Effects of Retarding the Ignition Timing on Horsepower

Changing your ignition timing can have far-reaching effects, so it's important to understand the process. When a piston reaches top dead center (TDC), it's at the end of its stroke and ready to go back down the cylinder bore. Retarded timing fires the spark plugs as the pistons pass TDC and are ready to begin their trip down the cylinder. If the timing is retarded too much, the pistons will be on their way down before the plugs fire. This will reduce the energy that combustion is able to apply to the piston and slow its downward motion. The result is incomplete combustion since the spark happens too late in the cycle, lost power, increased emissions because of the weak combustion and more wear to the cylinder bore.

A normally aspirated engine doesn't use a compressor or nitrous oxide to enhance combustion. Since the fuel-air mixture in its combustion chamber is less dense, it takes longer to ignite. Vehicle manufacturers typically set the timing at a default that will lower engine temperatures and reduce emissions. Advancing this setting increases the engine's horsepower by firing the plugs earlier in the compression cycle. If your plugs fire too early in this cycle, you get so much pressure and heat in the cylinder that the fuel-air mixture can auto-ignite and damage your engine. Turbocharged, supercharged and nitrous-injected engines are already running higher pressures and temperatures than normally aspirated engines, so the consequences of advancing your timing can be more severe. Retarding the timing on these engines compensates for the denser fuel-air mixtures by firing the plugs later in the compression cycle. This prevents premature combustion and allows the engine to run at peak performance without destroying itself in the process.

If you ever tried to kick start an old Harley, you know that the kickback can throw you over the handle bars. This happens when the plugs fire midstroke, so experienced riders retard the timing to minimize kickback. Kickback can happen with any gas engine, and in a vehicle, it can damage the starter. It's also easier to start a vehicle with retarded timing since temperatures and pressures have had more time to build in the combustion chamber so they'll ignite quicker. The downside is that retarded timing lowers engine speed, possibly causing a rougher idle. As you accelerate to cruising speed, the pistons in your engine will naturally move faster. This really robs your power if the timing is retarded since increasing piston speeds reduce the time needed for combustion and your plugs need to be firing earlier not later. That's why you need to adjust your timing to balance your startup and idle performance with the responsiveness you want from your engine at cruising speeds.