The pushrod seat in solid...
The pushrod seat in solid lifters (right) is deeper than those of hydraulics (left), so we ordered the correct length longer pushrods to compensate.
Testing a hydraulic versus solid isn't as simple as it may seem. While it may seem like just a matter of ordering solid- and hydraulic-lifter camshafts with the same specifications and running a test, there are a few considerations that are not apparent at first. Beginning with the advertised duration numbers, solids and hydraulics are rated by completely different standards. For instance, in the Competition Cams line, hydraulics are rated for duration at .008-inch lifter rise, while solids are typically rated at .020 inch.
Comparing a solid to a hydraulic by advertised duration is like comparing apples to oranges. In regard to lift, things are a little simpler. But again, a direct comparison of specs would be misleading. The lash needs to be subtracted from a solid cam's specs to arrive at the true lift at the valve, which can then be compared to the hydraulic cam's specs. Finally, we have duration at .050. While both types of cams are rated in the same way, for the duration at .050 spec, the numbers can't be directly compared. Duration at .050 is measured in crank degrees at .050-inch lifter rise on the opening and closing side of a lobe.
We had checked both cams'...
We had checked both cams' installed centerline prior to the test, so on dyno day all we needed to do was line up the dots. Both were set to 106 degrees intake centerline.
The engine isn't interested in how far the lifter is moved, but rather only cares what is happening at the valves. With a solid, the lash will take up some of the lifter's motion before there is any valve motion. In fact, with the 1.6:1 rocker ratio in our test engine, the solid's duration at .050 reads as if the duration were taken at .0313-inch lifter rise as compared to hydraulic terms. That's a significant difference. A solid cam will behave like a hydraulic with approximately 10 degrees less duration at .050-inch lift.
All this makes it difficult to accurately match a solid- and hydraulic-lifter cam; matching the numbers in a cam catalog or on a spec card certainly can't do it. Our hydraulic cam was one of Comp's latest hydraulic profiles-an Xtreme Energy 275HL. These cams are ground with lobes specifically designed for high lift with a Mopar .904-inch tappet diameter. To match the fast rate of lift, we ordered a custom-ground solid cam based on Comp's MM-series .904 tappet-diameter lobe profiles. On the intake side, we went with the 6581 lobe, with a 6583 lobe chosen for the exhaust. The numbers on our solid cam seemed much larger in duration at .050, smaller in advertised duration, and very close on lift after compensating for lash. In fact, these two profiles were as close as we could approximate with the solid lobes available to us. We expected the idle vacuum and quality, cranking compression, and low-end output to be very similar between these two cams (see cam spec chart).
With cams and lifters in hand, we made the trip to Westech's dyno facilities for our little experiment. Our test engine was a 440 Chrysler wedge, actually measuring 446 ci after a .030-inch overbore, with a set of out-of-the-box Edelbrock heads, a Performer RPM intake, and 10.2:1 compression. For our baseline runs, we installed the Comp hydraulic XE275 camshaft and corresponding hydraulic lifters. The valvetrain included a set of 1.6:1 Comp Cams aluminum roller-rocker arms and pushrods. The engine combination was dialed in for timing and jetting on the dyno, with a Demon 950-cfm annular booster carb providing the air/fuel mix. The 440 idled with a lope at 900 rpm, and we recorded 12.7 inches Hg of vacuum.