What should the timing be on a small block 350?
For a typical Chevy small-block 350 with a conventional ignition system, aim for an initial timing of roughly 6–12 degrees BTDC and a total timing around 32–36 degrees BTDC. Vacuum advance will add some timing at idle depending on the setup.
The question of timing a Chevy 350 hinges on the engine’s cam, compression, fuel, and ignition system. Getting the numbers right improves idle, throttle response, and power while avoiding detonation. This article explains the basics, common targets, and a step-by-step method to set timing on a classic small-block 350.
Understanding timing and what affects it
Timing refers to when the spark plug fires relative to the piston’s position in the compression stroke. The base or initial timing is the spark at idle, while the total timing accounts for all advances (mechanical/centrifugal and vacuum) as RPM rises. Different engines require different baselines depending on cam profiles, compression ratios, fuel quality, and ignition systems.
Key concepts
BTDC stands for degrees Before Top Dead Center, indicating how far before the piston reaches TDC the spark occurs. Mechanical timing advance comes from the distributor’s weights and springs, while vacuum advance adds extra advance based on engine vacuum, usually at idle or light load. The goal is to optimize efficiency, power, and safety from detonation.
What influences the ideal timing
Cam duration and lift, compression ratio, fuel type (octane), altitude, enrichment/leaning of the mixture, and whether the engine uses a carburetor, EFI, or aftermarket ignition all shape the best timing. A stock small-block 350 often tolerates a different baseline than a heavily cammed or high-compression variant, so follow engine-specific guidance and verify with a timing light on a warmed engine.
Typical timing ranges for common configurations
Below are general targets for common Chevrolet small-block 350 setups. These assume a carbureted engine with a conventional distributor (often HEI) and a functional vacuum advance. If your setup differs, use these as starting points and adjust based on performance and detonation observations.
- Stock cam, carbureted small-block with vacuum-advance: initial timing 6–12 degrees BTDC; total timing 32–36 degrees BTDC. Vacuum advance typically adds ~8–12 degrees at idle, depending on the system.
- Mild performance cam (moderate duration) with carburetor and vacuum advance: initial timing 8–14 degrees BTDC; total timing 34–38 degrees BTDC.
- High-performance cam with heavier overlap or higher compression: initial timing around 4–10 degrees BTDC; total timing 34–40 degrees BTDC. Vacuum advance may be reduced or disabled in some aggressive setups.
- EFI or multi-port injection with ECU-controlled ignition: base timing typically set by the ECU; consult the vehicle’s manual. Common practice is to aim for a conservative initial timing and rely on ECU control for total timing; ranges can vary widely (often 0–12 degrees BTDC base, with ECU determining final timing).
Keep in mind that these are general guidelines. Always verify with a timing light on a warmed engine and adjust to prevent pinging or detonation. Factors such as fuel octane, engine temperature, and altitude can shift the optimal settings.
Step-by-step method to set timing
Before starting, understand the approach: set a baseline initial timing with the vacuum advance disconnected, then verify total timing at higher RPM, and finally reconnect the vacuum advance and confirm the idle timing remains within the target range.
- Gather the right tools: a timing light, the appropriate wrench for the distributor hold-down, and a thermometer or gauge to confirm proper engine temperature.
- Warm the engine to operating temperature (typically around 180–200°F / 82–93°C).
- Disconnect the vacuum line to the distributor (if equipped) and cap the line so vacuum can’t affect the base timing.
- Connect the timing light and loosen the distributor hold-down slightly so you can rotate the distributor without it slipping.
- Rotate the distributor to set the initial timing to your target (start with 8–12 degrees BTDC for a stock engine). Tighten the hold-down once the marks align and the timing light shows the desired initial timing.
- Reconnect the vacuum line to the distributor. If your engine uses vacuum advance, expect some change in idle timing; ensure the idle speed remains acceptable after reconnection.
- To verify total timing, bring the engine speed up to about 2,500–3,000 RPM with the vacuum line disconnected (as per your distributor’s guidance) and observe the timing. Adjust the distributor to achieve the target total timing (commonly 32–36 degrees BTDC for many street setups).
- Recheck idle timing with the engine at idle and the vacuum line reconnected. Make any final minor adjustments to keep idle smooth and free of detonation.
After completing the steps, take the car for a careful test drive to ensure smooth acceleration and no signs of pinging under load. If detonation is observed, back off timing slightly or switch to higher-octane fuel.
Practical tips and caveats
These practical notes help ensure safe and reliable timing adjustments on a Chevy 350:
- Always set timing with a warmed engine and stable idle; timing can shift with temperature and load.
- If using a performance cam, you may need to adjust the initial timing within a broader range. Listen for knock or ping and back off if needed.
- Vacuum advance lines should be checked for leaks and proper routing; faulty vacuum lines can throw off idle timing.
- For EFI and aftermarket ignition systems, follow the manufacturer’s timing guidelines, as ECU control can override simple mechanical timing targets.
When in doubt, consult the service manual for your exact engine and ignition setup, or work with a qualified technician to ensure you don’t risk engine damage.
Summary
For a typical Chevrolet small-block 350, a practical baseline is initial timing around 6–12 degrees BTDC and total timing around 32–36 degrees BTDC, with vacuum advance contributing additional idle timing. Adjustments depend on cam choice, compression, fuel, and ignition system. Always verify with a timing light on a warmed engine, and test under load to prevent detonation. These guidelines offer a solid starting point, but your engine may require slight deviations based on its specific configuration.
