VTEC Explained
Don't know what VTEC is? It's the acronym honda uses for Variable valve Timing and Electronic lift Control. This system controls the cam profile that controls the valves. This technology was first seen in America in the 1990 NSX. The NSX had a 3.0 liter V6 engine. Japan was the first place VTEC was released. In 1989 it was put in the Honda Integra XSi and RSi in the form of the B16a. In being able tocontrol which cam profile was used the best cam profile for low and high rpm's can be used. At low rpm's the valves duration is controled by the less aggresive cam profile. At high rpms, when the rpm where VTEC kicks in is reached, oil forces a piston in the rocker arm to move into the adjacent rocker arms connecting them. When this piston moves it makes the rocker arm ride on the VTEC part of the cam. That was the easiest way I could put it. I'll go into it deeper as you read on.
If you think about why they made VTEC it really makes you realize how genious it really is. The wilder the cam, the worse the idle. The problem of sacrificing performance for a smooth idle was solved when VTEC was invented. In having a VTEC motor you get the wild lobed cams and the smooth idle. This is achieved by Honda through the 3 different types of VTEC. The systems found in the DOHC's, the SOHC's, and the one found in the Civic VX. I'll start wit the the system found in the DOHC's. If you look at the diagram above you notice 2 different cam profiles. The one in the center is a wilder cam lobe, hence the valves will open earlier and close later. That would be the VTEC lobe. Each cylinder has 4 valves, and 6 cam profiles. 2 of these being the more aggresive VTEC cam profiles. The low rpm lobes control the valves with the rocker arms. The VTEC cam profile has it's own feeler but it rolls along the cam lobe not affecting anything. As the motor accelerates thru the low rpms it waits for when to kick in the high rpm cam profile. The computer is what tells it when to do this. When the computer tells the motor to activate the VTEC system, 2 pins in each grouping of rocker arms locks all 3 rocker arms together. When they are locked together the center high rpm cam profile controls the valves. The pins move when the signal is sent because the signal electronically opens a spool valve. This let's oil pressure go to the sliding pin. This process only takes milliseconds. But in those very few milliseconds the valvetrain totally changed allowing you to use the high rpm, more aggresive, cam profile that would of totally killed your rpm. In the SOHC VTEC engines only the intake valves are changed. This is because the motors are built more for smooth power deliver than horsepower. The exhaust valves are changed conventionally. This reduces the power that full VTEC add. In this simplified form of VTEC, instead of the 3 rocker arms locking together, only 2 do. The system found in the Civic VX is the most interesting. This setup, at low rpms uses stagared timing. This is when 1 intake valve is opened more than the other in each cylinder. This makes the mixture swirl better on it's way in. The car also is set to burn lean. These 2 together up the gas mileage. When VTEC kicks in on these cars it switchs to a more mild cam grind than those found in the other VTEC systems. This is meant mostly for merging and passing.
 
-4thgencivic
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