You’ve got questions. We’ve got answers. We work with the Summit Racing tech department to tackle your automotive-related conundrums. This week, we’re exploring why a 427 c.i.d. big block Chevy engine would stop increasing power once it hits 4,000 rpm.
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Q: I recently built a 427-cubic-inch big block Chevy with the following parts:
- 4-bolt tall block, steel crank, 9.0:1 pistons
- COMP Cams Magnum hydraulic cam (230 degrees duration at .050, .520 inches of lift)
- Stock open chamber heads with COMP 1.7-ratio Magnum roller rockers and shimmed valve springs
- MSD Pro Billet distributor, 6AL ignition, and Blaster 3 coil
- Hedman coated headers and a three-inch dual exhaust
The engine runs hard up to 4,000 rpm, then stops pulling. It doesn’t miss or act like it’s running out of fuel. The fuel pressure is set at 5.5 psi at the carburetor, and I’ve checked for full-throttle travel and made sure the secondaries are opening. I’ve also tried setting the ignition advance at 36, 38, and 40 degrees total, with no change. The truck has a TH-400 transmission with a 2,400 rpm stall converter, a posi rear axle with a 4.11 gears, and 31-inch tall tires, if that helps.
A: We don’t know what type of valve springs you are using, but we recommend taking a good look at them. Big block Chevy engines are famous for wearing out valve springs. When the springs are too weak to follow the cam profile, one of the several things usually happens:
- Engine rpm will flatten out (not increase after a certain point)
- Engine will pop through the intake or exhaust
- A spring will hit the valve seat and bounce into the piston, or
- The spring will simply break
It sounds like you have the first symptom of fatiguing springs. We suggest replacing them with a set of COMP Cams springs (part number CCA-911-16). These 1.525-inch O.D. springs are matched to your cam profile, have 125 pounds of seat pressure, and an installed height of 1.90 inches.
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