The Lotus 900 series is a family of internal combustion engines designed and built by Lotus Cars of the United Kingdom. Successor to the Lotus-Ford Twin Cam, the 900 was the first complete engine developed by Lotus. Production ran from 1972 to 1999.
Background As early as 1964, Lotus recognised the need to find a replacement for the Lotus Twin Cam engine. Colin Chapman issued a brief that listed the features to be required in a new engine, including 'high efficiency, flexibility, torque and smoothness which was suitable for hand assembly'. Unable to find this combination in any existing engine, the company used outside consultants and internal resources to define the characteristics of the next Lotus engine. After having rejected a 120° V6 due to being too wide for Lotus' chassis and a 60° V6 as too tall for the intended bodywork, the engineers determined that a 2-litre inline-four engine was the optimal choice. This future engine would have four valves per cylinder (16 valves total) operated by belt-driven dual overhead cams and develop . The block would be angled at 45° from vertical to permit a lower bonnet and simplify development of a 4-litre V8 version for future use in Indianapolis racing.
The design team was headed by Steve Sanville, Lotus' Head of Powertrain Development, and Ron Burr, formerly of Coventry Climax. Even though the team was able to complete the design for the new cylinder head and start work on the engine block and crankshaft, it became apparent that Lotus' racing programme and concurrent move to a new larger factory would limit the resources available for the new engine project.
Vauxhall At the 1967 Earl's Court Motorshow Vauxhall unveiled their new Victor FD model. The car included an all-new Vauxhall Slant-4 engine that shared many characteristics with the engine Lotus was developing. The Vauxhall engine was an inline 4 cylinder engine with a belt-driven overhead camshaft. The block was slanted at 45° from vertical and a V8 was planned but never realised. Most importantly for Lotus, the bore centres of the Vauxhall slant-4 were the same as those Lotus had determined for the 900 series.
After seeing the new engine at the show Chapman arranged a meeting with John Alden, Vauxhall's Engineering Director, where he negotiated the purchase of ten slant-4 blocks and four complete engines. Lotus would accelerate development of the 900 engine by using the Vauxhall iron blocks as test-beds for their new cylinder head while design of their own engine block was under way. For their test engines Lotus installed a crankshaft with a slightly longer stroke.
After it became known that Lotus was using Vauxhall's iron block in their engine development programme the rumour began to circulate that the Lotus engine was based on the Vauxhall design, even through the 900 series was entirely a Lotus design.
Later Vauxhall used Lotus' cylinder head as a starting point for the design of their own DOHC cylinder head for the slant-4 block. Until that head was available some Vauxhall rally cars used the Lotus cylinder head on the Vauxhall slant-4 block.
In 1974 Scottish Vauxhall dealer SMT, part of Dealer Team Vauxhall DTV aimed to win the Scottish Rally Championship. Lotus Sport supplied heads, created the first hybrid Lotus 907 head on a Vauxhall 2279cc block. This was fitted to a HC Viva Saloon and rebranded a Magnum. Developed and driven by Andrew Cowan who later worked on the Lotus Sunbeam rally development. With valve train issues causing a DNF Bill Blydenstein took over the engine builds and fitted the Lotus 907 head to the DTV Group 2 rally Vauxhall Magnum Coupes.
Vauxhall later went rallying in Group 4 with the Vauxhall Chevette HS. The road-going versions used the Vauxhall DOHC cylinder head but early rally cars used a modified Lotus 907 head on a 2,300 cc block. These early HS models were homologated with the Lotus 907 head in November 1976 before the requisite 400 cars had been built. A rule change in 1978 by FISA made the Lotus-head cars ineligible to compete and Vauxhall switched to their own DOHC head.
Engine development As design of the new engine began, Lotus saw the need for a new sports car engine for endurance races of between . This prompted the company to split the 900 project into two versions; one with smaller ports and a 51° included angle between the valves for touring applications and one with larger ports and a 41° included angle for racing. Tony Rudd, Engineering Director for Lotus, identified the following six engine types created or planned during the early stages of development and production:
Type 904 — Iron block 2 litre racing engine with fuel injection. Type 905 — Iron block 2 litre touring engine. Type 906 — Sand-cast aluminium block 2 litre racing engine with fuel injection. Type 907 — Die cast aluminium 2 litre touring engine. Type 908 — Aluminium 4 litre racing engine. Type 909 — Aluminium 4 litre touring engine.
Early testing of the type 904 engine did not reveal any fundamental deficiencies, although problems with the horizontally mounted distributor vibrating and the timing belt jumping off the inlet cam were identified. The Vauxhall blocks developed cracks around their main bearing bolt bosses, so a special batch were made with thicker castings. This change was applied to all subsequent slant-4 blocks.
The 905 engine experienced wear on the connecting rod against the crankshaft webs, which was solved by boring rather than honing the bushing on the small end of the con-rod so that it held more oil. Vibration problems with the distributor, mounted vertically on the 905, appeared on this engine as well, and it was found that a larger battery was needed to start the larger engine. Mechanical noise from the engine and noise from the air intake were excessive.
