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:
http://www.howstuffworks.com/engine.htm
On top of that, you must know how a
camshaft works:
http://www.howstuffworks.com/camshaft.htm
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. |
