The Purpose Of 1.6 Rockers

raymond said
Does anyone know what the current theory is on how much duration you pick up going to a 1.6 rocker? I have an OLD Crane master catalogue and it states that going from a 1.5 rocker to a 1.6 it is the same as going to a 3 to 4 degree bigger cam. Anyone got any thoughts?

Well to be truthfull, the duration (both advertised and at 0.050") is totally independant of the rocker arm ratio. These are camshaft specifications and the measurements ares taken at the lifter, not at the valve. So it stays constant regardless of wheather you have 1.5, 1.6 or even 1.8 to one ratio rocker arms. But what DOES change is the amount of valve lift at that 0.050" point.

Here is an example. At 0.050" of cam lobe lift with a 1.5:1 rocker, the lift at the valve is 0.075". Now with a 1.6:1 ratio rocker, it would be 0.080" lift at the valve. That is a difference of only 0.005". But if you were to rotate the camshaft a little, you can find where the valve is open 0.075" again. The amount you had to rotate the cam in degrees is the effective duration that the 1.6 rockers add. This amount will not be constant from one camshaft design to the next. It is totally dependant on the ramp of the cam lobe.

So to say 1.6 rockers always add 4 degrees of effective duration would be total BS. You would literally have to measure a specific camshaft to determine this value. Not only that, but it is very likely the intake lobe and exhaust lobe would have different profiles. So you would need to measure both.