May 2011 Chevy High Performance Q&A - Performance Q&A

Love It When A Plan Comes Together

Kevin McClelland Mar 29, 2011 0 Comment(s)

Most people think that when you increase the lift you get piston-to-valve clearance issues. Increasing the duration and/or reducing the valve separation angle will cause clearance reductions greater than the actual max lift value. When you’re running a stock-type camshaft in a big-block you have duration values in the very low 200-degree range at 0.050-inch tappet lift. When you increase the duration to, let’s say, 240 degrees at 0.050-inch tappet lift you have started the valve lift event 20 degrees sooner and ended 20 degrees later. This change puts the valves much closer to the piston than the actual max lift. If you have two camshafts with a max lift value of 0.500 inch, the camshaft with 200 degrees of duration at 0.050 will have a piston-to-valve clearance of 0.200 inch at the critical valve-to-piston location, which occurs right around 15 degrees before top dead center on overlap for the exhaust valve, and 15 degrees after TDC for the intake. A camshaft with 240 degrees and the same max lift of 0.500 inchbecause the valve event started 20 degrees soonerthe valve could be open 0.120-inch more than the smaller camshaft at the critical checking points. The main difference in the two camshafts is that the cam lobe dwells at max lift for 40 degrees longer with everything else being equal. Now you throw in the tightened valve separation angles and advanced intake centerline, the piston-to-valve gets even closer.

So a mild-performance camshaft will fit into your big-block without a problem. If you wish to go over 230 degrees at 0.050-inch tappet lift, we’d start checking things. Whatever you do, a safe piston-to-valve clearance will run around 0.080 inch for the intake valve and 0.100 inch on the exhaust. As for the valvesprings, go with the camshaft manufacturer’s recommended valvespring and retainer to ensure that the valvespring pressure is correct for those cam and lifters. The factory stock rocker arms will accept 0.533-inch max lift. Finally, with the cam manufacturer’s retainers in place, make sure you have at least 0.050-inch valve seal-to-retainer clearance at max lift. Many of the cam manufacturers have taken the increased lift possibilities into their retainer designs giving you the maximum clearance between the seal and retainer.

Good luck with the mild hop-up of your big-block. They can be fun, torquey little (big) engines. Many a tire company has made a fortune on BBC-equipped vehicles!

It’s All About The Boost!

I have long been a subscriber to Chevy High Performance and to one of your sister publications. In that magazine, I often see articles on Buick Grand Nationals with turbocharged V-6 engines. It’s hard to imagine a fullsize car running 9s or 10s with a V-6, but they apparently do. Since Chevrolet has always built higher-performance motors than Buick, why don’t we have any Chevrolet V-6 motors pushing Monte Carlos or Camaros into 9- or 10-second timeslips? How do we get big powerand hopefully good mileageout of the Chevy V-6 that is so plentiful? I cannot tell you how much it pains me that Chevy performance is being bested by a Buick!
John Caldwell
Albuquerque, NM

Having owned many of the GM products over the years, we had a ’70 Buick Grand Sport convertible with a 455-cid big-block. The Buicks were always referred to as the Businessman’s Hot Rod, with their upscale appointments and big power. The Stage 1 455 was only rated at 360 hp (far below real output for insurance reasons) and 510 lb-ft of torque. This would move the Skylarks and GSs right with any of the LS6-equipped Chevelles of their day.


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