Why Some 4-Cylinder Engines Use 8 Spark Plugs

In most cases, the number of spark plugs in a gasoline engine is typically represented by the number of its cylinders. For instance, it's perfectly normal for a V8 engine to have eight spark plugs, a V12 to have 12 plugs, and a four-cylinder engine to have four. But in some cases, engines have double the number of plugs in each cylinder. The first to do so was the 1914 Alfa 40/60 Grand Prix; its engine was so revolutionary that it made waves before the automaker even became known as Alfa Romeo.

Indeed, the company and its proprietary Twin-Spark technology are intertwined like bees to honey. The beating heart of the Alfa 40/60 Grand Prix is a 4.5-liter inline four-cylinder dual-ignition engine with two spark plugs per cylinder, which brings the grand total to eight. With eight spark plugs, the engine should produce more power, right?

Well, that 4.5-liter four-banger only produces 39 horsepower and 44 pound-feet of torque. It's not much by 2025 standards, but it was plentiful in 1914. They can indeed help improve performance, but dual-ignition engines are more about efficiency, fuel economy, and emissions.

Alfa Romeo isn't alone in the dual-ignition game. Japanese automaker Nissan used a specialized dual-ignition system known as NAPS as far back as the 1980s, while Ford and its 2.3-liter Lima engine combined a single overhead camshaft design with two spark plugs per cylinder. Other big names — like Mazda, Mercedes-Benz, Chevy, and Dodge – also dabbled with dual-ignition engines.

Meanwhile, Honda unveiled its next-gen 1.3-liter i-DSI four-cylinder engine in 2001, and it eventually made its way to models like the 2003 Civic Hybrid. While some twin-spark designs engage the spark plugs simultaneously, Honda's intelligent dual sequential ignition (i-DSI) engine has two spark plugs per cylinder that fire in different orders depending on engine load and engine rpm.

Furthermore, the spark plugs are placed diagonally in the combustion chamber to combust the air-fuel mixture more quickly, enabling Honda to make the most of every drop of gas. The design enables the engine to achieve a higher compression ratio and more intense combustion at any rpm while minimizing engine knock. Honda's 1.3-liter i-DSI engine produces around 85 horsepower, which is more than the vintage Alfa, but its highlight comes from the fuel economy. Combined with Honda's Multimatic S continuously variable transmission (CVT), the i-DSI engine could achieve 54 mpg without spewing many harmful emissions.

Besides potentially having more power while remaining fuel efficient, four-cylinder engines with eight spark plugs are perceived to be more reliable due to their redundant design. If one plug fails, the engine will keep running. It's why some aircraft engines have two plugs per cylinder as an added safety net when either one fails mid-flight.

As for the cons of having double the number of spark plugs, the most glaring downside is the cost of maintenance. Spark plugs come in many types, and some iridium plugs would set you back around $8-$15 each. For a conventional four-cylinder engine, it could cost around $60 to replace all four plugs. But if you have a dual-ignition engine, the cost would be doubled, potentially putting it over $100.

The price is not too much of an issue if you're willing to replace the plugs yourself, but you can expect to pay almost double the labor in a professional shop for dual-ignition systems. Moreover, eight spark plugs means eight coil packs, and replacing all of them won't be cheap. For reference, you can pay anywhere from $35 to $300 per coil, and that doesn't include labor.