Leading vs. Trailing Spark Plugs: Why Rotary Engines Need Two Types
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
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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.
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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.
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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 |
|
| Mazda 20B | 20 | 18,000 |
|
| Mazda 13B-REW | 24 | 22,000 |
|
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.
Can I use the same spark plug for both positions?
Technically you can, but you’ll likely see fouling on one side and overheating on the other, which reduces mileage and may damage the engine.
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How often should I replace the spark plugs on a rotary?
Most experts recommend checking them every 20,000 miles and replacing them around the 40,000‑mile mark, or sooner if you notice mis‑fires.
Got a rotary story or a plug question? Drop a comment below – I love hearing how fellow enthusiasts keep their rotaries humming.
Source: Read Official News
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