strawtarget

NOTE: I don't know the proper names for engine parts, so my question is not expressed very well. I hope you can figure it out.

I've seen photos of an open 1.8T, and you can see the chain and wheel used to vary the cam timing. When you pop the hood, you can see a bulge on one side of the engine.. that bulge contains this assembly.

My basic understanding of a cam is that it's a rod with bumps on it. As it turns, the bumps lift the vales. My understanding of cam timing is weak. Basically, it controls how soon (or late) the valves open? Does the optimal valve open/close point change at higher RPMs?

How does the chain assembly work? Does it simply rotate the cams (we have 2, right?) forward or back a small amount according to what the ECU asks for? Perhaps by some kind of pulley?

Thanks. Just trying to learn a thing or two.

raVen

<center><img src="http://pictureposter.audiworld.com/410/vvt.jpg"></center><p>

hoTTub

<b>Does the optimal valve open/close point change at higher RPMs? </b>

Yes, that is why the Honda V-Tech makes so much more power (relatively speaking =P) compared to the non V-Tech. It changes the valve timing for the optimal induction/expulsion of air.

<b>How does the chain assembly work? </b>

I am sure that someone will correct me if I am wrong but I think that the chain (which does spin the cams) has a variable tensioner that changes the length of the chain, which in turn changes the timing of open/close cycle.

As a side note, turbo motors DO NOT like valve overlap (when the intake/exhaust valves are open at the same time) since the boost is blown right through instead of loading up the combustion chamber

HTH

hoTTub

helped me =P

raVen

<center><img src="http://pictureposter.audiworld.com/410/vvt2a.jpg"></center><p>

strawtarget

Very much so! Thanks!!

Agent911

as you know cam is the brain of the engine (some must disagree, but shut up

you have two cams, one for intake, and one for exhaust.

cams are rods with bumps, those bumps press on the spring to open/close valves which open to let air/fuel in, and other vales open to let exhaust out.

cams are mechanical, and fixed to the engine speed by a chain rotating ralative to each other. the duration of the open-close period can not be changed.

problem arises when your engine is in high rpm... the valves open-close just too fast for intake and exhaust - you lose torque and untimatly power.

the ferraris, and new 7 series bmw have true variable cams - ferrari = mechanical, bmw = electrical. they change the open-close timings for max torque in each RPM range - makes good power out of small engine and saves fuel.

v-tech have two sets of cams. one for lower rpm - and when you reach near high rpm, different sets of cams kick in - they call this v-tech, but really it should be just called 2 stage cams.

any other variable cam timing - i.e. changing when cam opens - but can not set different open - close time, are not variable cam timings. and frankly, they are more or less useless.

its like this - if you want huge power out of 1.8T, it would be possible on the higher RPM. so you would get lower compression pistons (to let more fuel and air in) then you would get bigger turbo for more air flow, and ultimatly a different set of cams that stays open maximum period to let most air/fuel in and exhaust out. and hey, you could do 15,000 RPM with peak HP at that point.. but i doubt anyone would do it.