Cam Timing and Duration Explained for the masses

[Zak33]

Because VTEC and other tricks are causing such confusion I'd like to try to break down some of the more commonly misguided beliefs about how an engine breathes.

As with all Zak33(tm) discussions of the past, if you point out I'm wrong, and I find that you're right , I'll admit it, thank you and then remember for future.

So don't be shy about joining in.

OK....lets take a single valve. It's an inlet valve. Lets ignore all the rest of the engine, except the cam shaft above it that pushes it open.

When a cam shaft turns, the lobes on it (bulges on the sides engineered and ground into it) push the top of the valve stem and therefore the valve moves down and lets air and fuel into the cylinder.

A large spring connected to the valve then wants to lift the valve again, pushing it shut, and it can only do that when the camshaft turns further, moving the lobe or bulge back around and off the top of the valve allowing it to shut.

In between the camsgaft lobe and the top of the valve stem is a tappet. It is there to bridge the gap between the two, and they are normally hydraulic so that they can adjust themselves over the course of the engines life to adjust for little layers of wear and tear. They literaly to stop the camshaft battering the valve stem to death, while tapping it on the head and they stop it making a tapping noise

There are different sorts of this arrangement, but this is the main one in use today.

This is a side profile of a cam lobe at the top of the picture, and the funny box shape below it is the tappet (more on that later). As that camshaft spins, the lobe acts upon the tappet, which pushes the valve down and makes it open.


So.....a camshaft turns, and the bulges on it's sides (called lobes) push down on the top of the tappet, which in turn pushes down on the top of the valve stem, opening it.

More soon

[Zak33]

This is a beaut of a picture. Cam shafts along the top, pushing down on tappets, and you can even see the spring that wants to lift the valave back up

[Zak33]

Now....imagine if we were to make the "lobe" a different shape

Imagine if we were to make it stick out further. Well....sod me if that doesn't make the valve open further

Or....if we made the lobe extend further around the camshaft, so the valve was open for longer

Guess what?

Lift

Duration

See anything there....? Camshafts with extra lift or duration are often part of permformance kits They make the valve stay open for longer and open further, allowing more air in.

So why not fit them to everything?

Because its not that simple!!!

More soon

[Zak33]

Right... the general public imagines thje following is true:

A piston goes down, and the inlet valve magically opens in a split second....as the piston goes down, it sucks air in. At the bottom of its' travel, the air just stops sucking in, and the inlet valave shuts in a trillionth of a second, like magic. Then the piston comes up, and squishes the air in such a perfect way, that at the top dead centre, the spark goes "spark" (ran outta words there!!!) and the petrol/air mix goes "THWOOM" (better....but sad) and down goes the piston with force, and just at the bottom, the exhaust valve flies open in a quadrillionth of a second, and the piston merrily rams its way up, and pushes every last drop of nasty exhaust fume out. Atg the very top of it's travel the exhaust valaves slams shut so fast you cants begin to believe it, and the exact same moment the inlet valave shoots open, ready for the piston to rush back down and suck in all that lovely sweet oxygen.

Let me be the first to tell you it's not that pretty! In fact....it will never be that pretty, because its an engine, not a piece of software. The camshafts need to turn, the valaves ned to travel, the springs needs ot then recose them and the engine has to turn all this stuff under immense friction.

As all my engineered biddies here will now attest, at high revs, the piston shoots down so damn fast, the air simply doesn't all get inside. Even if you hacksawed the entire top of the engine off, and had no valves at all, the speed a pston goes down the bore at 10,000 revs is mind numbing!

And it creates a short term vacuum.

Try this....hold a drinking straw in your mouth, seal it between your lips....and then breath in really fast! Short of breath? Damn right.

Now, imagine a car at 6000rpm....still accelerating. The piston has gone all the way down, and now on it's way up AND ITS STILL SUCKING!!!!! Because the valve could'nt let enough air in!

