Lotus 907 / 910

Lotus's own four — the slant four, 907 through 912 — in the Esprit, Elite, Eclat and Excel, and in the Jensen-Healey.

⚑ It is one of very few engines a small manufacturer designed entirely alone, and it shows in both directions: an all-aluminium sixteen-valve twin-cam in 1972, years ahead of the industry, and tolerances and a parts supply that never had volume behind them.

It is canted over at 45 degrees to fit under a low bonnet, which is where the name comes from and why access to anything on the exhaust side is awkward.

Cooling is the thing to respect. These engines run hot in mid-engined cars with long coolant runs, and an all-aluminium block with wet liners does not forgive an overheat — the failure is the head gasket, then the liners.

The 910 in the Esprit Turbo is boosted without an intercooler on the early cars, so intake temperature is a real variable, and timing belt intervals on this family are short and taken seriously by everyone who runs one.

Parts and specialist knowledge are the constraint, not the engineering — and both are concentrated in a handful of specialists.