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Old 03-30-2006, 06:17 PM
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cam info

can you guys tell me what these camshaft numbers mean, espesally the deg lobe cen. thanks its going in a 357 chevy.

@0.050 lift=230-230, advertised duration=284 284

gross valve lift=.452 .452, deg.lobe cen=114

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Old 03-30-2006, 11:23 PM
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Yeah, I'll take a shot at it.

Cam manufacturers have used all sorts of clearances through the years to compare one cam to another. The standard used to be SAE J604d, which used, I think, 0.004" intake clearance and 0.006" exhaust clearance. In other words, the actual duration of the particular lobe wasn't figured until the clearance has been taken up with a dial indicator. If you put the indicator on the heel of the intake lobe (base circle part of the lobe) and turned the cam so that you had a lift of 0.004", that would be your zero point to begin measuring duration from and you would continue to add duration to that lobe until you got back around to the closing ramp and found 0.004" again on the backside of the lobe.

This worked fine until all sorts of hot rod grinders began to enter the marketplace and used what they considered to be the correct clearance. Some used 0.010", some used 0.020", some used 0.060", etc., etc. Somewhere along the line, all the grinders got their heads together and agreed to publish a duration at 0.050" lift in addition to still using their "short lift " figures, now called "advertised duration". These can be figured at any lift the manufacturer wants to use, so the only scientific way to compare cam lobe design is by the lift @ 0.050" figure. You can tell how agressive a lobe is by subtracting the 0.050" duration figure from the advertised duration figure and dividing by 2. This will give you the clearance ramp duration on each side of the lobe, opening and closing. A lobe which has a small difference between the 0.050" figure and the advertised figure will be a very agressive lobe design. A lobe which has a large difference between the 0.050" figure and the advertised figure will be a relatively "lazy" lobe design. Now, before you condemn the lazy design, consider the beating the cam lobe takes in jerking the lifter from a standstill to max lift and think about how much easier the lobe would have it if it could take a little extra time to do it. Making max horsepower is nice, but you also have to consider the hardware and try to make it live to make horsepower.

Valve lift is the total movement of the valve from its seat to maximum lift. It is the measurement of the eccentric of the cam lobe multiplied by the rocker arm ratio. If, for instance, a cam has an eccentric measurement of 0.300" (base circle is 1.000", measurement from heel to nose of the lobe is 1.300") and the rocker arm ratio is 1.5, then 0.300" times 1.5 = 0.450" valve lift.

Lobe center is the amount of camshaft degrees between the point of maximum lift on the intake lobe and the point of maximum lift on the exhaust side of a given pair of cam lobes. As an example, lobe center is calculated for cylinder number one only and does not deviate between number one and the other cylinders in the powerplant. To make a representative sketch of lobe center, draw a line between the very center of a camshaft intake lobe and then draw a similar line through the respective exhaust lobe for the same cylinder. The number of camshaft degrees between these two imaginary lines is the camshaft lobe center.

Also called lobe displacement angle (LDA), the lobe center has a definite bearing on how a particular camshaft will operate in a specific powerplant. If the lobe center of the camshaft is increased, the valve overlap will be decreased. The overlap decrease is created because the exhaust timing will occur earlier and the intake timing event will occur later in relation to crankshaft position. Conversely, if the lobe center or displacement angle is decreased, overlap increases. Note that lobe center cannot be changed once a camshaft has been ground.

The intake centerline and the exhaust centerline are added together and divided by 2 to arrive at the LDA. A narrow LDA such as 104 will result in low manifold vacuum and a choppy, erratic idle. The motor will make good low end power but will max early in the rpm range. A wide LDA such as 114 will give excellent manifold vacuum and tend to give added power in the upper rpm range.

Now that I've gone this far with explaining the workings of a cam, you should know that in order to properly mate a cam to a motor, you must know the static compression ratio of the motor. The reason is that the most important point on any cam is the intake valve closing point. If you have a cam that closes the intake valve early, then you must use a relatively low static compression ratio on the motor to prevent building excessive cylinder pressure. If you use a cam that closes the intake valve later, then you must use a relatively high static compression ratio to allow sufficient cylinder pressure to make power.

Crane Cams has a very good explanation of the mating of cams to compression ratios. I recommend studying some of their grinds and learning how the compression ratio works with the intake closing point.

Any other takers????

Last edited by techinspector1; 03-30-2006 at 11:34 PM.
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