Are long duration cams good in a big block Chevy?

The big-block Chevrolet engine (BBC) was introduced in the early 1960s with an intended performance application in NASCAR racing. However, it also found success in regular production vehicles and trucks, as well. From the beginning, GM offered high-performance versions which, among other things, included camshafts featuring greater valve lift and longer duration. Depending on the application, long duration cams can offer meaningful performance gains.

Function

The purpose of the camshaft is to precisely time the opening and closing events of the intake and exhaust valves so that they allow the air/fuel mixture to enter the cylinder, be ignited and then forced out through the exhaust port.

Significance

Duration--the length of time a valve remains open--defines the character of the engine. Less duration restricts the operating range of the engine to lower rpms, while longer durations move the power band higher in the rpm range. Generally, each additional 10 degrees of duration moves the power band higher by 500 rpms. Standardized measurements of duration are compared at .050-inch valve lift.

Standard Performance

Intake and exhaust durations from 200 to 220 degrees at .050-inch lift are representative of factory (stock) type camshafts. These provide a smooth idle and reliable operation from idle speeds to approximately 5,000 rpms.

Street High Performance

Longer durations of between 230 and 260 degrees shift the power band higher at the expense of low rpm torque, but the BBC generally has an abundance of low-end torque anyway. Idle quality suffers with longer durations, but power is significantly increased at higher rpms. Additional engine modifications may be required.

Competition Applications

Extremely long durations are not advisable for anything but racing applications. Durations from 270 to more than 300 degrees at .050-inch valve lift are typical of engines that operate in the 4,000 to 8,000+ rpm range.

Tip/Warning

"Too much cam" can have a negative impact on performance and reliability if not properly matched for the intended application. It must be complemented by several other factors, as well. When offered a choice between two or more camshafts, the milder one is often the better choice.

Source:

Comp Performance Group (Factory-type)

Comp Performance Group (Street/High Performance)

Comp Performance Group (Race/Competition)

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