I've been trying to figure this out, but still don't clue. How is "advertised duration" measured? I've been trying to compare some import cams to my LE custom grind, such as the famous HKS "272/272" cam which doesn't even tell you the duration at .050 lift. I understand how duration is measured from .050 inch lift, which makes sense, but don't fully understand what advertised duration can be measured. Also was trying to figure out why these imports have barely any lift on their cams, what would be the purpose of running a lot of duration with only .400 lift? I've been looking at cams for my SI (daily driver) and the cam I am looking at is around 250/252 duration @ .050 lift, but still only has .510/.470 lift even with that much duration, I was trying to figure out why that wouldn't be a higher number.

 

ttt

 

Manufacturers are free to choose any "standard" for the lift point at which they measure duration of their cams. Some may use 0.015, some will use 0.020 or 0.030. The use of 0.050 is an attempt to come up with a uniform measurement system, so you can make some rudimentary comparisons of duration.

 

"Advertised Duration" is usually pretty close to the total duration of the lobe. Because the initial part of the ramp is so gentle, it's difficult to tell exactly where the lift starts. Different manufacturers rate it at different points. Comp uses .006 lobe lift, which is .009 - .011 valve lift depending on the rocker ratio. Some other manufacturers use .002 lobe lift for "advertised".

By the time the lifter has moved .050 the opening ramp is past, so you can get a better idea of how various lobes compare using .050 lobe lift. Some folks use .020, but that's not as common, as Fred just mentiioned.

An advertised duration of 270 is usually fairly close to 220-225 degrees @ .050.

Smaller valves don't need to lift as far to get their max flow as larger valves. 25% of valve diameter is a good starting point. That's lift/diameter or L/D often called "L over D"

If you look at the L/D ratio of a .400 lift on a 1.5 inch valve compared to a 2.0 inch valve, you'd lift the 2.0 valve .533 to have the same ratio.

 

Plus the fact that the vast majority of these import engimes have two intake valse per cylinder. With two intake valves per cylinder you don't need as much lift to get the air into the cylinder.