Brent V

I've just been doing misc research and was curious how the OEM cams are configured (overlap, lift, duration) and how changes could be made to maximize the benefit of having a larger turbo that theoretically should be creating less exhaust backpressure. Anyone have a link or cared to do any of the same kind of research?

201-A4

Inside a turbo motor the pressure in the exhaust manifold is greater than the pressure in the intake manifold. If the valves are allowed to overlap for too long, reversion can occur. For this reason, turbo cars are typically equipped with relatively mild overlap, short duration cams. This limits the opportunity for exhaust gasses to reenter the combustion chamber. Unless a larger turbo can reduce the exhaust manifold pressure to less than the intake manifold pressure (no turbo can do this), a longer duration, larger overlap cam is not going to do your motor much good. This is one of the reasons that you see a lot of the 1.8T guys (and many others) making over double the factory rated horsepower on stock cams.

PS: If what I have written here is old news to you, disregard it. I just thought Id share some info that I learned in my turbo-machinery class. :-)

Modern_man

<img src="http://bobqzzi.imgbay.com/ExhaustCamChart.jpg">
<img src="http://bobqzzi.imgbay.com/InletCamchart.jpg">
Thia was posted at VWVortex in the 1.8T engine forum.....

Ray Khan

you thinking of playing with cam timing? or going with cams?

Brent V

Since my exhaust leaks like a **** and my car runs like **** I'd like to know if the leaks are what's causing the performance hit and why.

Some people I've talked to say it's the oxygen sensor giving skewed readings. Other things I've read have indicated that if leaks are present and too much backpressure is lost there is a chance that the intake charge will have a higher pressure than the exhaust and blow fresh air/fuel right past the exhaust valves while there's overlap.

Still others believe that it is impossible to have less exhaust pressure than intake pressure and that an open exhaust behind the turbo is best way to make more power.

I think that low-end tq will suffer from any kind of leak and that even though there's a turbo restricting the flow of exhaust gasses, it's only when the exhaust gas volume reaches a certain point that the turbo begins to be a restriction and adds to the exhuast pressure.

What does everyone else think?

Ray Khan

I know what you mean, I hear all these confilcting theories that all seem to make sense the way people describe them.