In previous articles, we’ve talked about the importance of Considering Individual Timing Events When Choosing a Cam to help us determine the duration, lobe separation and advance needed. This is especially important for cars with electronic injection which are dependent on idle vacuum to run correctly.
To effectively choose the best cam for your application, it’s helpful to understand the four-stroke engine’s motion, and how each of the four key piston motion and valve events interact with each other.
Here’s a quick recap of the four events in order of importance:
1. Intake Valve Closing (IVC)
This event has the most control over the powerband. We trade low-end torque for high-end horsepower when we close the intake valve later after BDC.
2. Intake Valve Opening (IVO)
This is the first component in overlap and the most important to idle quality. The idle gets rougher when we open the valve earlier BTDC, but we net more airflow on the intake stroke.
3. Exhaust Valve Closing (EVC)
This is the second component in overlap and is a compromise between reversion at low rpm and part throttle and the positive effects of scavenging higher in the power band and WOT.
4. Exhaust Valve Opening (EVO)
This is the least critical of the four events due to high residual cylinder pressure blasting out of the cylinder even before the piston hits BDC. It helps rid the cylinder of burnt gasses, but it’s a compromise. Opening too early before BDC decreases pressure on the piston that could be used to turn the crank, but reduces pumping losses. Pumping losses are caused by pressure working against the piston when it’s on the upstroke after BDC.