Yes, I did a search, but there are so many threads that have "vtec" in them I could spend a year sifting through them. Does anyone care to explain in relatively simple terms how vtec works? ie what happens? Thanks!!

 

The absolute best source, where you will find the most complete, clear and nicely illustrated explanation of how VTEC works is in a wonderful website called "howstuffworks.com"
go in there and search VTEC, you will not be disappointed.
Cheers
Daniel

 

There are extra lobes on the cam shaft that take over the valves above 6k RPM. Essentially you get two completely different cam profiles for the high end and the low end. This gives you a very flat torque curve across your entire RPM band.

 

Imagine if you will, an engine from the factory that is smooth at idle through midrange but lacked top-end power. The owner wants more power, so (among other things) he swaps the street camshaft for a racing camshaft. Now the car has a sputtering idle and poor low-end throttle response, mainly because of too much overlap.

Now consider a person who wants the best of both worlds. What to do? Cut both cam profiles on the same camshaft, and find a reliable way to switch between them at the crossover RPM of the two cam profiles: street-cam at low RPM then racing-cam at high RPM.

Before Honda's VTEC came along 15 years ago, I don't think anybody had a way of doing this in a production vehicle. If old hot-rodders took a look at what VTEC does, they would be impresssed, and wonder "why didn't *I* think of that?"

 

Its when the engine produces its maximum performance, at low, and high RPMS.

 

Switching Camshafts
When the computer decides to switch camshafts, it closes a valve that forces oil along passageways through the camshaft into the third rocker. It has little pistons which are forced outwards (I'm a bit fuzzy here, but I think this is right) into the outer two rockers. All three rockers are then locked together and operate as one. The middle cam lobe has more lift than the outer two so it then controls the lift and duration of the set of valves. When switching back to the low speed cam the ECU just opens the valve, lets the oil out of the rockers, the pistons unlock the rockers and everything operates as before.

When to Switch Camshafts
The ECU is constantly comparing the torque curves of the low and high speed camshafts. It calculates the expected volumetric efficiency of the engine based on the current environmental conditions (air temperature and pressure) and the engine conditions (temperature, engine load, throttle position), and then derives the expected torque from the volumetric efficiency for each camshaft. Most of this has to be done anyhow in order to
determine how much fuel to inject.

When conditions are right (the revs are over about 4500-6000 rpm (depends on model), the engine is warm, there is enough oil pressure to activate the pistons and the car is moving) the ECU will switch from the low to high speed camshaft when the expected torque of the low speed camshaft equals the torque of the high speed camshaft. The ECU closes a solenoid valve that then forces engine oil, under pressure, along the camshafts to active the third rocker arm


UL can probably explain it better.