* Compression Ratio: A .060" overbore increases compression slightly. You need to know the *exact* final compression ratio to select the proper heat range. Higher compression generally requires a colder plug.
* Cam Profile: A performance camshaft will affect combustion chamber temperatures and thus plug heat range needs. A more aggressive cam might require a colder plug.
* Ignition System: The type of ignition system (points, electronic ignition, etc.) and its tuning also impact plug selection. A more powerful ignition system might necessitate a different heat range.
* Fuel Type: The octane rating of the fuel used influences combustion temperature.
Instead of recommending a specific plug number, I'll guide you on how to choose one:
1. Determine your compression ratio: This is crucial. A machine shop that did the overbore should have this information. If not, you'll need to calculate it.
2. Consult a spark plug manufacturer's cross-reference chart: ACDelco, Autolite, Champion, and NGK all offer extensive charts. These charts typically list heat ranges and recommend plugs based on engine displacement, compression ratio, and application.
3. Look for plugs with a "reach" appropriate for your engine: This refers to the length of the plug's threads. A longer reach might be needed if you've done head work that changes the spark plug well depth.
4. Consider heat range: Heat range is a measure of the plug's ability to dissipate heat. A colder plug dissipates heat more effectively, preventing pre-ignition in high-compression or high-performance engines. A hotter plug retains heat better, ideal for lower-compression engines or those that run at lower RPMs. Start with a recommendation from the chart and may need to experiment slightly, observing plug appearance after some driving. A proper running plug will show a light brown to tan color on the insulator. Too light (white) is too hot; too dark (black) is too cold.
5. Consider the electrode material: Standard plugs generally use copper, while performance plugs might use platinum, iridium, or other materials for longer life and improved ignition. This often comes at a higher cost.
In short: Don't rely on a simple plug number recommendation without knowing your specific engine characteristics. Use the steps above to find the best plug for your modified 454. A reputable parts store or performance shop can also provide assistance with plug selection if you supply them with the necessary engine information.
