Any high performance engine will need to have the pushrod length taylored for that particular engine combination. Longer valves, smaller cam base circle, valve seat work, etc., etc. will affect the pushrod length. An adjustable pushrod is used to determine the correct pushrod length for a particular engine. The best way to determine the right pushrod length takes a little time and patience, but the results are well worth the effort:
Set up a dial indicator on cylinder #1 intake spring retainer and install a degree wheel on the crankshaft. Install the adjustable pushrod and adjust lash to 0. Rotate the crankshaft 5 degrees at a time, measure the lift and record the amount. Do this in 5 degree increments throughout the entire lift of the lobe.
Now shorten the adjustable pushrod length by .050" and repeat the process. Each time, plot the results of your data on a graph, which will show you the difference in lift rate. Keep shortening the pushrod in .050" increments until the peak lift starts to decrease. The point where the peak lift just starts to decrease is usually the point that provides the greatest area under the lift curve. At this time you can tweak the adjustable pushrod, be lengthening it by .025" to see if there is and distinguishable difference in peak lift. This pushrod length will be the exact pushrod length that maximizes performance and power. Repeat the process for the exhaust valve.
The adjustable pushrod can now be measured and exact length pushrods ordered.
As a final check, also look closely at rocker tip travel across the valve stem. For high spring rates and high RPM drag race engines I prefer to have the rocker nose wheel just inboard of the stem centerline when the valve is closed. Then at mid-lift the nose wheel is just outboard of the stem centerline and at full lift the nose wheel is close to the valve stem centerline.
