I found this info on two seperate 318 builds done by Engine Masters magazine in their winter 2006 and winter 2008 issues. Just thought I'd share the results.
Maximum lift for the least duration is part of the formula thats helping produce the power levels of today's modern engines; the valves spend more time at higher lifts where the cylinder heads are flowing more cfm.
The first comment needs to be: A solid lifter cam (flat tappet or roller) does not have this limit as to how fast the valve can be opened for a given duration.
The 2006 318 build used OEM 360 heads, which use 3/8 valve stem diameter. The cam was a Comp XE275HL which is designed for mopar's 0.904 lifter diameter. Cam specs are 275/287 gross duration, 231/237 at 0.05 tappet rise, lift is 0.525/0.525 using 1.5:1 rocker ratio. Valve springs were Comp 972 with 140 lb seat and 310 open. They tried 1.6 rocker ratio but the effective limit of the engine dropped from over 6700 rpm to 6100. They tried zero lash to stop lifter pump up but it didn't help. The problem was there is a limit ot how much intensity a hyd lifter can handle (flat tappet or roller); IE the cam had a very high lift considering the short duration.
0.904 lobes are 8% faster than chevy high intensity lobes, and the 1.6 rocker adds another 7%. The 1.6 rocker results in valve lift of 0.560 with only 231 duration at 0.05 which is well into the range of an aggressive solid roller lobe profile. The result is valve train instability; increasing valve spring load does not help.. instead valve train weight reduction is required.
So in the second 318 build done in 2008, they used Engine Quest magnum cylinder heads which have smaller 5/16 diameter valve stems, rather than the older 3/8 diameter. Valve O.D. stayed basically the same with 2.02 and 2.03. Just reducing the weight of the valves made the difference.
IN the second build they used the same cam, Comps XE275HL, but used 1.6:1 rocker ratio common to the magnum heads. The engine ran easily to 6400 rpm.
The valve springs used on the first build had 140 lb seat pressure, which Crane cams said is pushing the limit for street use. They said the springs used on the second build were Comp 901-16 springs and had 101 lb seat load, but their spring data did not make sense as they were pushing the springs into coil bind with the valve lift. Either a typo for the spring part number, or very poor check and assembly proceedures.
So the fast acting hyd cam lobes need a light weight valve train, which really wouldn't hurt a solid cam either. As Smokey Yunick said, you need to take the mass off the valve side of the rocker arm, which is exactly what lighter valves and retainers do. Note how heavy roller lifters are compared to flat tappet lifters. Smokey is right, extra weight on that side of the rocker doesn't have the same effect.
Topic: Small Block hydraulic lifter maximum lift vs. duration (Read 1938 times)
