Leading vs. Trailing Spark Plugs: Why Rotary Engines Need Two Types

Image: Leading vs. Trailing Spark Plugs: Why Rotary Engines Need Two Types – Performance Comparison and Specifications

Bottom line: Rotary engines use separate leading and trailing spark plugs because each side of the combustion chamber sees a different air‑fuel mixture, and the right plug heat range keeps the engine clean and reliable.

Design & Looks

Unlike a piston engine, a rotary’s housing spins in a figure‑8 shape. The spark plugs sit at opposite ends of the same chamber – one at the “leading” edge where the fresh mixture first enters, and the other at the “trailing” edge where the burnt gases exit.

The leading plug sees a cooler, richer mixture, while the trailing plug fires into a hotter, leaner zone. This visual quirk means the plugs look identical, but they behave very differently under heat.

Performance & Mileage

Choosing the wrong plug for either position can cause mis‑fires, fouling, or premature wear. A hotter plug on the leading side will over‑heat and melt the electrode, while a colder plug on the trailing side can’t burn off carbon, leading to deposits that choke power.

When tuned correctly, the engine runs smoother, reaches its advertised mileage (often 20‑25 mpg for a 13B‑renault), and avoids costly repairs.

Price & Rivals

Rotary engines are a niche market. The extra cost of buying two plug types is small—usually $5‑$10 per plug—but it adds up compared to a single‑plug V‑engine. Competing sports cars with piston engines don’t have this complication, which is why many buyers overlook the rotary’s unique needs.

Typical plug price: $5‑$10 each
Overall rotary maintenance: higher than a comparable piston engine
Key advantage: high rev range and smooth power delivery

Engine	Mileage (mpg)	Price (USD)	Top Features
Mazda 13B	22	15,000	Compact size High rev limit Unique sound
Mazda 20B	20	18,000	More torque Turbo‑ready Durable internals
Mazda 13B-REW	24	22,000	Twin‑turbo Racing pedigree Improved cooling

FAQ

What is the difference between leading and trailing spark plugs?

The leading plug fires into a cooler, richer mix, so it needs a colder heat range. The trailing plug fires into a hotter, leaner mix and requires a hotter plug to stay clean.