Lotus invested £550,000 in a new machining facility for the new engine, and a series of changes were made to the design to adapt it to the numerically controlled milling machines. One change was to split the case along the crankshaft centre-line and incorporate a separate one-piece bearing cap and engine skirt girdle. This change eliminated the need to machine deep main bearing saddles and restored some stiffness to the block assembly. The camshaft housings were kept separate from the cylinder head assembly, which simplified machining operations.
Development of the touring engine diverged into a version for the United States and another for the home market and the rest of the world. The main differences were in the carburettors used and compression ratios. Engines for the US received Zenith-Stromberg carburettors and an 8.4:1 compression ratio due to emissions requirements and California law penalising engines with ratios above 8.5:1. Engines destined for the UK and Europe had Dell'Orto carburettors and a 9.5:1 compression ratio. Pistons for UK and European engines were flat-topped and fly-cut for valve relief, while Federal engines had an additional indentation milled out of the centre of the piston top.
Twelve sand-cast aluminium engines were made and installed in the test fleet vehicles, which were joined by a Bedford CF van that continued to be used for deliveries while testing the engine. The alloy blocks were mechanically noisier than the iron engines. Another issue was that the alloy blocks used wet liners which were poorly supported in the initial design and this caused a high rate of cylinder head joint failures. This was dealt with by increasing the torque loading on the studs and increasing the thickness of the liners. An electrolytic corrosion issue was dealt with by changing to a stainless steel gasket and using a chemically inhibited coolant. The engine tended to show low oil pressure at idle which, while not dangerous, was expected to worry car owners and was solved by installing an oversized oil pump.
While preparing engines for Jensen other problems came to light. Transmission vibration thought to be caused by insufficient beam stiffness in the engine/transmission assembly was addressed by adding two lugs to the lower edge of the crankcase. Two oil-related problems surfaced. The first was oil being introduced into the air box during sustained high-speed cruising. An external oil-separating breather was added until the housing for the rubber-lip crankshaft seal could be re-shaped into an oil-separator chamber, which became the permanent solution. The other problem, which was seen under similar conditions, was that the oil pressure would drop precipitously. This was caused by oil being held up in the cam boxes and not draining back into the sump. This was due to mismatched drain holes, casting flash, and sticking or incorrect relief valves. The short-term solution was to phosphate the camshafts, and the other causes were dealt with as assembly problems.
Jensen-Healey The engine was complete by 1970, but Lotus' existing cars could not be adapted to use the new engine, and Project M50 would not become the Elite Type 75 for several more years. At the same time Norwegian-American businessman Kjell Qvale had taken a controlling interest in Jensen Motors and teamed up with Donald Healey of Austin-Healey fame and his son Geoffrey Healey to design a car to be called the Jensen-Healey, using Vauxhall Viva GT components. They were looking for a suitable engine for the car, having decided that the Vauxhall slant-4 would not be powerful enough after being certified for US emissions.
Chapman approached Qvale and offered to supply Jensen with 60 of the new 900 series engines per week. This initial offer was declined but, after a second offer by Chapman, in October 1971 Jensen announced that they would be using up to 15,000 Lotus 907 engines per year in the Jensen-Healey. The engine would be certified to 1973 Federal standards, but use the European Dell'Orto carburettors and be rated at .
The 900 series engine first appeared in production form as the 907 with the March 1972 debut of the Jensen-Healey. It was the first mass-produced multi-valve engine available to the general public, appearing one year before the 16-valve SOHC Triumph Dolomite Sprint and three years before the 16-valve DOHC Chevrolet Cosworth Vega.
The engine in the Jensen-Healey experienced a series of problems. In addition to a high rate of oil consumption, distorted cylinder liners occurred. An updated Jensen-Healey Mk2 was introduced late in 1973 with a revised engine having a redesigned crankcase, but by this time the car had already acquired a reputation for poor reliability and sales never reached expected levels. Production of the Jensen-Healey ended in 1976, but by this time Lotus was using the type 907 in their own cars.
Chrysler/Talbot In 1977, Desmond "Des" O'Dell, Director of Motorsport for Chrysler UK, approached Chapman about having Lotus supply a 900 series engine for a special project. O'Dell wanted to develop a rally version of the Chrysler Sunbeam to take up the competition role formerly filled by the Chrysler Avenger. It would be competing against the then-dominant Ford Escorts.
The engine Lotus supplied had the same bore as the 907 but at O'Dell's request displacement was increased to by lengthening the stroke to . Tony Rudd developed a flexible flywheel to dampen increased vibrations from the larger size. This engine was called the type 911. In 1978 Chrysler sold their European operations to the PSA Group, so when car the car debuted in 1979 it was called the Talbot Sunbeam-Lotus. Ultimately 2,298 road-car versions were sold.
In the rally car the 911 engine was tuned to be rated at . Driver Henri Toivonen won the RAC Rally in 1980, and teammates Guy Frequelin and Russel Brookes were third and fourth. The next year, the car won the Argentine Rally and placed second at Monte Carlo, Portugal, Corsica, Brazil and San Remo, winning the 1981 Championship of Makes for Talbot and earning Frequelin second place in the Drivers series.