hello everyone and welcome i am
currently surrounded by vtech there's
vtec in this s2000 in this honda accord
and in this honda civic type r engine in
front of me so in this video we're going
to be getting into what is vtec and how
does it work now in order to understand
vtec we need to understand engines and
engines need at least three things to
function they need air they need fuel
they need compression and in the case of
gasoline engines like this civic type r
engine you also need a spark now vtec
deals with the air portion of this
equation of those four things that the
engine needs so now we know what an
engine needs and it operates on four
strokes so the first stroke is intake
where the piston moves down and it pulls
in air and fuel the second stroke is
compression where that piston moves up
and it compresses that air and fuel
mixture the third stroke is the power
stroke where a spark plug ignites that
air and fuel mixture pushing the piston
down
and then the final stroke is the exhaust
stroke where the piston moves up pushing
out those spent exhaust gases now vtec
can optimize air flow during those
strokes so both during the intake stroke
as well as during the exhaust stroke you
can use vtec to maximize and optimize
the airflow that you get going through
the engine now before we look inside of
the type r engine let's look at a
simplified example so here we have a
cylinder head and we have valves to
allow air to come in and out of the
cylinder so here we have the intake
valve which allows for air to go in and
then here we have the exhaust valve
which allows for air to go out now how
much these valves actually open is
dependent on the camshaft so the
camshaft is going to be responsible for
opening and closing those valves so you
can see these cam lobes on the camshaft
right here so this is rotating and as it
rotates it will force that valve open
now the greater the height on this lobe
right here that forces that valve open
the more it's going to lift and so the
more that valve lifts open you can see
the more airflow it will allow to come
in or out of the engine now if you can
put more air into an engine you can put
more fuel into that engine and then you
can make more power so now let's take a
look at how the valves work on the type
r engine
so here we can see two different
camshafts this rod running along right
here and right here and a traditional
camshaft will look like this one right
here where you have just a single cam
profile that is used to open and close
the valves versus a vtec camshaft which
you have right here which has multiple
profiles of different heights and
different lengths that will open and
close those valves so here we can see
the v-tech camshaft and here are the two
valves that this camshaft is responsible
for opening and closing and as you can
see it has two different cam profiles to
choose between so it can use this inner
cam profile which has a higher lift on
it or it can use this lower cam profile
here on the outside it'll use both of
these simultaneously to open these two
valves so it can switch between these
two profiles so why might you want this
lower profile well you'll use the lower
profiles at lower rpm so at lower rpm
you don't need as much air and so
because of that you use a lower profile
and that means that the air is coming in
faster by restricting the airflow the
air is forced to speed up and by
speeding up that air it mixes with the
fuel better and because it's mixed with
the fuel better you get more efficient
combustion now at high rpm you want to
make as much power as possible so you
want as much air coming in the engine as
possible so you use the higher lift cam
profile and that allows that valve to
open up more and you can force more air
into the engine and thus use more fuel
and make more power so how does vtec
actually work well at lower rpm this
outer profile as mentioned will be
responsible for opening and closing the
valves and it does that by pushing these
outer rocker arms down this center
rocker arm is disconnected from these
two outer rockers and so this center cam
profile is moving this center rocker up
and down but because it's not attached
to the outer rockers which are attached
to the valves it's not actually
influencing the lift of the valves so
they're lifted with the lower cam
profile now once the engine starts
spinning up faster you're going to want
to start to use this higher lift cam
profile and so what happens is oil
pressure at those higher engine rpms
will force a piston to lock these rocker
arms together and so where previously
this cam profile was just moving this
rocker arm up and down and it wasn't
doing anything now it is locked to these
rocker arms which are of course attached
to the valves so now this center cam
profile is responsible for opening and
closing the valves and because it has a
higher lift the valves will then have a
higher lift and you can continue to use
oil pressure to change if that piston
has locked or unlocked these rocker arms
to choose which cam profile you want to
use now if you've been paying close
attention you'll have noticed that vtec
only exists on this engine on the
exhaust side so only for operating the
exhaust valves versus the intake valves
up top which just have a single cam
profile different from many of honda's
previous performance engines which have
vtec on both the intake and the exhaust
so why in this case is their vtec only
on the exhaust side well the reason is
that this engine is turbocharged as you
can see right here so the turbocharger
makes it really easy to force air into
the engine it's easy to get that air in
without using that extra lift profile
but then once you've had combustion
occur and those gases have expanded you
have a really high pressure in that
cylinder and so by using the higher lift
profile it's easier to force those
exhaust gases out and the other thing
you have to work against not only do you
have that higher pressure after
combustion you also have to force that
air through a turbocharger so by using
the higher lift profile it's easier for
that airflow to move out move through
the turbocharger and out the exhaust
thank you all so much for watching and
if you have any questions or comments of
course feel free to leave those below