* Engine's specific application: Was it in a Bonneville, Grand Prix, Catalina, etc.? Different models might have slightly different specifications.
* Engine modifications: Has the engine been modified in any way (camshaft, heads, carburetor)? Modifications drastically alter ideal ignition timing.
* Fuel type: The recommended timing can vary slightly depending on whether it's running on regular, mid-grade, or premium fuel.
To find the correct information, you need to:
1. Consult your owner's manual: This is the most reliable source for the original specifications. Look for sections on ignition timing and spark plug specifications.
2. Check a Pontiac service manual: A Haynes or Chilton manual (or a factory service manual if you can find one) for a 1973 Pontiac with a 455 will provide detailed specifications. These manuals often provide different timing settings for different engine conditions (idle, various RPM ranges).
3. Look at the spark plug itself: The plug's heat range and recommended gap should be stamped on the plug itself. This won't tell you the ignition timing but will give you the right gap for the plug type.
General Guidelines (but NOT substitutes for looking up the correct specifications):
* Ignition Timing: You'll likely find a base timing specification (usually measured in degrees BTDC - Before Top Dead Center) at idle. The exact number will vary but is likely in the range of 8-12 degrees BTDC. The owner's manual or service manual will indicate how to set it correctly. Setting the timing incorrectly can cause severe engine damage. A timing light is required for this procedure.
* Spark Plug Gap: A typical gap for a 1973 455 would likely be in the range of 0.035" to 0.045 inches. Again, check the spark plug itself or your service manual for the precise specification.
In short: Do not attempt to set the ignition timing or spark plug gap without consulting the correct service manual or owner's manual for your specific 1973 Pontiac 455 vehicle. Incorrect settings can cause engine damage or poor performance.
