July 2012 Chevy High Performance Q&A

Chain Reactions, Part II

Kevin McClelland May 29, 2012 0 Comment(s)
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Big-block float

I have a Gen VI big-block Chevy with a hydraulic roller. It has a camshaft with 234/240 degrees of duration at 0.050-inch tappet lift, and 0.595-inch max lift. I’m running Crower 1.75 stainless rockers, Manton 3/8-inch pushrods, and performance Morel lifters. I also have Isky valvesprings (PN 8205), COMP Cams steel retainers on Brodix heads, and an Edelbrock RPM Air-Gap intake. My issue is I think I lose valve control at revs much over 5,000 rpm. The car feels as if it has a governor on it that kick on at about 5,200-5,300 rpm in high gear. It is smooth, just flat. It plains out about at the 1,000-foot mark and no matter what, goes 107 mph and it should be around 115 mph. I just pulled some springs off and I have 130-pound seat and 350 pounds over the nose. My question is what kind and how much spring do I need? I feel confident I don’t have fuel issues and I may scrap it all and install a solid flat tappet or a solid roller. I’m sick of it. Thanks for any help!

Montgomery, AL

Valve control has always been a problem with hydraulic roller big-blocks. The valvetrain weight of the big-block doesn’t lend itself to the increased weight of the hydraulic roller tappet, and the valve weight of 2.19-inch stainless valves. You can usually pick up 200-300 rpm of limiting speed by installing titanium retainers over standard steel retainers. COMP has released a nice line of Tooled Steel retainers that give you the weight savings of Ti, but with the durability of tooled steel.

As for your valvesprings, I checked out the spring book on Isky’s website. The 8205-Plus spring specs out at 128 pounds on the seat at an installed height of 1.950 inches, and you should be seeing around 430 pounds at your over the nose lift of 0.595 inch. Either you don’t have PN 8205 springs, or you have killed them by sitting at a fuss point waiting for the finish line to come. One interesting thing that I have found over the years is that engines will accelerate through unstable points of the valvetrain, but if the engine can’t rev past them quickly enough they will stop revving at those fuss points. This is what your problem sounds like. Changing the spring design by its damping style, diameter of spring, number of coils, all changes the natural frequency of which a spring resonates. You must have a valvespring with a natural frequency that goes into instability just above 5,000 rpm with the mass of the valvetrain, and acceleration rate of your camshaft. In the past I’ve increased the seat pressure by shimming the springs over 30 pounds without any change in the fuss point. I would recommend going with a set of COMP Cams valvesprings (PN 930-16), which is a dual valvespring with a flat wire damper. The springs spec out at 1.550 inches in diameter and have an installed height of 1.900 inches and a seat pressure of 153 pounds. At your 0.595-inch max lift you can expect an open pressure just above 400 pounds. Running the seat pressure much over the 15-pound mark you can run into issues with the lifters bleeding down from the excessive pressure. Along with the 930-16 springs we would run the matching Tooled Steel retainers (PN 1732-16). These retainers weigh only 2-4 grams heavier than comparable titanium retainers. These tool steel retainers utilize COMP 10 degree Steel Super locks and you will need PN 613-16 for 11/32-inch valves, and PN 612-16 for 3/8-inch valve stems.

Finally, switching to mechanical flat will take care of your problem. If you decide to go this route we would still recommend putting your valvetrain on a serious diet. You will benefit in the long run from any mass you can drop off of your valve action. Good luck.

Source: compcams.com


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