How to Calculate a Camshaft Duration

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Camshaft duration has a meaningful effect on an engine's power-band--the RPM range where it creates its power. Duration is the length of time--in crankshaft degrees--that the inlet or exhaust valve is open. Factory engines have camshaft duration figures that are relatively short compared to performance applications. As a result, they exhibit good low RPM power and reliability. High RPM competition engines, on the other hand, use longer durations that flow more air and fuel into the engine to generate more power. While camshaft manufacturers publish these specifications for their cams, physically measuring duration requires the engine to be partially disassembled.

Things You'll Need

  • Dial indicator gauge and base
  • Camshaft degree wheel and pointer
  • Positive piston stop
  • Harmonic balancer removal and installation tool
  • Crankshaft socket and socket wrench

Locating Top Dead Center (TDC)

  • Remove the harmonic balancer from the front of the engine so that the crankshaft snout is fully accessible. This allows for the attachment of a degree wheel and use of a crank snout socket to turn the engine over. These procedures are not required for an engine build that is not yet installed in a vehicle.

  • Attach the degree wheel to the front of the crankshaft using the balancer bolt to secure it snugly in place. Attach the pointer to a bolt or nut, such as a timing cover or water pump bolt, and position it so that it is pointing to 0 on the degree wheel. Remove the valve cover from the cylinder head for the number 1 cylinder bank then remove the number one spark plug. Insert the positive stop in place of the spark plug.

  • Rotate the engine clockwise with a crankshaft (snout) socket and wrench until it stops turning. Readjust the pointer to 0 on the degree wheel. Rotate the crankshaft in the opposite direction until it stops and note this value. Remove the stop and continue to rotate the crankshaft to a reading precisely halfway between this new value and 0. Re-adjust the pointer to 0 again. This is exact Top Dead Center (TDC).

Determining Duration

  • Place the magnetic base of a dial indicator gauge securely on a flat portion of the cylinder head near the two forward-most valves (number one cylinder) and attach the dial indicator gauge so that it is sitting on the rocker arm directly above the push rod. Set the indicator to 0, make one full revolution of the crankshaft and stop again at TDC. Reset the dial indicator to 0.

  • Rotate the crankshaft until the dial indicator indicates exactly .050 inch tappet lift (also known as lifter rise). Note the reading on the degree wheel. Continue rotating the engine until maximum lift is indicated and stop the rotation at exactly .050 inch tappet lift again then note the reading on the degree wheel. Continue rotating to TDC then stop. Subtract the smaller figure from the larger one to calculate camshaft duration at .050 lift.

  • Reposition the dial indicator to the adjacent rocker arm for the same cylinder and follow the same process as above. Values can also be measured at other lifts, such as .008, .020 and .200 inch lift, to determine camshaft "intensity" as well, but .050 inch lift is the most meaningful and common measurement.

Tips & Warnings

  • It is a good idea to locate both degree values on each side of TDC in each direction of rotation more than once to be certain accurate readings are made (unless a very accurate digital dial indicator is used) to account for possible timing chain slop. If there are discrepancies, multiple readings will yield a more accurate average.

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