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How does VTEC work?


Advise: is by no means responsible for information that may be incorrect or inconsistent.  This information was put together by various members of the community.  We provided this information by reference only!

VTEC - Variable valve Timing and lift Electronic Control

To fully understand VTEC, you first have to know basically how a traditional 4-stroke piston engine makes power. For that, refer here:

On top of that, you must know how a camshaft works:

After you have read these links, or if u already know basically how an engine and camshaft function, continue...

Traditionally, on DOHC non-vtec motors, a typical camshaft looks like the following:

that's a VW cam, but a Honda non-vtec cam looks pretty much the same.

2 Lobes per cylinder, a primary and secondary lobe. Each lobe acts upon the rocker arm (pushes down on it). When the rocker arm is pushed down, the other end of it, where the valve is connected to, the valve opens up a certain distance from its resting position. This is called Valve Lift

Along with a valve lift specification, cam lobes are also measured in what is called Duration which is, basically how long the lobe keeps the valve open over a 360 degree range, since the cam rotates, they measure it in a circular style, like this: And so the degree measurement is of the blue. The red is the time when the lobes are not acting on the rocker arms...that's why they stay closed...not because some additional lobe is keeping them closed.

So here's what a normal VTEC cam looks like. I think these are ITR cams, but I can't tell from this pic:

Notice the BIG cam lobe that is sandwiched between the primary and secondary smaller lobes. That big meaty cam lobe is the VTEC lobe.

Now think about it. Normally, the primary and secondary lobes just rotate and open valves, and there's no middle lobe to complicate things. But now, you have a middle lobe...but how does that lobe take control over the non-vtec lobes?

This is where you hear the term VTEC crossover...the point at which the VTEC lobe is engaged.

This is achieved by the ECU reading a variety of different sensors, most importantly, Oil Pressure, Water Temperature, Engine Speed, Throttle Position, and some others. When all these systems show ready, a PIN is pushed through all three of the Rocker Arms per cylinder, so that it is now in reality one BIG rocker arm. And since the VTEC lobe is much bigger than either the primary or secondary lobes, its like there is only one single cam lobe instead of the previous two.

This brings me to some side points that I'd like to address:

- Overlap - When you have aggressive cams geared towards making power w/o the aid of Forced induction, they tend to have high lift and long duration. Along with the long duration, you have a long overlap - the amount of time both the intake AND exhaust valves are open. Long overlap tends to make the motor lose a bit of compression, since the exhaust valves are still open when the piston begins its compression stroke, some air/fuel mixture is lost out the exhaust valves. Hence, when upgrading to aggressive cams, increasing the compression ratio to make up for this loss is a good idea, or else you won't be taking full advantage of those cams.

- Upgrading the Valvesprings - When you upgrade to a cam with higher lift characteristics, you obviously want the valve train to handle it..otherwise, the springs bind and you don't achieve the maximum lift of that cam.

- Setting VTEC Crossover point -VTEC engagement on a stock car is OPTIMIZED. Do NOT mess with it because all you are really doing is taking away power, depsite what it "feels" like.

Now that we have that cleared up...YES, i know that bringing the VTEC crossover up or down around 500 or even 1000 rpm up or down may indeed improve your power curves. Again, this is something u can only really prove if you are on a dyno and u can see the immediate results.

So, why do you need a vtec controller? U don't, really. Unless you are using aftermarket cams, or have upped the compression, or some other internal modification that demands a shift in the VTEC x-over to again be optimized. AGAIN, something you should really be doing on the dyno, as every motor is different, not to mention weather conditions, grade of gasoline, general condition of the motor, etc....

So when you hear people say "I got a vtec controller and I set vtec to 3,000 rpm so im mad fast now, u know they're a not bright.

Why? This leads into my explanation on how vtec works but, basically on a vtec motor there is a vtec cam lobe and non vtec primary & secondary cam lobes. Those non vtec lobes are optimized for low end power delivery. However, @ upper RPM, the small lobes cannot deliver enough air nor keep the valves open long enough to continue to let the motor make power.

Hence, the VTEC crossover...where the VTEC cam lobe takes over duties in opening and closing the valves from the secondary and primary lobes. Since the vtec cam lobe is much taller, it can now provide more air and duration the motor needs to keep making power @ higher RPM.

Now think about it. What does setting VTEC at 3,000 rpm do? It makes a cam lobe designed to work well at HIGH rpm to work at LOW rpm. Conversely, setting VTEC too high will cause the motor to start dying out, then JUMP foward when the VTEC lobes take over.

If u have or do get a vtec controller, try it one day.

Set vtec to 3,000 and see how much of a pig the car is down low. Then set vtec to say 7,500 rpm or something. See how the car accelerates, slows down, then springs foward again at vtec x-over.  You may think that is the car making more power, but if you were to look at dyno graphs, you would see the torque curve fall off, then rise back up @ your specified vtec cross over point.  The above is also a good way to determine a starting point for an optimal vtec x-over when you have done some internal mods and u would like to set vtec @ a certain point. Take a dyno pull with vtec set low, then another pull with vtec set really high.

Overlap the graphs and where the curves intersect where power falls off and where power comes up is where you should start to mess around with a crossover point.  Now, I-VTEC is VTEC, with a twist. Toyota's VVTL-i is the same thing as i-VTEC. Basically, in addition to VTEC, the ECU reads and modifies both intake and exhaust cam timing to an optimal point at every point on the RPM band. This ensures the most efficient cam timing to achieve optimal hp and torque. Infinitely Variable valve Timing and lift Electronic Control. This is why the new RSX-S motors, although only making 5 more hp than the old Type-R motors, are making around 12 more lb-ft of torque - because of i-vtec.

Also realize that VTEC and i-VTEC, although refined and marketed by Honda, is nothing THAT new. I think some Mercedes models first used variable valve timing back in the 50's or 60's...and in today's market, Ferrari, BMW, Toyota, Nissan, Honda, and many other manufacturers use some variation of variable valve timing. If you think about it, although complex and expensive both to research and develop, a variable valve and cam timing system has no competition from a fixed valve timing and fixed cam timing system. Pretty soon, all cars in the market will have variable valve and cam timing, not only does it help with making power, as I'm sure you're all concerned, but since it is more efficient, it cuts down on pollution and smog as well.


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