When a car is "cammed," it means it has a performance camshaft installed. A performance camshaft is designed with different lobe profiles and durations compared to the stock camshaft. The lobe profiles determine the timing and lift of the engine's valves, affecting how air and fuel flow into and out of the combustion chamber.
The choppier sound at idle that is often associated with cammed cars is primarily a result of the camshaft's aggressive lobe profiles and longer duration. Here's how it happens:
Overlap: Performance camshafts typically have a longer duration of valve overlap compared to stock camshafts. Valve overlap refers to the period when both the intake and exhaust valves are partially open simultaneously. This allows some exhaust gases to be drawn back into the combustion chamber during the intake stroke. The presence of overlap at idle creates an uneven and choppy exhaust note.
Reduced idle vacuum: The increased valve overlap and the resulting choppier exhaust note can affect the engine's idle vacuum. Vacuum is generated by the downward movement of the pistons during the intake stroke. In cammed engines, the reduced vacuum caused by valve overlap can lead to a rougher and more erratic idle.
Impaired cylinder filling: The aggressive lobe profiles and longer duration of performance camshafts can impact the engine's volumetric efficiency at low speeds. Volumetric efficiency refers to how well the engine can fill its cylinders with air and fuel. Camshafts with increased duration and lift may hinder efficient cylinder filling at idle, contributing to a choppy idle sound.
It's important to note that while the choppier sound may enhance the perceived performance and character of a cammed engine, it doesn't necessarily indicate more power or efficiency. Camshafts are part of a larger system that includes other engine components, such as intake and exhaust systems, that must be properly matched for optimal performance.