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Why Your Engine Has Multiple Timing Marks

Crankshaft

If you've ever pulled the timing belt cover off the front of your engine, you've likely noticed that there are several timing marks on the crankshaft and (usually) the camshaft as well. Ever wonder what all of those marks are for? Most would assume that the top-dead-center (TDC) mark is all that really matters.

In most cases, the TDC mark is the one that will matter the most to you. When you change the timing belt or otherwise work with the engine's timing during maintenance or repair, TDC is where you'll focus your efforts in lining things up to get them put back together. The other marks, however, are important for a few reasons.

Engine Marks, Above and Below TDC

Most engines have two or three marks. These will be on the engine block and will coincide with a mark (usually a hash or line) on the inner crankshaft pulley and, often, on one or more camshaft pulleys. Timing is set when the mark on the pulley and the engine block match. On many vehicles, the goal is to set the crankshaft and camshaft to TDC simultaneously to finalize the installation of the timing belt or chain.

Normally, the TDC mark is labeled as either "TDC" or "0" and it's usually the central mark on the lineup. Marks to one side or the other denote "off-times." The below top dead center (BTDC) mark will be on the left side and above top dead center (ATDC) to the right. Other marks added to the left or right will be designated as Below-Below TDC and Above-Above TDC respectively. They usually have numbers on them which will be how they're designated (e.g. "3 below TDC").

Each of these marks designates a degree of departure from top dead center. Normally, the number given to them denotes that degree of departure. So the 3 given to the mark just below TDC is actually three degrees of departure from TDC.

Why Move Timing?

Moving timing from one mark to another, which usually means both the camshaft and crankshaft are moved identically, changes how the engine operates and where it's peak outputs and fuel efficiency points will be.

If you envision an engine cranking, with the crankshaft turning, lifting and lowering pistons in sequence as the belt or chain from its pulley turns the camshaft, which in turn opens and closes valves, you can see how this would affect things. Moving the timing like this affects the position of the piston when the valves to inject fuel and air open.

The lower the piston, the larger the cylinder volume when air and fuel are injected. When ignition happens, there is less piston travel for the combustion to push, meaning there is less energy being given to the crankshaft from each cylinder ignition. This changes how much power is being produced for the amount of fuel being used and it affects when the power translates to the crankshaft during the piston's travel. All of this affects torque and horsepower curves at different RPM levels.

Tuners can use this to tune an engine to a specific type of output - say faster off-the-line performance or higher top speeds. It also changes when fuel economy is at maximum in terms of RPM rates and vehicle speeds. In short, efficiency is sacrificed at one end of the spectrum in order to gain some at the other. Normally, TDC is the best middle ground for the vehicle and its engine in terms of overall fuel efficiency.

Now you know why there are timing marks other than TDC on your engine.

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