Only at low revs does the engines cylinders fill all the way up by the time the pistion has got to the bottom.

So,....you see.....to make a really high revving engine we may well need a cashft with totally different lobes on than on a low revving engine. We may well want the lobes to start earlier or finish later too.....we might want them to open further...or maybe....less.

At tick over...with a high lift and long duration camshaft....the car just stalls. Lots of really highly tweaked cars don't "tick over" in the way your mum's fiesta does. They have their tickover set at 3500 rpm!

[Zak33]

And the reason that Fiesta wont rev to 8000, EVEN DOWN HILL WHILE BEING PUSHED BY AN EVO VIII while the fiest is in 2nd gear is that....

air is darn hard stuff to force into and out of places where it cant get!

Now the cleverness of a VTEC.

At town speeds, and up to the pre determined revs of that engines design, the lobes above the two inlet and two exhaust valves(per cylinder) have been set to open and close at seperate times, a fairly short distance, so that they have time to open and close. Dont forget....these valves need to travel. Its not instantaneous.

But...at high revs, we need them to open and close earier and later and be open for longer.

SO we need to put a third lobe on the camshaft, BETWEEN THE OTHER TWO, which looks like the Himalaya's The other two are the foot hiils....this one is Everest!

Now..we need the middle one to stay disconnected from the valves while the outer two do their work. BUT as the revs hit the sweet magic spot, deemed correct by the Honda dudes...an increase in oil pressure pushes a series of rods along inside the tappets, making the central lobe suddenly actually act on the valves and the two outer ones are literally just flapping free

So all of a sudden, we have rally profile cams shafts.

And ...magic happens the other way too as the revs drop the bolds slide back and the central lobe stops effecting the valves

[J4MES]

vid of valves

[Zak33]

Try this little Honda beauty.

OK,....its a slow download.....and it covers lots of engine stuff.....but its got a stunning cam shaft section

[J4MES]

your vid > my vid

but mine had supercharger power...... ggggggggrrrrrrrrrrrrrrrrrrrrrrrrrrr

[Konan555]

except the cam shaft above it that pushes it open

AHEM - or below for the cave engines around here

They literally to stop the camshaft battering the valve stem to death, while tapping it on the head and they stop it making a tapping noise

Also worth a mention they allow for expansion of materials involved?

And the reason that Fiesta wont rev to 8000, EVEN DOWN HILL WHILE BEING PUSHED BY AN EVO VIII while the fiest is in 2nd gear is that....

There's a rather angry rev limiter in place so you don't send bits of engine all over the place

[Zak33]

There's a rather angry rev limiter in place so you don't send bits of engine all over the place

the other stuff you wrote is right.....but not that bit!

The rev limiter only cuts the ignition at a certain limit...it does NOT put a brake on the engine

I'm pointing out that IF the valves opened far enough and for long enough, the engine would rev high enough. But IF you blocked the exhaust, welded the valves shut and left it in gear you would NOT push that car unless the tyres were skidding.

Please guys...remember the difference between the different things that create engine power. Inlet and exhaust gas flow are essential. So is ignition timing...BUT cutting the ignition (or fuel) to limit the engines revs is NOT the same as it running out of breath due to lack of air from gas flow

Tell you what...as an example: With the ignition ON, so that sparks still flow, and with a full tank of fuel......go and totally block the air intake to your engine. TOTALLY BLOCK IT. SO ZERO air can get into it.

NOW try to push it

Bet it goes nowhere

[Konan555]

I was trying to say you'd normally hit the limiter (be it a spark cut, fuel cut or half fuel limit) before you hit the limit of aspiration.

8000 sounded a little low, that was all. And there'd be bits of con rod and stuff to worry about first.

I know this is in the context of cams etc etc, it was a tounge in cheek point (bit like OHV).

