Guide: BMW E31 M8
Background
At the Frankfurt Motor Show in September 1989, BMW unveiled their new range-topping flagship: the E31 8-series.
Developed at vast cost, the 8-series effectively replaced the long-running E24 6-series. However, the new machine was a bigger, grander and much more expensive offering.
Initially the 8-series was sold in just one configuration: the 850i Coupe.
The 850i was powered by BMW’s all-alloy five-litre Type M70 V12 engine that, in its very conservative state of tune, produced 300bhp at 5200rpm and 332lb-ft at 4100rpm. Two different gearbox options were offered: a six-speed Getrag manual or four-speed ZF automatic.
BMW had planned to offer an elegant 850i Cabriolet from the start of production. However, this plan was eventually abandoned; although it would have made a great rival for the Mercedes SL and Jaguar XJS, BMW thought an 8-series Cabriolet unlikely to recover its development cost.
As soon as 850i production was underway, BMW Motorsport began work on a much-talked about M8.
It was well known that the M70 V12 offered spectacular tuning potential; BMW Motorsport created an enlarged all bells-and-whistles motor that pumped out nearly twice as much as the original. They also went to town in every other department so as to create the fastest and most extreme BMW road car yet.
Developed in utmost secrecy, the prototype M8 emerged for the consideration of BMW’s executives in early 1992.
Engine / Gearbox
At the heart of the M8 was an extraordinary super high performance iteration of BMW’s Type M70 five-litre V12.
Dubbed S70, it was the first in a series of S70 engines that went on to power the McLaren F1 (S70/2) and the BMW V12 LM / LMR racing cars (S70/3).
The standard 850i engine was equipped with single overhead camshafts, two valves per cylinder and an 8.8:1 compression ratio. It displaced 4988cc thanks to a bore and stroke of 84mm and 75mm respectively. Engine management was via the latest Bosch Motronic.
Peak output was 300bhp at 5200rpm and 332lb-ft at 4100rpm.
For the now dry-sumped S70 engine, BMW Motorsport took the displacement out to 6064cc by enlarging the bores by 2mm (to 86mm) and extending the stroke by 12mm (to 87mm).
New four valve cylinder heads with dual overhead camshafts were installed. New roller valves were used instead of conventional throttle valves. Valve timing was made continuously variable.
The compression ratio was hiked to 11.0:1 and the oil reservoir was moved to the trunk for more favourable weight distribution. Oil lines ran through the roof to the engine bay.
All told, the new S70 motor produced a formidable 549bhp at 7500rpm and 479lb-ft at 5600rpm.
The only transmission on offer would be a Getrag six-speed manual.
Chassis
The standard 8-series’ unitary steel bodyshell offered pillar-less styling when the side windows were lowered. To stiffen the shell, the M8 came with fixed B-pillars. To save weight, the new side windows were formed from synthetic Lexan.
Suspension-wise, the existing 8-series’ MacPherson strut front and multi-link rear axles were enhanced with stiffer springs, firmer dampers, thicker anti-roll bars and uprated bushes. Electronic Damper Control was imported from the base model. Ride height was 10mm lower than normal.
Whether the four-wheel steering system from the standard 850i was also fitted is unclear.
The brake configuration was uprated with ventilated discs all round (345mm front / 328mm rear). In comparison, the 850i ran 342mm discs all round (ventilated front / solid rear).
Handsome new bi-colour split rim wheels were fitted. They were the same size as the optional wheel upgrade for the 850i (8 x 17 front and 9 x 17 rear).
Bodywork
The M8’s Bright Red exterior panels were fabricated from a mixture of firbreglass composite and carbonfibre reinforced polymer.
Attention-grabbing new features included a deep front spoiler, flared wheelarches, side skirts and large intake blades behind the doors. The blades fed fresh air into the oil coolers for the engine and differential.
The 8-series was one of the last clean sheet automative designs to come with pop-up headlights. For the M8, these were ditched to reduce weight and improve aerodynamics. Fixed headlights were now integrated within the bank of supplementary lights either side of the BMW kidney-gille nose intakes.
The new single piece hood was redesigned with a more prominent power bulge and integrated air vents for the front-mounted radiator.
Small body coloured exterior mirrors were fitted for driver and passenger.
Impractically, on the prototype the Lexan side windows were all fixed in place with the exception of the driver’s door window which had a small sub-section in the corner which could drop down a few inches.
Interior
BMW Motorsport gave the M8 arguably the most radical cockpit overhaul they had attempted to date.
The rear seats were deleted. Instead, the rear quarters were upholstered in a mix of light weight carpet and BMW Motorsport-branded suede alcantara. The same high-grip suede was used to upholster the dash, centre console, door panels and new single-piece bucket seats.
The Recaro buckets could be manually adjusted fore / aft only and came with Sabelt four-point harnesses instead of conventional seatbelts.
The centre console / transmission tunnel was completely redesigned. An additional three gauges (for oil pressures and temperature) were mounted at the top of the centre console, behind which was a repositioned, short-shift gear lever.
The rest of the instrumentation was as per standard (large read outs for road and engine speed flanked by a petrol gauge to the left and a water temperature gauge to the right).
Weight / Performance
In base trim, the 850i tipped the scales at a not inconsiderable 1790kg.
Thanks to its many weight-saving features, the M8 came in at 1443kg (an astonishing reduction of 347kg).
If equipped with a six-speed manual gearbox, the 850i could hit 62mph from a standstill in 6.4 seconds. The M8 had a projected 0-62mph time of just 4.2 seconds.
A top speed of around 200mph was predicted (if the standard 155mph electric speed limiter was removed).
Cancellation & Subsequent History
Before ever embarking on any serious testing or development, it was clear the M8 in its current guise was not a viable production proposition.
It would have retailed at nearly twice the price of a Porsche 911 Turbo and there was simply an insufficiently large market to cover the development costs. Economic conditions had turned sour pretty soon after the 8-series September 1989 launch.
To placate those customers that wanted more power, the 850 CSi (which BMW Motorsport had a hand in developing) was launched at the Frankfurt Motor Show in September 1991.
The finished M8 was presented to the BMW board in early 1992, but by this time the decision on its fate had long since been made.
Afterwards, the one-off prototype (chassis 031EG1100LH231590) was dispatched to a BMW warehouse where it spent the best part of two decades under wraps.
Text copyright: Supercar Nostalgia
Photo copyright: BMW - https://www.bmw.com
