Why is it when we talk about cam duration, we always go by the duration at .050, instead of advertised duration. For example here are two cams, from Crane....

Advertised Duration: 276°/284°
Duration @ .050: 208°/216°
Lobe Separation: 113°
Gross Lift: .530''/.530''
RPM Range: 1500-5500

Advertised Duration: 272°/280°
Duration @ .050: 208°/216°
Lobe Separation: 113°
Gross Lift: .500''/.500''
RPM Range: 1500-5200

The only thing different in these two, is the lift. The duration at .050 is the same. Yet, the advertised is different. Maybe I am old school, but I always knew what someone was talking about when they said they had a 272 or 292 duration cam. All the talk about duration at .050 confuses me. Makes it sound like the cam is itty bitty.

 

.050 duration numbers are used because there is very little head flow below .050 lift on a N/A engine.

 

.050 is used so everyone is on the same page. If you look at Crane, they do their advertized duration at .006, but comp does theirs at .004 (I think). Having everyone compare at .050 is to make the numbers consistent. But you are right, the .050 number doesnt give you the complete picture.

The issue is the lobe profile, or its shape. Some cams will gradually open, then slowly close after peak. More aggressive cams will open faster, have higher lift, then snap shut. As in your example, both cams could have the same .050 duration, but have completely different results. The "lasy" lobe will be more quiet and easier on the valvetrain. But the lazy lobe will not make as much power. The aggressive lobe will open the valves higher for more air, make more power, but will be harder on the springs and you'll hear the valves contacting the heads. Crane's retail LS1 cams look like they have less duration when compared to the comp cams at .050. When in reality, the Cranes begin to open earlier (and slower causing more overlap) and close later because of the longer advertized duration. This will bleed off low end power and cause more lope. So its all a function of the lobe - the big lift cams will have more aggressive lobes, but you need both the adv duration and the .050 numbers to get a feel for the cams profile.

 

So I guess I made a good choice out of the two, right?