How We Rate Spark Plugs
Disclaimer:
Our rating system is a rough estimation designed to compare spark plug designs relative to each other in terms of performance and reliability. It is not intended as an absolute measure of product quality.
We developed this simplified scoring system to help users navigate large cross-reference lists when choosing a suitable replacement spark plug. In many cases, these lists can be extensive and difficult to evaluate. Our system aims to highlight the most relevant and high-performing options more efficiently.
The ratings use a 1 to 5 scale, with 5 representing the best overall performance based on key design factors such as ignitability, reliability, and durability.
While browsing the cross-reference table, you may notice a small arrow icon — this indicates a direct comparison with the currently selected (active) product.
Materials
Materials play a crucial role in spark plug performance and longevity. The two key characteristics we consider are:
- Conductivity: This refers to the material's ability to allow electricity to flow, which directly impacts spark efficiency and ignition performance.
- Durability: This is the material's resistance to heat and fouling, affecting the spark plug's overall lifespan and reliability.
Below is a table showing the materials used for spark plug electrodes, along with their assigned scores based on these two important parameters.
| Material | Conductivity | Reliability | Overall Score |
|---|---|---|---|
| Iridium | 4 | 5 | 4.5 |
| Platinum | 4 | 5 | 4.5 |
| Ruthenium | 4 | 5 | 4.5 |
| Silver | 5 | 3 | 4 |
| Palladium | 3 | 4 | 3.5 |
| Copper | 5 | 2 | 3.5 |
| Gold | 5 | 2 | 3.5 |
| Tungsten | 2 | 4 | 3.0 |
| Titanium | 2 | 3 | 2.5 |
Note on Electrode Composition
Spark plugs typically have two electrodes: the center and the ground. Since both are essential to spark plug function, we treat them equally in our rating system. The overall material score is calculated as the average of the center and ground electrode materials.
Note on Alloys
Some spark plugs feature composite electrodes made from multiple metals (e.g., Iridium-Platinum). In such cases, we treat each material separately. For example, a spark plug like the Autolite XP63DP2, which has an Iridium-Platinum center electrode and a Platinum ground electrode, is rated based on a combination of Iridium, Platinum, and Platinum, with the scores averaged across all three materials.
Design
New electrode designs are meant for specific engine designs (e.g. rotary) or to improve spark plug lifespan, performance and decrease a chance of misfire.
Each manufacturer uses different names, but core spark plug design principles for performance and reliability are largely the same:
- Reduced Ground Electrode Area: Designs with minimal ground electrode mass—such as taper-cut or cut-back ground electrodes—allow the flame kernel to expand more freely, improving combustion efficiency and throttle response.
- Fine Center Electrode Design: Slim center electrodes (regardless of material) require less voltage for the spark to jump the gap. This increases ignition reliability and reduces the chance of misfires, especially under high load or cold start conditions.
- Deep Spark Positioning: Designs that place the spark deeper into the combustion chamber—such as side discharge or extended reach configurations—bring the ignition point closer to the center of the air-fuel mixture, enhancing burn speed and power output.
- Multi-Ground Electrode Designs: Some plugs feature two, three, or four ground electrodes. This doesn't improve performance directly, but it improves durability and ensures consistent sparking even as one electrode wears, enhancing long-term reliability.
- V-Groove or Notched Electrodes: These designs split the spark path or concentrate the spark energy at a single point, improving ignitability and flame propagation.
- Projected Tip Design: A protruding electrode extends further into the combustion chamber, optimizing the spark location for better ignitability and combustion efficiency, especially in modern, fast-burn engines.
Not all designs are better than conventional. Some designs have limited use cases (e.g. rotary engines) and not exactly an improvement, but designed to operate in specific conditions. Also, some designs (e.g. Star-Shaped) are experimental and not proven to improve performance nor reliability.
Here's a scoring table for each center and ground electrode design:
Applications* - niche use cases different from conventional 4-stroke engine
Ground electrodes
| Design | Ignitability | Durability | Reliability | Score | Applications* |
|---|---|---|---|---|---|
| Fine Wire | 5.0 | 3.5 | 4.0 | 4.2 | |
| U-Groove | 4.5 | 4.0 | 4.0 | 4.2 | |
| Side Discharge | 4.5 | 4.0 | 4.5 | 4.3 | Rotary engines, flame-propagation optimization |
| Slant | 4.0 | 4.0 | 4.0 | 4.0 | |
| Cut Back | 4.0 | 3.5 | 4.0 | 3.8 | |
| Multi-Ground | 3.0 | 5.0 | 5.0 | 4.3 | Fleet, marine, or long-service-interval engines |
| Square | 3.5 | 4.0 | 3.5 | 3.7 | |
| Shield Strap | 3.5 | 4.0 | 3.5 | 3.7 | Engines in high EMI environments (e.g., aircraft) |
| Projected Pad | 3.5 | 4.0 | 3.5 | 3.7 | |
| Surface Discharge | 3.0 | 5.0 | 4.0 | 4.0 | High-compression, forced induction engines |
| Flat | 3.0 | 3.5 | 3.0 | 3.2 | |
| Semi-Surface Discharge | 3.0 | 4.0 | 3.5 | 3.5 | High-RPM, detonation-prone applications |
| Hybrid | 3.0 | 4.0 | 3.5 | 3.5 | Custom/tuned setups with mixed requirements |
| Low-Angled | 3.0 | 3.5 | 3.0 | 3.2 | |
| Bridge | 2.5 | 3.5 | 3.0 | 3.0 | Marine or dual-fuel systems |
| Oversized | 2.0 | 4.0 | 3.0 | 3.0 | Older industrial or agricultural engines |
| Push Wire | 2.5 | 3.5 | 3.0 | 3.0 | Small engines, scooters, or motorcycles |
| Projected Square | 2.5 | 3.5 | 3.0 | 3.0 | Engines requiring deeper spark exposure |
| Standard | 2.0 | 4.0 | 3.0 | 3.0 |
Center electrode design
| Design | Ignitability | Durability | Reliability | Score | Application* |
|---|---|---|---|---|---|
| Fine Wire | 5.0 | 3.5 | 4.0 | 4.2 | |
| V-Cut | 4.5 | 4.0 | 4.0 | 4.2 | |
| Taper Cut | 4.0 | 4.0 | 4.0 | 4.0 | |
| Standard | 3.5 | 4.5 | 4.0 | 4.0 | |
| Necked Down | 3.0 | 4.0 | 3.5 | 3.5 | Rarely used, sometimes in performance-tuned |
| Oversized | 2.5 | 4.5 | 3.5 | 3.5 | Older industrial or low-speed engines |
| Bar | 2.0 | 4.0 | 3.0 | 3.0 | Vintage motorcycles, agricultural applications |
| Star-Shaped | 2.0 | 4.0 | 3.0 | 3.0 | Experimental or multi-spark configurations |
Overall Score
The overall score is derived by averaging the material and design scores. This composite score provides a general evaluation of the spark plug's performance and reliability.
The overall score does not factor in value for money. In some cases, a spark plug with a lower score might offer better value for the price, depending on specific use cases or budget considerations.
Our scoring system is designed to offer a relative and approximate estimate that helps you easily compare products and identify higher-performing options from a long list of cross-referenced spark plugs.