Editing VTEC (section)

== Context and description ==

[[Japan]] levies a [[Road tax#Japan|tax based on engine displacement]],<ref>{{cite web |url=http://www.pref.aichi.jp/global/en/living/taxes/tax.pdf |title=Overview of Automobile Taxes |work=Aichi Prefectural Government Office |publisher=[[Aichi Prefecture]] |access-date=2017-08-04 |archive-date=2017-07-13 |archive-url=https://web.archive.org/web/20170713090456/http://www.pref.aichi.jp/global/en/living/taxes/tax.pdf |url-status=live }}</ref> and Japanese auto manufacturers have correspondingly focused their [[research and development]] efforts toward improving the performance of their  smaller engine designs. One method for increasing performance into a static displacement includes [[forced induction]], as with models such as the [[Toyota Supra]] and [[Nissan 300ZX]], which used [[turbocharger]] applications, and the [[Toyota MR2]], which used a [[supercharger]] for some model years. Another approach is the [[Wankel engine|rotary engine]] used in the [[Mazda RX-7]] and [[Mazda RX-8|RX-8]]. A third option is to change the cam timing profile, of which Honda VTEC was the first successful commercial design for altering the profile in real time.{{Citation needed|date=August 2018}}

The VTEC system provides the engine with valve timing optimized for both low- and high-RPM operations. In basic form, the single [[Cam (mechanism)|cam lobe]] and [[Rocker arm|follower/rocker arm]] of a conventional engine is replaced with a locking multi-part rocker arm and two cam profiles: one optimized for low-RPM stability and [[fuel efficiency]], and the other designed to maximize high-RPM power output. The switching operation between the two cam lobes is controlled by the [[Engine control unit|ECU]], which takes account of [[engine oil]] pressure, engine temperature, vehicle speed, engine speed, and throttle position. Using these inputs, the ECU is programmed to switch from the low-lift to the high-lift cam lobes when certain conditions are met. At the switch point, a [[solenoid]] is actuated that allows oil pressure from a [[spool valve]] to operate a locking pin, which binds the high-RPM [[rocker arm]] to the low-RPM ones. From this point on, the valves open and close according to the high-lift profile, which opens the valve further and for a longer time. The switch-over point is variable, between a minimum and maximum point, and is determined by engine load. The switch-down back from high- to low-RPM cams is set to occur at a lower engine speed than the switch-up (representing a [[hysteretic]] cycle) to avoid a situation in which the engine is asked to operate continuously at or around the switch-over point.

The older approach to timing adjustments is to produce a camshaft with a [[valve timing]] profile that is better suited to low-RPM operation. The improvements in low-RPM performance, which is where most street-driven automobiles operate a majority of the time, occur in trade for a power and efficiency loss at higher-RPM ranges. Correspondingly, VTEC attempts to combine low-RPM fuel efficiency and stability with high-RPM performance.