* Cam Shaft: The camshaft profile significantly influences the optimal timing. A performance cam will require different timing than a stock replacement cam.
* Ignition System: The type of ignition system (points, electronic) and its condition affect timing needs.
* Fuel System: The carburetor or fuel injection system and its tuning will influence the best timing.
* Engine Modifications: Any modifications like headers, intake manifold changes, or compression ratio alterations will change ideal timing.
For a stock, unmodified 1985 Chevy 350 with a stock ignition system:
The base timing is usually around 10-12 degrees BTDC (Before Top Dead Center) at idle. However, this is just a starting point.
Finding the Correct Timing:
You'll need to consult your engine's specific specifications, ideally found in your owner's manual or a repair manual specific to your year and model. Even then, those values are starting points - fine-tuning is often necessary.
The best way to find the optimal timing is through dynamic timing adjustments. This involves:
1. Setting the base timing: Use a timing light to set the base timing to the manufacturer's specification as a starting point.
2. Vacuum Advance: Ensure the vacuum advance mechanism is working correctly. It should add timing at higher RPMs.
3. Tuning: This is where you'll fine-tune the timing for optimal performance and efficiency. This often involves using a vacuum gauge to find the best timing at various RPMs, looking for peak vacuum. Incorrect timing will show up as poor fuel economy, rough idling, and decreased power.
Without a proper repair manual and some mechanical experience, attempting to fine-tune the timing yourself can potentially harm your engine. Consult a qualified mechanic if you are unsure about how to proceed.
