I wish I had better pictures to show the Variable Volume Induction System (VVIS) but I’ll show what I’ve got:
Below is the picture of the magnesium intake chamber (lower portion of the picture). This chamber becomes part of the intake path when the butterfly valves open as manifold pressure rises above (vacuum decreases) a certain value (~4800 rpm on a stock motor).
In the upper portion of this picture, you can see the butterfly valve array. This piece bolts between the intake chamber and the intake manifold.
So the VVIS tunes the engine for better high-rpm breathing AND better low-rpm breathing. It makes this switch around 4800 RPM. VVIS has nothing to do with cam timing or valve lift (VTEC)--only intake path geometry.
Between 5800-6000 RPM, depending on a few factors like throttle position and a few others I think, VTEC will engage the high-profile cams. Note: if the engine management system measures a water temperature below a certain value then the high-profile cams are not engaged and the redline fuel cutoff is lowered to safeguard the engine.
Some (most) of us use the word “VTEC” to indicate that the engine is using the higher cam grind—this is a misnomer. An engine either has VTEC (Variable Timing and Lift Electronic Control System) or it doesn’t. It doesn’t go in and out of VTEC. It’s understandable why this naming convention exists but if someone really understands how VTEC works they will probably not use the word VTEC to describe a specific cam grind. That is, VTEC is the system that switches between two cam profiles; it is not the name of a cam profile. But, it’s more sexy to say “when VTEC kicks in this car moves!” than to say “when the higher cam profile is selected by VTEC, this car moves!”
Blow is a picture of the 3 intake rocker arms for the #4 cylinder. These rocker arms can operate independently of each other OR they can be “locked” together as one. VTEC is in the business of locking and unlocking these arms—that’s it. When the head is assembled, these rocker arms are sandwiched between the cams and the valves. The cams actuate the pads on the top of the rocker arms and the rocker arms in turn actuate the valves. The two outside rocker arms are the ones that move the valves up and down—the center arm does not directly move a valve. In a similar non-VTEC engine, there would only be two rocker arms, but the design of VTEC is all about the center arm. It’s the high-lift profile cam lobe that operates this center arm. When the arms are operating independently of each other, the valves are opened and closed according to the cam profiles of the lobe above the rocker pad, these are the low-profile or smaller diameter cam lobes. BUT when the rocker arms are locked together, the higher-profile cam grind that actuates the center rocker arm is left to operate the valves.
Here are the cams. See how the center cam lobe is "taller" than the two on either side (ok this picture doesn't show it too well but the center is a little bit taller). This is the high-profile cam grind that the valves see when the rocker arms are locked together.
The arms are locked and unlocked by pins in a similar way to keyed door locks (well, kind of). Local oil pressure (controlled by VTEC) actuates these pins. If VTEC wants to lock the arms together then it increases the oil pressure that these pins see which pushes them away from their natural unlocked positions. In the picture below, just above my thumb is a little doughnut shaped thing where the pin of the middle rocker arm goes. This doughnut shaped thing is connected to a spring which is overcome by the oil pressure when the arms are locked. When VTEC drops the oil pressure, then the spring returns the pins to their unlocked position. The amount of oil required to move these pins is very small and in my opinion cannot come close to accounting for the sudden drop in oil pressure some claim to see on their gauge when revving into the VTEC switchover range.
Here’s a shot of a pin extending from the center rocker arm.
Man, this got a whole lot more involved than I intended. Hopefully this post helped to clear up what VTEC does and doesn’t do, but I’m not so sure I accomplished this task. Please note the engineering precision required to lock and unlock these arms at 6000 rpm! Pretty cool, eh?
DanO
[This message has been edited by DanO (edited 08 March 2002).]
