The Willys-Knight Model 70A is a fascinating example of an idea that once promised to rival the conventional petrol engine.

Built by Willys-Overland in Toledo, Ohio, this 1927 model used a six-cylinder Knight sleeve-valve engine rather than the poppet valves found in most other cars. The result was an engine renowned for smoothness and quiet operation, and one that gave Willys-Knight a distinctive identity in the American market.

What Is a Sleeve-Valve Engine?

The most unusual feature of this car is hidden inside its engine.

Instead of using conventional inlet and exhaust valves operated by a camshaft and valve gear, the Knight system used thin cylindrical sleeves surrounding each piston. These sleeves moved up and down inside the cylinder and opened or closed ports in the cylinder wall at the correct point in the combustion cycle.

This meant the engine could breathe without the usual poppet valves, springs and rocker gear.

The system was mechanically complex, but it produced exceptionally smooth and quiet running, qualities that were highly valued in luxury and upper-medium cars of the period.

What You Can See Under the Bonnet

The engine bay of this car gives a good clue to its unusual internal design.

Six spark-plug leads can be seen along the top of the engine, confirming its inline six-cylinder layout. What is less obvious is what is missing. There is no exposed rocker gear or conventional overhead-valve mechanism because the valve action takes place internally through the moving sleeves.

That gives the engine a cleaner and more enclosed appearance than many contemporary designs.

The Model 70A Engine

The Model 70A was powered by a 2,915 cc six-cylinder sleeve-valve engine producing approximately 53 horsepower.

For a medium-priced American car of the late 1920s, that gave respectable performance while retaining the refinement that Willys-Knight promoted so heavily.

Power was sent through a three-speed manual gearbox, giving the car a conventional driving layout despite the unusual engine design.

Why Knight Engines Were Different

The Knight sleeve-valve system had several advantages over early poppet-valve engines.

Because the ports opened and closed through the movement of sleeves rather than valves being repeatedly lifted and slammed back onto their seats, the engine could operate very quietly.

The design also allowed relatively large gas-flow openings, which could help breathing and smooth power delivery.

However, the system demanded precise manufacturing and careful lubrication. As conventional poppet-valve engines improved, they became quieter, more reliable and cheaper to produce, gradually eroding the advantages of the sleeve-valve design.

A More Refined Willys

Willys-Knight was positioned above the cheapest cars in the Willys-Overland range.

The Model 70A Special Six offered buyers a combination of refinement, unusual engineering and respectable performance without moving into the highest-priced luxury-car market.

Its 113¼-inch wheelbase gave it substantial proportions, and the model was available in several different body styles.

For buyers in 1927, the appeal was not simply power. The smoothness and quietness of the Knight engine were major selling points.

Nearly Half a Million Willys-Knights

Willys-Overland used Knight sleeve-valve engines for many years and built hundreds of thousands of Willys-Knight cars before production ended in the early 1930s.

The system had once seemed like a serious alternative to conventional valve design, and several respected manufacturers around the world adopted versions of the Knight principle.

By the end of the 1920s, however, advances in conventional engines were making sleeve-valve systems harder to justify economically.

The End of the Sleeve-Valve Era

Willys-Knight production came to an end in the early 1930s as Willys-Overland faced serious financial difficulties during the Great Depression.

The company increasingly concentrated on simpler and less expensive cars, and the more complicated sleeve-valve models disappeared from the range.

This marked the end of one of the most interesting engineering experiments of the early automobile era.

This Car

Built in 1927, this Model 70A Special Six represents a period when automobile engineers were still exploring very different solutions to the same basic problems.

Its 2.9-litre six-cylinder engine looks relatively conventional from the outside, but internally it operates in a completely different way from most engines of its era.

That contrast is what makes the Willys-Knight so interesting. Beneath the bonnet is an engineering idea that was once praised for its refinement and sophistication, but which ultimately lost out to the simpler poppet-valve engine that became the industry standard.