Editing Z31/camspecs (section)

=== CENTERLINE ===

The cam's centerline specification is used to tie the valve timing to the crankshaft's rotation. This spec is expressed as the number of degrees the crankshaft must rotate from top dead center until the cam has rotated to the peak (or centerline) of the lobe.

The centerline spec and the duration spec can be used to calculate when the valves open and close in relation to the crankshaft's rotation. When the valves open (or close) relative to the crankshaft's rotation is known as valve events or valve timing. Some cam manufacturers will provide valve event information and others only provide duration and centerline information.

Understanding the effects of valve events or valve timing is the real secret to understanding engine performance. For the engine to run at its peak performance, the valves must open and close at the correct time in relation to the piston's position and the crankshaft's speed.

==== FINDING INTAKE LOBE CENTER ====

Remove ALL lash. With your dial indicator on the retainer or follower, rotate the engine in the direction it would normally turn, and come up to .050 inches of lift. Write down that figure from the degree wheel. This is your opening figure. This is when the intake opens BEFORE Top Dead Center. Example would be 10 degrees on the degree wheel BTDC.

Now go over the top on the lobe until your indicator is .050 inches off the Base Circle. Now you should be where the intake closes AFTER Bottom Dead Center. Keep in mind to continue turning the engine in the same direction it runs and DO NOT BACK UP. Example would be 39 degrees on the degree wheel ABDC.

You can now calculate your duration. The valve opens at 10 degrees, plus it closes 39 degrees, plus 180 degrees (the distance in degrees between TDC and BDC). Your duration at .050 inches of lift would be 229 degrees.

You can now calculate your lobe centerline. Divide your total duration by 2 and subtract your intake-opening figure. This would normally be the smaller number of the two figures.

104.5° would be your lobe centerline.

'''Please Note:''' If you have a low overlap cam, the intake opening may be AFTER TDC, if so, you will have to SUBTRACT that figure from the closing number and add 180. This will be the duration at .050 inches of lift.

You can now calculate your lobe centerline. Divide your total duration by 2 and add your intake opening figure. This would normally be the smaller number of the two figures.

114.5° would be your lobe centerline.

==== FINDING EXHAUST LOBE CENTER ====

Once again remove ALL lash. With your dial indicator on the retainer or follower, rotate the engine in the direction it would normally turn, and come up to .050 inches of lift. Write down that figure from the degree wheel. This is your opening figure. This is when the exhaust opens BEFORE Bottom Dead Center. Example would be 44 degrees on the degree wheel BBDC.

Now go over the top on the lobe until your indicator is .050 inches off the Base Circle. Now you should be where the exhaust closes AFTER Top Dead Center. Keep in mind to continue turning the engine in the same direction it runs and DO NOT BACK UP. Example would be 8 degrees on the degree wheel ATDC.

You can now calculate your duration. The valve opens at 44 degrees, plus it closes 8 degrees, plus 180 degrees (the distance in degrees between TDC and BDC). Your duration at .050 inches of lift would be 232 degrees.

You can now calculate your lobe centerline. Divide your total duration by 2 and subtract your exhaust closing figure. This would normally be the smaller number of the two figures.

'''Please Note: '''If you have a low overlap cam, the exhaust closing may be BEFORE TDC, if so, you will have to SUBTRACT that figure from the closing number and add 180. This will be the duration at .050 inches of lift.

You can now calculate your lobe centerline. Divide your total duration by 2 and add your exhaust closing figure. This would normally be the smaller number of the two figures.