NefMoto

Hello to all, i recently had a conversation about using stock cams from a NA car (1.8 20V) to a turbo engine (1.8t 20VT). It is a popular low budjet conversion here in Greece. Has anyone done something similar is there any power gain? What should i change to tune such a modification?

I am not convinced running NA cams is a good idea.  Generally with turbos and cams, Duration is the single most important event, with overlap second, and lift third.

Usually turbo cams use more exhaust duration than intake duration, to help prevent reversion (exhaust gas entering the intake).  Basically you are trying to manage the pressure ratio in the exhaust.  By using split duration cams with more duration on the exhaust side, it will help to prevent burned gasses from exiting the combustion chamber into the intake (reversion), since the intake pressure is lower than the exhaust pressure.  NA cams typically have more intake duration than exhaust duration, which is opposite of what you need with a turbo setup.

Overlap also plays into this - Part of optimizing cams for an NA setup is using the exhaust velocity to create a scavenging effect where the exhaust and intake valves are momentarily open at the same time.  Using this cam timing strategy, exiting exhaust gasses under high velocity actually creates a vacuum in the combustion chamber with both valves open, which aids in filling the cylinder with more fuel/air.  On a turbo motor, the optimal cam timing is keeping the intake valve closed, and then opening the exhaust valve after top dead center, to prevent reversion, which also lowers the combustion chamber pressure below intake manifold pressure.

When it comes to lift - if the heads don't flow more at .400" lift than they do at .300" or .350" lift, you're not going to gain much of anything.  But if your heads flow more at higher lift, then you will definitely see improved performance.

If you have a big turbo, retarding the stock cams will shift the power band into the upper rev range, and you will lose some low end torque in sacrifice.

Really good info!  :)

Thanks Jason...The setup that most of the guys seem to go for is the stock turbo intake cam and 'ADR' NA exhaust cam...

Thank you very much for the answer it was a great expanation. :)

totaly correct Jason!

Greeting from Greece,
Fotis