car that would not look or feel out of place taking part in a suburban Post Office armed robbery.

The now-classic E28 M5 was once the world’s fastest four-door saloon

Not so the M635CSi. As the firm’s flagship road machine, it was probably deemed prudent to put the emphasis on civility in a car that would appeal to richer, older customers than the M5.

Both are easy to drive smoothly, bereft of driveline shunt and with easy to control – if rather heavy – clutch and throttle pedal inputs.

The pleasingly consistent, shock-free steering also feels quite heavy, while the four-wheel disc brakes are good, but just a shade less pokey than their specification promises.

BMW’s neat dashboard layout (left) was the envy of rivals; tri-colour details (right) adorn the M5’s cabin

The M88 unit is as silky, flexible and even-sounding as any other BMW straight-six, although the tone of cultivated mechanical throatiness emanating from their large-bore tailpipes tells anyone with the necessary empathy that there’s something special moving these big BMWs down the road.

In both cases, it is the way this magnificent engine winds out so strongly from 4000rpm to its redline that is so thrilling, with no obvious ‘step’ in the power delivery.

Easily roused, it lunges these cars forward with a fierce, turbine-like whistle and a frantic, relentless gathering of pace.

The BMW M5’s Recaros are firm but highly adjustable

First gear takes you to past 40mph, second to just shy of the legal limit and third to something over the ton.

Using the full, scalp-prickling 7000rpm, you could be travelling at 130mph in either car before selecting fifth – a situation that can be presented in less than half a minute from rest.

Apart from a longish movement into bottom gear, the Getrag ’box – with fifth on a dogleg – is slick, accurate and hard to fault: you find yourself looking for excuses to use it.

Near-centrally mounted twin tailpipes protrude from the BMW M5’s rear bumper

With identical final-drive and gear ratios, the marginally lighter M5 might be a shade more lively, but the impression is of a slightly harder-feeling car.

Both of these BMWs ride firmly – by 1980s standards – on their Bilstein dampers, but they are nowhere near what you’d describe as harsh, while never being floaty or flabby.

Perhaps the extra weight of the coupé takes the edge off the way the wheel movements are checked, but poor surfaces do not take away from your ability to corner rapidly in either car.

On the road, the M635CSi (behind) is a little softer than the lighter M5

Somehow, BMW managed to dial unerring straight-line stability into their speed-sensitive power steering while maintaining superb turn-in.

The E24 and E28 M-cars hold understeer and body roll in abeyance to a level that allows you to urge them along very quickly.

There is a gear for every eventuality in the closely stacked ’box, and abundant torque to balance the attitude of the car just where you want it.

Flared wheelarches dress both M-cars, joining the 415mm-diameter split-rims of the M635CSi and the M5’s optional bodykit

Lairy, easily held powerslides – a recurring theme of effusive M5 and M635CSi magazine road tests in the ’80s – feel as if they are a downchange and a twitch of the right foot away, should you wish to indulge.

For every reason you can imagine, I elected to give such heroics a miss, but the option is there for those with the nerve and skill to fully exploit these finely honed examples of the way BMW fed its hard-won competition nous into the cars it sold to the public.

These are very fast, very forgiving cars that might not want to put you backwards into a hedge or across a field, but offer no electronic insurance against such an eventuality should your luck, ability or judgment – or the weather – fail you at the critical moment.

‘Lairy powerslides feel as if they are a downchange and a twitch of the right foot away, should you wish to indulge in them’

Prettier cars have come out of Munich before them, and faster ones since, but I cannot think of any others that so perfectly encapsulate the spirit of what the firm, at its ’80s best, was all about.

Not preening exotica or hoodlums’ street racers, but well-groomed, well-wrought cars with a bright halo of competition pedigree.

More than just transport but essentially practical, the M635CSi and M5 were high-performance BMWs built for mature adults, with business to attend to but nothing to prove.

Images: Luc Lacey

Thanks to: David Rapoport

Factfiles

BMW E28 M5

Sold/number built 1984-’88/2241
Construction steel unitary
Engine iron-block, alloy-head, dohc 3453cc ‘six’, Bosch fuel injection
Max power 286bhp @ 6500rpm
Max torque 251Ib ft @ 4500rpm
Transmission five-speed manual, RWD
Suspension independent, at front by MacPherson struts rear semi-trailing arms, coil springs, telescopic dampers
Steering power-assisted recirculating ball
Brakes discs, with servo and ABS
Length 15ft 2in (4620mm)
Width 5ft 7in (1700mm)
Height 4ft 7in (1400mm)
Wheelbase 8ft 7in (2625mm)
Weight 3152Ib (1465kg)
0-60mph 6 secs
Top speed 147mph
Mpg 17-22
Price new £34,850 (1988)
Price now £40-75,000*

BMW E24 M635CSi

Sold/number built 1983-’89/5855
Construction steel unitary
Engine iron-block, alloy-head, dohc 3453cc ‘six’, Bosch fuel injection
Max power 286bhp @ 6500rpm
Max torque 251Ib ft @ 4500rpm
Transmission five-speed manual, RWD
Suspension independent, at front by MacPherson struts rear semi-trailing arms, coil springs, telescopic dampers
Steering power-assisted recirculating ball
Brakes discs, with servo and ABS
Length 15ft 7¼in (4755mm)
Width 5ft 8in (1725mm)
Height 4ft 5in (1353mm)
Wheelbase 8ft 7¼in (2625mm)
Weight 3742Ib (1505kg)
0-60mph 6.1 secs
Top speed 158mph
Mpg 15-17
Price new £45,780 (1988)
Price now £45-100,000*