Fixed valve timing is always a compromise. An engine that idles smoothly at 800 RPM needs different valve events than one screaming at 7,500 RPM. Variable valve timing (VVT) systems solve this by adjusting when valves open, how long they stay open, and in some cases how far they open — all while the engine is running.

The core problem comes down to volumetric efficiency. At low RPM, you want short valve overlap (the brief period where both intake and exhaust valves are open simultaneously) to keep the idle stable and maintain low-end torque. At high RPM, you want more overlap and longer intake duration to take advantage of the intake charge's momentum and ram more air into the cylinder.

Cam phasing (continuous VVT) is the most common approach. Toyota's VVT-i, BMW's VANOS, and most modern systems use oil pressure acting on a vane-type actuator bolted to the camshaft sprocket. This rotates the entire cam relative to the crankshaft, advancing or retarding all valve events simultaneously. Typical adjustment range is 30–50 degrees of crank rotation. It changes when the valves open but not how long they stay open.

Switched-profile systems take a different approach. Honda's VTEC uses two or three different cam lobes per cylinder — a mild lobe for low RPM and an aggressive lobe for high RPM. A hydraulically actuated locking pin engages the high-lift rocker arm at a set RPM (typically around 5,500–6,000 RPM on the B16A). This changes both lift and duration in a single discrete step. That's what produces the famous "VTEC crossover" surge.

Combining both strategies is where modern systems shine. Honda's i-VTEC pairs cam phasing with lobe switching. Toyota's Valvematic and BMW's Valvetronic add continuously variable valve lift, which lets the engine control load by adjusting valve opening instead of using a throttle plate — reducing pumping losses by up to 10%.

Rule of thumb for cam phasing: every 10 degrees of intake cam advance shifts peak torque roughly 300–500 RPM lower in the rev range. So a system with 40 degrees of authority can effectively move the torque peak across a 1,200–2,000 RPM window, giving you a much fatter powerband than any fixed-timing cam.

A real-world example: the Toyota 2ZZ-GE (found in the Celica GTS and Lotus Elise) uses both VVT-i cam phasing on the intake side and a VVTL-i lobe-switching mechanism. Below 6,200 RPM it runs a conservative cam profile. Above that threshold, it switches to high-lift lobes with 11.2mm of lift and 245 degrees of duration, pushing peak power to 8,200 RPM. The cam phaser fills in the mid-range that the aggressive lobes would otherwise sacrifice.