I'm changing my rocker arm ratio and am familiar with the mathematical method to figure lift but I want to know how duration will change. Is there a method for figuring duration that will get me in the ball park?

 

Duration isn't affected by changing rocker arm ratio, amigo. Just lift. What cam are you running anyway?

 

A degree wheel plus some math will give you the duration.

 

Duration (at the valve itself) is most definitely affected by rocker ratio changes however its fairly minor. Seat to seat timing stays the same but you get "more area under the curve" i.e. more duration at @ a given lift (.050", .100", .150", etc.)

 

It does change slightly but it is not enough to need measuring

 

Thanks everyone. I knew that the duration change was only minor but it's looking like a degree wheel is the only way to get it accurate. The cam is a Corvette LS1 #0968 cam. I picked up a surprisingly good deal on a new Comp Cams Pro Magnum kit with the 1.85 ratio. I thought using the cam and the rocker kit for a 5.3 truck would give it a nice boost. I think the reason I picked it up so reasonably is the 1.85 ratio is really too much of a lift change for most modern LSx cams. Performance cams are built now days with so much ramp speed that changing to a 1.85 ratio would cause all sorts of valve train havic I would guess.

 

As a rule of thumb it is advised not to use 1.8+ with cams that have lifts over .550
Now this is relative to the ramp rate of the lobe. The lazier the lobe the more it accepts higher ratio.