Ignition Timing For Better Fuel Economy (part 1) [Archive] - El Camino Central Forum : Chevrolet El Camino Forums

: Ignition Timing For Better Fuel Economy (part 1)

01-31-2009, 07:59 PM
Ignition Timing For Your Hot Rod for Better Fuel Economy

Finding Fuel Efficiencies and Peak Power


Whether you're tuning for power or fuel efficiency, igniting the air/fuel mixture at the correct time is critical. A good timing light is your first step to getting your engine where it needs to be.
Improving fuel economy or gaining horsepower means making the engine more efficient. Fuel economy or maximum power is only obtainable when the correct air/fuel mixture or ratio (a richer mixture for power and a leaner mixture for economy) is ignited by the ignition system at the correct time. In this way, all the energy of the richer air/fuel mixture is converted to power; similarly, the leaner mixture gains fuel economy because its charge is burned completely.A high-performance engine will only perform at its best when the initial timing and the ignition advance curve are tailored to suit the engine, the fuel, the owner's driving style, usage, etc. High-performance carburetors, intake manifolds, cylinder heads, camshafts, and other tuning components are all dependent upon correct ignition timing; if the spark is not delivered at the proper time to the combustion chamber, the quest for power or economy is for nothing.
Optimum air/fuel ratios are usually calculated by an engine dynamometer. These ratios vary not only when tuning for power or economy, but also from engine to engine, load conditions, altitude, etc. For unleaded fuel at sea level, the stoichiometric value (the ideal ratio of air and fuel that is required to provide the complete burn) is 14.7:1; that is, 14.7 lbs of air to 1 lb of fuel. However, because of operating losses in the induction system due to intake runner and cylinder wall wetting, a more realistic air/fuel ratio for maximum horsepower will be a richer 12.2:1 to 13.5:1 and leaner for maximum economy. Rich mixtures are safer due to their cooler charge and slightly longer burn time. Once established, an air/fuel ratio meter, like the Innovate Motorsports LM1 Air/Fuel Ratio Meter, could help monitor the figures.

Advance curves can be tuned with bushings and springs. The bushings limit the amount of ignition advance, and the springs control the rate of advance.
Igniting the correct air/fuel ratio at the correct time is the responsibility of the ignition system. Ignition timing consists chiefly of three parts: initial timing, timing advance or curve, and total timing. Checking and adjusting these elements of timing for maximum power and efficiency are fairly easy and inexpensive. It's also satisfying to uncover untapped power in a performance engine, knowing that those gains can improve the performance of other components. Certainly, a carburetor cannot perform to its optimum unless the ignition timing is correct. The basic rule of carburetor tuning is ignition first. Once the ignition advancing mechanism is correct, the air/fuel mixture can be tuned for improved power or fuel efficiency.
The chief function of initial timing is to provide a clean idle and crisp throttle response. One of the best guides to determine the initial ignition timing of V-8 engines can be found in the Barry Grant Inc. catalog or on their Web site under the Demon Carburetor Guide. Typically, they recommend 10 to 12 degrees of initial timing when the duration of the camshaft is less than 220 degrees @ 0.050-inch of valve lift; 14 to 16 degrees of initial timing with a camshaft duration of less than 240 degrees @ 0.050-inch; and 18 to 20 degrees of initial timing when the camshaft duration is less than 260 degrees @ 0.050-inch of valve lift. To check initial timing, clean the TDC (top dead center) line or indentation on the harmonic balancer at the front of the engine and, if necessary, identify it with chalk or crayon to make it more visible. Run the engine at idle, aim a timing light at the harmonic balancer, and note the initial timing of ignition to the number-one spark plug. To adjust the initial timing, slightly loosen the bolt that secures the distributor to the engine, and slowly rotate the distributor body until the initial timing agrees with the appropriate figures mentioned above. Retighten the distributor to the engine. To verify that the harmonic balancer and the pointer are correctly aligned on a new or rebuilt engine, bring the number-one piston to TDC on the compression stroke and confirm that the indentation on the harmonic balancer and the pointer are in alignment on zero degrees.

Although not as commonly used today as it once was, a distributor tester is a valuable resource to identify the amount of mechanical and vacuum advance as well as the rate at which it occurs.