Works for me....
"VALVE TIMING DIAGRAMS"
HOW CAN THEY LIE TO YOU?
What is a valve timing diagram? Simply put, the numbers like INTAKE OPENS at 20 degrees BTDC (Before Top Dead Center) and CLOSES at 60 degrees ABDC (After Bottom Dead Center), EXHAUST OPENS at 60 degrees BBDC (Before Bottom Dead Center) and CLOSES at 20 degrees ATDC (After Top Dead Center).
This timing may also be written as:
20 - 60
60 - 20
Of course, most of the time these four data points are NEVER ACCOMPANIED WITH THE EXACT CAM LIFT THAT THESE NUMBERS WERE TAKEN AT!
Many times the timing diagram fails to specify the total duration of the timing diagram. Let's calculate the total duration of the intake by simply adding together 20 plus 60 and adding the constant 180 which totals 260. The exhaust will calculate as 60+20+180=260. Let's assume these numbers were taken at .050 inches of cam lift.
Now the timing may be written as:
20 - 60 = 260.0 @ .050" cam lift
60 - 20 = 260.0 @ .050" cam lift
Now let's play GAMES WITH THIS TIMING DIAGRAM. If we ADVANCE THE CAM 5 crankshaft degrees the SAME timing diagram becomes:
25 - 55 = 260.0 @ .050" cam lift
65 - 15 = 260.0 @ .050" cam lift
Notice that the left side numbers INCREASE by FIVE and the right side numbers DECREASE by FIVE.
If we RETARD THE CAM 5 crankshaft degrees from the ORIGINAL timing diagram it becomes:
15 - 65 = 260.0 @ .050" cam lift
55 - 25 = 260.0 @ .050" cam lift
Notice that the left side numbers DECREASE by FIVE and the right side numbers INCREASE by FIVE.
Remember, all three of these timing diagrams are from the SAME CAMSHAFT! Only the ADVANCE AND RETARD of the camshaft IN REFERENCE to the CRANKSHAFT has been changed!
Let's play another camshaft "SELLERS" GAME. (Sellers don't make cams, only package other manufacturers camshafts). Using the SAME intake and exhaust lobe specifications, the MAKER of the camshaft CLOSES the LOBE CENTERLINE (SEPARATION) 5 CAM DEGREES and the NEW timing diagram becomes:
25 - 55 = 260.0 @ .050" cam lift
55 - 25 = 260.0 @ .050" cam lift
Notice that each of the "OVERLAP" numbers INCREASE by FIVE crankshaft degrees. Overlap should be called "CROSSOVER AREA" to be correct.
The original 20-60-60-20 camshaft had a LOBE CENTERLINE of 110.0 CAMSHAFT DEGREES.
This "NEW" camshaft has a LOBE CENTERLINE of 105 camshaft degrees.
This CENTERLINE is BUILT INTO THE CAMSHAFT and cannot be changed by the engine builder. But, he may ADVANCE OR RETARD the camshaft in reference to the crankshaft. This WILL NOT CHANGE THE "LOBE CENTERLINE" but will change the MAXIMUM LIFT POINT (sometimes also called CENTERLINE).
If we RETARD this new "TIGHT CENTERLINE" Camshaft 5 crankshaft degrees from the ORIGINAL timing diagram it becomes:
20 - 60 = 260.0 @ .050" cam lift
50 - 30 = 260.0 @ .050" cam lift
Before cam duration can be matched to an engines operating range, the EXACT CAM LIFT must be specified AT WHERE THE DURATION IS COMPUTED.
If a statement is made "this cam has 260 degrees of duration", without adding "260 degrees at .050", the duration number of 260 degrees is TOTALLY USELESS.
That's why knowledgeable engine builders usually select a camshaft based on its duration at .05000" cam lift. In addition to being more accurate than "advertised duration" as an indication of a cams performance potential, duration at .05000" lift applies almost universally, regardless of camshaft make, model or manufacturer.
When comparing camshaft specifications, the best way to cut through the confusion is to focus on duration at .050" lift and lobe separation. These two figures will provide a solid indication of a cams performance characteristics.
"HAPPY CAM ANALYSIS TO YOU"
