Advertised duration is either actual "seat to seat" timing or the timing at .006" VALVE lift. I think most of the major aftermarket cam manufacturers use the .006" valve lift duration for their "advertised" specs. DD2000 seems to work best with the .006" valve lift timing, which is an SAE measuring standard.
Valve timing is probably the weakest link in DD2000, and the industry in general. It doesn't really tell you anything about flank acceleration, which can make a big difference in actual area under the lift-crank angle curve for cams with the same timing specs.
Also, rocker arm ratios are NOT what they are advertised to be. I take lift crank angle diagrams for the cams that interest me and measure the actual rocker ratio. Not only is the rocker ratio NOT as advertised, it's NOT even constant, but varies with the amount of lift, and because of this, the actual timing numbers in the engine will be less than advertised.
Also, with mechanical lifter cams, the actual rocker geometry means that specified clearances based on theoretical rocker ratios are too loose.
Thanks, I looked over my spec card found that I should have the sepcs ar .004 valve lift. Put that into the DD2000, and I lost about 25hp. I guess time will tell as to what actual power my engine will be making.
On another note, I am trying to check rocker clearance wo make sure I won't be hitting my valve covers since I had to put a nice dent in the top of it. Anyways I currently place the head on the block and then pour a little oil into the lifter and place the pushrod and rocker on. Tigten this all down and then place some putty on the valve cover where the issue is. put it on, rotate the engine, and I have not come into contact. However how does a hydro lifter work? does the lift increase at low RPM or high rpm?
Corvette Store
Stupid question! regarding cam specs.
