Honda VTEC vs i-DSI

by Dennis Costa

Through the years, Japanese auto and motorcycle manufacturer Honda has gained fame not only for its line-up of vehicles, but also for its innovations in optimizing engine performance. Chief among these innovations is Variable Valve Timing and Lift Electronic Control, also known as VTEC, and Intelligent Dual and Sequential Ignition, or i-DSI. But is one system better than the other?

Camshaft Basics

Internal combustion engines depend on intake valves to let in air/fuel mixture into the cylinder for combustion and exhaust valves to let the burned mixture out of the cylinder. Valves have to open and close at specific intervals, and a camshaft takes care of that. A camshaft is basically a long metal shaft with protruding lobes. Every time the camshaft rotates, the lobes actuate other mechanisms such as lifters, pushrods or rocker arms in order to push some valves open and letting other valves close via a spring mechanism.

Timing Problems

High-performance car enthusiasts commonly found a problem with their fast cars: they weren’t that good at low-speed performance. That is because racing engines accustomed to operate at high speeds have very different needs than engines designed to go at lower speeds. For example, racing engines have a longer timing in the opening of its intake and exhaust valves. Valve lift also plays a key role: the higher a valve lift opens, the more air-fuel mixture enters the cylinder and the more it combusts, which yields more power. However, standard engines tend to have lower valve-lift numbers.

Variable Valve Timing

This dilemma was solved with the arrival of the Variable Valve Timing, or VVT, system. Before this, designers tweaked with the placement and shape of the cam lobes in the camshaft, reaching a compromise in performance between low engine speeds (measured in revolutions per minute or RPM) and higher RPM speeds. VVT basically allows cam timing to change, delivering greater efficiency and power over a wider range of engine RPMs.


Previous variable valve timing systems existed, but Honda’s VTEC system stood out from the pack. It works by employing several cams of different sizes in the camshaft, as well as several rocker arms placed next to each other. At lower RPM speeds, only some of the cams actuate on the rocker arms, which in turn lift the valves. These cams are shaped in a way that only the proper amount of air-fuel mix enters the cylinder at one time to optimize acceleration. But when the engine reaches a certain number of RPM’s, a control module activates what is basically a pin that locks in the rocker arms. This enables every cam lobe in the arrangement to exert force on the rocker arms, including those with higher profiles. This lifts the valves to a higher degree, and optimizes performance at high-RPM levels.


Meanwhile, the i-DSI system aims to deliver the same results as VVT technologies, but using a whole different approach. Rather than tinkering with cam designs to determine valve timing, i-DSI plays with the timing of the spark plugs that ignite the air-fuel mixture once it enters the cylinder. Standard engines use one spark plug per cylinder to ignite the fuel at key moments; i-DSI uses two per cylinder, arranged in a diagonal pattern.

How i-DSI Works

The first spark plug, located next to the intake valve, fires just as the mixture has entered the cylinder. As the mixture starts to burn, the second spark plug ignited, further expanding the flame rapidly into the whole area to achieve complete combustion. The timing between the sequences of ignitions varies according to engine speed for maximum fuel economy and power output. For instance, at mid-range RPM speeds, the interval between the first spark plug ignition and the second is more pronounced, while at high RPM speeds, the system supplies an almost simultaneous ignition on both ends.


Both systems employ ingenious ways to basically squeeze the performance out of relatively small engines all across the RPM range. While VTEC is associated more with high-performance vehicles, i-DSI is more linked to compact cars that still need to have some power under the hood. Both technologies serve their own ends, and thus it is difficult to declare one technology as better than the next. However, in terms of impact and design influence, VTEC casts a larger shadow, with plenty of other manufacturers having their own version of Honda’s innovation, albeit under different names.

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