[Zad]

Some big diesel truck engines use an exhaust brake which effectively closes the exhaust pipe, and it most certainly does not stop the engine turning! On a petrol engine with a much lower compression than a diesel, the air would just act like a spring, the engine would continue to rotate and provide minimal retardation.

Drive a Fester to 50mph, drop it into 1st, and the engine *will* do 8000 rpm. Possibly not for long though. On many engines, they will not deliver meaningful power above a certain speed because of valve bounce. The spring cannot shut it in time and so compression is lost (spring resonance may also be a problem). Fit stronger springs and the same engine may well achieve 8000 rpm (although at lower power output due to friction and may suffer oil starvation etc etc). By the way, the theory about hydraulic valves is fine, but what usually happens is that a film of varnish eventually builds up on the "bucket" (or piston) and they can't operate properly and start rattling.

One big reason that performance engines idle at such a high speed is that the flywheel has been lightened to improve response.
Zak edit: this bit here \/ is beautifully worded
On many engines, when the piston has gone past bottom dead centre on the induction stroke, the inlet valve will remain open. The inrushing fuel/air has inertia and will continue to flow inwards for a short time. Similarly, the exhaust will still be flowing out after top dead centre on the exhaust stroke. This inertia can also be used to suck the air/fuel in, so it is not unusual to see exhaust/inlet cams overlap. This overlap can be increased / reduced by using variable valve timing.

Some "multi valve" engines such as the Duratec V6 24V use a different approach, each of the 2 inlet valves is fed from a different source, and 1 of the inlets has a butterfly valve in it, which closes it off below 3500 rpm or so, making it work like a conventional single cam engine with better low end response, fuel mixing and economy. Above that, and its a snarling 4 valve per cylinder engine with better breathing. Another method, that BMW use is a variable valve lift to replace the inlet throttle butterfly altogether.

Mike (ready for questions and any corrections...)

Incidentally, I think that the animation of that supercharger is wrong, the air in that sort of supercharger doesnt go in the front and down to the back like a fan, it goes around the sides.

[Shad]

"Closeup of a Honda VTEC Rocker Arm Assembly - 3 rockers for each pair of valves. At lower RPM's the outer two rockers operate off stock camshaft lobes. At higher RPM, the VTEC solenoid kicks in and allows oil pressure to flow to the center VTEC rocker. This rocker rides on a higher lift and duration camshaft lobe. The oil pressure now engages tiny 'clutches' that grab the two outer rockers, and open the valves to the now higher cam specs."



Zak edit: NICE PICCY MAN

[badass]

Incidentally, I think that the animation of that supercharger is wrong, the air in that sort of supercharger doesnt go in the front and down to the back like a fan, it goes around the sides.

That animation is correct. Thats a twin screw whipple type superchrager. Notice the 2 rotors are sifferent shapes.
Its roots type ones where it sort of goes around the sides.

[Zak33]

Zad, cheers for the input

The exhaust brake on a truck WILL eventually stall the engine, if left on for long enough IF IT WAS A STRONG ENOUGH SEAL. But they're not. It does not stop inlet, only exhaust, so the theory would be that eventually it simply had too much air in the bores to move anymore!

I have tried to turn over a car with all the valves shut (no cams in it), by hand with the biggest lever on the crank case, after I'd ground the valves by hand all afternoon. It was a Sunbeam Ti engine with a fiercesome compression ratio and beautiful valves and head.

It would not budge easily. It was quite astonishing how much resistance it put up. The air finally made it out, round the piston rings and past the valve stems, but man it was hard.

The flywheel theory is fine if it's been lightened sufficiently, but in normal context its the cams that are causing the trouble. Leaving the valves open with such long overlap (as per your very excellent explanation which kicks mine in the ass ) causes a very unstable engine and the tickover needs raising.

[jon bda]

Leaving the valves open with such long overlap (as per your very excellent explanation which kicks mine in the ass ) causes a very unstable engine and the tickover needs raising.

Which equals lumpy and rorty and sounds sexy...