So I received a request to make a video explaining the differences between
dual overhead cams, single overhead cams and
overhead valves or more commonly known as pushrod.
So I'm going to go through those. You'll probably see things like
DOHC labeled on the side of cars, SOHC and you might not know what it means so
once again: dual overhead cams, single overhead cams and OHV stands for
overhead valves, but more commonly they're called pushrod.
So what a camshaft does is:
as it rotates - so it'll be rotating like this and as it rotates
in this direction - basically it's just a cylinder with a lobe on the end of it
and as this lobe rotates it'll come in contact with this rocker arm right here.
I've got everything color labeled so anything green is a camshaft, anything
blue is a rocker arm and anything red is a spring.
So this camshaft will rotate,
the lobe will press against this rocker arm which pivots at this point;
so as it pivots, it'll push down this valve, opening it up, letting air to either go
out or come in depending on if it's an intake or an exhaust valve.
So now I'm gonna go through each of the three types and some advantages of each.
So the first one: here is the push rod and how that works is you'll have a camshaft
that's in the center - this is gonna be a V engine - so from above you're going to
have the six cylinders here in the camshaft in the middle.
So as that camshaft rotates,
it will push against these lifters and these lifters are
connected to push rods - this black line right here. That push rod will push up on
the rocker arm, once again, it will pivot at that point and it will push the valve in.
So with the push rod typically you're going to have all your valves in a line
going back and so you'll have typically two valves per cylinder.
(It's) less-efficient to have two valves but that's just the way some companies do it.
It is possible now to have a rocker arm for each valve and have four valves
per cylinder but it's not quite as common.
Some advantages of this:
It's more compact because you can keep the camshaft inside with the engine rather
than putting it on top like the other methods.
It's less complex, therefore cheaper
and you can use higher weight oils because you don't have these
complicated rocker arm camshaft systems that require a great amount of lubrication.
One thing I should mention also is:
there (are) springs connected here, just valve springs
and so, as that rocker arm pushes the valve in,
it needs something to force it back up.
So once the cam is off the lifter that spring
will push the rocker arm back up and will close the valve.
Alright, so single overhead cam:
so you'll have overhead - meaning it's above the cylinders rather
than to the side of, so you'll have it - and this is just an inline four from
above just going to look like that: camshaft above four cylinders.
So how it works is:
say the camshaft is rotating counterclockwise here.
It'll hit this rocker arm, push the exhaust, let the exhaust go out;
it will keep rotating, hit the intake rocker arm
that will rotate here, push the intake in and as that
piston goes back down it can suck in air.
So it'll be a single camshaft above all
of the cylinders and then it will control both the intake and exhaust valves.
Now this is more common to see: three or four valves per cylinder
because it's easier to do with the rocker arms connecting here in the lobes.
So some advantages: you have less inertia forces.
So here with this pushrod
when you're going to have this cylinder, this pushrod,
moving back and forth there's nothing balancing that inertia.
Whereas here you're not going to have that great of a inertia force
just because the items are smaller and they're rotating a much smaller amount.
So with this push rod you've got this long pin and it's getting thrown back
and forth back and forth so at higher speeds it becomes a problem with the balancing.
So that's an advantage of single overhead cams or just overhead cams in general
and overhead cams in general are also more efficient,
main reason being you can have more valves per cylinder.
So, finally dual overhead cams:
so you'll have one camshaft for the intake valves
and one camshaft for the exhaust valves.
So each one will be dedicated for the intake or exhaust.
That's a nice thing to have and with that you can have better
performance and you can rev it much higher rpms because you're not having to
deal with these inertia forces and it's a better design.
So from above that's what it'll look like: you'll have the the two cam shafts and
typically, with dual overhead cams you'll always have four valves per cylinder.
It's even possible to have five valves if you want three intakes and two exhausts just
to smoothen things out and increase flow and just make it overall more efficient.