With the limitation on rpm, despite a wide range of different springs, it began to look as though the springs' ability to control the cam's action at the valves was not really the limiting factor here. The evidence began to point to the hydraulic lifter itself. To explore this possibility, we decided to run a diagnostic check, substituting a solid lifter for the hydraulic. Can a solid lifter run on a hydraulic stick? For testing such as this, the answer is yes. Hydraulic cams lack the clearance ramps required to ease a solid lifter from lashed clearance on the base circle to the flank where the lifter begins to really accelerate. The trick is to lash the solid at .003-.004-inch lash, hot, which will effectively allow it to follow the lobe similarly to a hydraulic. The difference is, it will not have the hydraulic mechanism we suspected to be the cause of instability in our setup. The lifters were swapped and lashed, and the results were telling. The engine pulled cleanly to the 6,000-rpm limit of our test and produced 607 hp at 6,000 rpm. Peak torque was now at 549 lb-ft at 5,200 rpm. The solid lifter allowed the cam to translate its design specifications to the valve more accurately, while it appears with the hydraulic, something was being lost in the translation.
While the engine handily met our 600hp by 6,000-rpm goal, the check with the solid lifter certainly isn't a long-term solution. Actually, it just pointed to the direction where we needed to do more work. The simple conclusion here is the combination of very intense valve action, high spring loads, and a relatively heavy valve and valvetrain is just too much for the hydraulic mechanism to cope with. At higher rpm, the lifter likely just collapses the internal plunger, losing lift and duration, making the power curve nosedive. The easy solution is to substitute a solid roller in place of the hydraulic flat tappet, since the roller will allow high spring loads, while the solid body will eliminate all question of hydraulic instability. These intensities are exactly what a solid roller is designed for. Our engine did meet the goals of mild manners at idle, running smoothly at 850 rpm without a big-cam lope, while generating over 14 inches of idle vacuum at 14 degrees of initial advance.
We are uncertain exactly where this project will lead until we get more testing done on the dyno. We'd like to see peak torque edge up to the stratospheric levels and improve the high-rpm dynamics without sacrificing the great idle we have now. It'll make for some interesting testing to come.