For decades, carmakers have searched for ways to make smaller engines deliver the performance and effortless torque normally associated with larger-displacement powerplants, without sacrificing their potential efficiency advantages. It is a difficult engineering compromise, particularly when extracting more performance traditionally means, before modern hybridization, increasing displacement or adding forced induction.
Mazda approached the problem from an entirely different direction, combining an unusual combustion cycle with a supercharger to overcome the inherent limitations. The result was a remarkably sophisticated engine concept that anticipated the efficiency-focused powertrains now becoming increasingly common, proving that some seemingly unconventional ideas were simply ahead of their time.
Mazda Wanted A Smaller Engine To Behave Like A Bigger One
When Mazda introduced the Millenia in 1994 as a 1995 model, the model was a near-luxury sedan, sitting above the company’s mainstream vehicles. Mazda aimed it at buyers considering entry-level luxury cars such as the Lexus ES 300 and the Acura TL. The car was the product of Mazda’s canceled Amati luxury-car program, giving it ambitions beyond those of a conventional family sedan.
The standard engine was a naturally aspirated 2.5-liter DOHC V6 producing 170 horsepower and 160 pound-feet of torque. The powerplant provided adequate performance, but Mazda had something considerably more ambitious in store for the Millenia S. Its supercharged 2.3-liter V6, which Mazda named the KJ-ZEM, ran on the Miller combustion cycle. It produced 210 hp and 210 lb-ft, despite having 200 cc less displacement than the naturally aspirated version.
The difference was particularly significant in terms of torque. The smaller engine produced 50 lb-ft more, while delivering maximum torque at 3,500 rpm, compared with 4,800 rpm for the naturally aspirated 2.5-liter. In other words, Mazda had created a smaller V6 that could deliver the output and low-speed pulling power expected from a considerably larger engine.
However, outright power was not the only objective. Contemporary descriptions of the engine specifically presented the Miller-cycle design as a way of combining big-engine performance with the fuel economy associated with a smaller engine. The result was therefore not simply a more powerful version of the Millenia’s conventional V6, but an attempt to rethink how a relatively small engine could provide the refinement, response and performance expected in a near-luxury sedan.
The numbers made the idea particularly convincing: less displacement, yet 40 more horsepower and 50 more lb-ft of torque. The interesting question was how Mazda achieved that without simply resorting to a larger engine.
The Miller Cycle Changed What The Supercharger Was Actually Doing
The Miller cycle changes the way a conventional four-stroke petrol engine handles its compression stroke. Instead of closing the intake valve around the point where the piston reaches bottom dead center, the valve remains open as the piston begins moving back upwards. In the KJ-ZEM, the intake valves close 70 degrees after bottom dead center, according to a contemporary technical description.
The benefit is that the engine does not have to compress the full amount of air in the cylinder, compared to the conventional way. By effectively shortening the compression stroke while retaining the full expansion stroke, the engine can reduce pumping losses. In simple terms, less energy is spent compressing the intake charge, while the combustion gases can still expand over the engine’s full power stroke.
However, there is a trade-off. Keeping the intake valves open as the piston begins its upward movement means some of the intake charge that would normally be trapped inside the cylinder can escape back into the intake system. That would normally reduce the amount of air available for combustion and, consequently, limit the engine’s ability to produce power. Mazda’s solution was to use a supercharger specifically to overcome this disadvantage.
Rather than functioning simply as a device for adding power, the Lysholm screw-type compressor supplied pressurized air to the intake system, helping maintain the required charge inside the cylinders despite the unusual valve timing. Mazda had developed the system with IHI, and contemporary US government documentation describes the M-Miller system as using a Lysholm compressor to boost intake pressure.
The relationship between the two technologies is particularly important for this application. The unusual valve timing created the efficiency advantage, but it also created an airflow problem. The supercharger effectively compensated for that problem, allowing Mazda to use the Miller cycle without accepting the corresponding loss in engine performance.
NHTSA later identified the 1995 model-year Millenia S as the first production vehicle offered with a Miller-cycle petrol engine, specifically citing its 2.3-liter KJ-ZEM and crankshaft-driven Lysholm compressor.
The Result Was A 2.3-Liter V6 With Big-Engine Output
The numbers were impressive, but the way the engine delivered them was just as important. The 2.3-liter V6 produced 210 hp at 5,300 rpm and 210 lb-ft of torque at 3,500 rpm, figures that gave it the output expected from a significantly larger naturally-aspirated engine. Ward’s contemporary testing described the torque figure as a particularly useful characteristic, with the peak arriving relatively low in the rev range.
Central to that character was the Lysholm screw-type supercharger, which used an intercooler to force a denser intake charge into the cylinders. This was not simply a case of fitting a supercharger to increase peak output. Its role was closely tied to the Miller-cycle engine’s unusual operating principle, supplying the air needed after the delayed intake-valve closing reduced the charge the engine could otherwise retain. The compressor helped completely fill the cylinders while producing more power than you’d normally expect from such a small engine.
The system also used electronic boost control, allowing the supercharger’s operation to be managed as part of the engine’s broader control strategy. That made the KJ-ZEM more sophisticated than a conventional small V6 with forced induction added for performance. The objective was to preserve the benefits of the Miller cycle while maintaining the airflow necessary for useful power and torque. On the road, the result was an engine whose 210 lb-ft peak at 3,500 rpm gave it a broad, accessible character rather than requiring the driver to chase high engine speeds for its strongest performance.
The engineering was significant enough to earn repeated recognition from WardsAuto. The Miller-cycle V6 appeared among the Ward’s 10 Best Engines from 1995 through 1998, while they later described it as a technically sophisticated example of powertrain engineering.
Mazda Was Quietly Inventing A Formula That Looks Familiar Today
The basic idea behind the Miller cycle did not disappear with the Millenia. Instead, variations of the same approach have become increasingly common in modern petrol engines, particularly in hybrids. Modern Atkinson-cycle and Miller-cycle engines use delayed intake-valve closing to reduce the engine’s effective compression ratio while retaining a longer expansion stroke. This can improve thermal efficiency, although it also reduces the amount of air trapped in the cylinder.
That makes the KJ-ZEM interesting in retrospect. Mazda applied the same basic efficiency principle to a production petrol engine in the mid-1990s, when most mainstream petrol engines still followed the conventional Otto-cycle approach. The company effectively found another way around the compromise created by the Miller cycle. Rather than relying on an electric motor, it used a mechanically driven compressor to supply the intake air that the unusual valve timing would otherwise sacrifice.
The significance is therefore less about the headline power figure and more about the way Mazda combined several pieces of technology into a functioning production powertrain. The supercharger was not simply there to make a small engine powerful; it also made Mazda’s Miller-cycle strategy practical in a production car.
More than 30 years later, that approach looks considerably less strange. Mazda’s experiment offers an early example of an idea that would eventually become an important part of the industry’s pursuit of greater combustion efficiency.
The Millenia S itself has not become an expensive collector car. Those available can be picked up for a few thousand dollars. The Millenia S has clearly remained well below the values of genuinely collectible Japanese performance sedans, reflecting its status as an overlooked near-luxury sedan rather than a sought-after classic.
Sources: Mazda, NHTSA, WardsAuto
