I'm wondering how you technically measure cams to get both the lift and the duration??

 

Are you asking what the specs mean, or how the specs are actually measured?

 

Usually you remove the cams from the car and put them on a stand, then you use a micrometer to measure the base circle and the lift -- basically the smallest diameter and the largest diameter of the cam profile.

This is a nontrivial amount of work.

 

everything you ever wanted to know about cams

http://auto.howstuffworks.com/camshaft.htm

 

Yes, how they are phisically measured. dewey seemed to answer the question, and I still have to check carguycw link, thanks guys.

 

lift is the height of the lobe on the camshaft

duration is the "fatness" or width of the lobe that determines how long the valve is open.

 

After you determine the base circle and max lift, using an index to measure cam rotation you measure the lift and degrees from the centerline of the lobe. Duration is usually specified at 0.010" or 0.050" lift from base circle.

Some quality shops and most quality cam grinders have a profilometer that does this automatically and stores the datafile on a computer.

 

just read something that says "when comparing camshaft profiles, always reference the duration at .050" valve lift."....... looks like in metric system, they use 1mm instead.

so it could be misleading to calculate the duration simply by looking at the cam degrees at opening and closing.

 

You are correct that when using metric measurements 1 mm lift is usually used to define duration.

How do you consistently define valve opening? Is the valve considered opened at 0.001", 0.010", 0.020" or 0.050" lift. Compounding this question, pushrod engines (and some OHC with cam followers) have rocker ratios other than 1:1. If you change the rocker ratio of SB Chevy from 1.6 to 1 to 1.7 to 1 you increae the max lift and change the point where the valve is 0.050" off the seat.

The best and most consistent method of specifying a cam is to give duration at 0.010" and 0.050" above the base circle and to give max. lift. It is better if you have the lift across the entir cam profile in 0.100, 0.050 or smaller increments, but this information is not always available.