How should I mount the fans?

frets

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Hey, I am building a new rig but I have to use a old case as a temporary case. So I have 2 fans one is a 120mm and the other one's a 80mm which one should be the intake and which one the outtake?
 

frets

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It's a old ATX case but would it be good and safe if I put the 80mm as intake casue if I want to put a 120mm in the front I have to do some drilling and cutting on the case.
 

frets

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If I but a 120mm fan on the side as a intake and the 80mm on the back as a exhaust would that be good?
 
Most cases are negative pressure.

Removal of heat from the inside of the case is more important than trying to pump it full of air that can't be removed quick enough after it's been warmed by passing over the components.

Use the 80mm fan as the intake fan and the 120mm fan as the exhaust fan if the 120mm fan has the higher air flow rate.