Deep Singh

I have no idea what cams do,can someone explain? They are defined by duration and lift,what does that mean?
Also I d'ont see the point of the variable cam on the STi7.On most cars it improves the driveabilty,most STi7 owners who have owned a classic STi will agree that it is less driveable(until remapped).Fuel economy is also no better so where is the advantage?

Pogue

The cams operate the valves via tappets pushrods or shims. The cam is linked to the crankshaft via a belt/chain and turns at half the revolutions of the crankshaft. The shape of the cam lobe determines the "lift" and "duration" (the time the valve is open or closed)

dhorwich

Here go mate..!!!

see here

Dan

john banks

The STi has variable intake cam timing mapped by RPM and load.

I am by no means a cam expert but my limited understanding is that this allows you to have less of a compromise between for example idle and high power/RPM areas, helping emissions and power from volumetric efficiency. You want low overlap at idle so that the hydrocarbon rich charge doesn't just go out of the exhaust and pollute. On a turbo engine you also generally want low overlap because you don't want the exhaust back pressure from the turbo (high on small turbos as fitted from factory) to cause mixture of exhaust gas and fresh charge since this reduces power and can lead to detonation. More lift and duration are helpful at high RPM so that you have a wide open channel for as long as possible to get the charge in in the shorter time available.

nom

So... the outcome is, it's likely that much of the valve goings-on with the sti7 are to allow it to pass emissions tests - so reach a wider market - rather than for performance. Although the cams can be made advantage of (performance-wise) afterwards once the emissions thing has gone (ie once it's sold).

hypoluxa

Camshafts control the opening and closing of the valves. This allows the air/fuel to enter and exit the combustion chamber.

You have 4 cams, 2 control the inlet valves and 2 control the exhaust valves). There are 4 valves per cylinder, 2 to allow the mixture in prior to the big bang and 2 to let the mixture (now exhaust gas) out.

If we ignore timing, duration and lift for the moment...

We would expect the inlet valves to open when the piston is at top dead center after the exhaust stroke, and close at bottom dead center (the induction stroke) This allows the air fuel mixture to enter the chamber. With the valves now closed the compression stroke takes place (piston travels up) followed by the power stroke (piston forced down). At the end of the power stroke (piston bottom dead center) the exhaust valves open to allow the spent charge to exit the cylinder and stay open until the piston reaches top dead center (exhaust stroke). At this point the cycle starts again with the inlet valves opening to allow the fresh charge in etc.

In the above example the duration of both the inlet and outlet cams is 180 degrees (of crankshaft rotation).

In the real world however the valve train has mass and the air/fuel charge inertia, so the ideal 180 degrees becomes simplistic, hence the inlet valves will open before TDC, and close after BDC, in a similar way the exhaust valves open before BDC and close after TDC.

The timing of all these events - opening, closing (and therefor duration), lift (how far the valve is lifted of its seat) and overlap (when all valves are open - end of exhaust stroke/beginning of the induction stroke) are controlled by the cams and are critical to engine performance.

MorayMackenzie

Deep,

If you were to swap your pesky overcomplicated variable cam overlapping jigger pokery engine for a super cool classic one, I suspect you might notice a difference. Especially considering the extra weight the later engine has to pull... remember when comparing the driveability "feel" of the v7 to that of a classic that the same capacity engine is pulling a fair bit more weight around in the v7. I suspect an STI7 engine in a classic would do a better job than a classic engine in the V7.

Moray

Turbo_Steve

Is the STi s variable timimng the same as VTEC, i.e. changing the cam lobes at specific revs / load to improve gas flow & power output (at the expense of torque, in the case of VTEC)?

Does this mean it can be used in combination with the turbo charger for a large power hike at higher revs?

john banks

It is far simpler. It is only on the intake side, lift is not adjustable, duration is not adjustable. The only thing adjustable is the advance of the intake cam - ie when it happens. It does affect overlap though because of this.

Deep Singh

Thanks guys,a little complicated for my low mass brain but I think I get it.
If it was for emissions(mainly) then this has'nt really worked because no where outside of Japan(?) got the 280 BHP version and they had to be further detuned/fettled to pass emissions.
John,have you tried playing with the cam timing when Ecuteking a 7?

john banks

Not yet.

MorayMackenzie

Deep,

I think that Pat has played with cam overlap on a 7... maybe you coulad ask him about it.

Moray

Tino

The clicky link from the third top post activated Spybot on my PC.It contains something called Avenue A. Inc (some kind of spyware)Just thought you'd like to know.
TTFN

dhorwich

By the way i had no idea about the link... if you want me to delete it i will..!

Dan

Deep Singh

Hi Moray.I did'nt realise he had yet! Feel a bit bad about ringing him up all the time when he's up to his eyes in bits of engines!