Leiden vs. Trailing: why rotary engines need two different types of spark plugs – Jalopnik

Perfectly tuning the heat range of an engine’s spark plugs is a dark art for all but the most experienced tuners and the technical eggheads at automotive OEMs. However, properly adjusting the spark plug in a rotary engine application is a whole other level of nerdy. Although it’s probably a bit redundant to say that, because rotary engines – and the enthusiasts who love them –are naturally nerdy. They give off the same vibe as people who insist on using a Linux operating system in the great 2026. Forever chided by piston pumpers that their cars are powered by so-called magically spinning Doritos, the rotor heads must remain steadfast in their dedication to Felix Wankel’s fantastic, but often misunderstood, engine layout. The first thing you need to know about rotary engines is that they hate running in a lean combustion mode, so avoid that at all costs. The second thing you need to know is how to properly adjust the front and rear spark plugs on each rotor.

To understand the value of the right spark plugs in a rotary engine, you first need to understand a little about how a rotary engine works in the first place. A Reuleaux triangular “rotor” rotates about an eccentric axis within the inner shell of a bilobed peritrochoid case. Each of the three corners of the rotor slides against the surface of the housing, creating three continuous stages of intake, compression, combustion and exhaust at all times. It’s as clear as mud, I know.

To put all this more simply, the front plug ignites about 95% of the air/fuel mixture, while the rear plug ignites about 10 degrees later in the rotor arc, consuming the remaining 5% of the unused hydrocarbons. There’s no hard and fast rule for this, but some Mazda nuts claim that about three to five percent of a rotary engine’s power comes from the addition of a tow plug. Trailing plugs are so important to extracting those last few horsepower that the company added a third spark plug, a “late trailing plug” to the R26B engine in its famous 787B Le Mans-winning prototype race car.

Do you already understand how it works? No? Okay, just watch this video of Mad Mike’s ridiculous “Mad Mac” rotary-powered McLaren and it won’t really matter anymore. Just listen to that ripper singing.