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.

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 150 hp (112 kW). 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.

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 2.0 L (1,973 cc; 120.4 cu in) 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 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.