Pressure in Valve Chamber

[EXtremeAccord]

What does it mean if you have air blowing out when you take your oil cap off?
I know there's too much pressure in the valve chamber, but why?

 

[highperboi]

i dont think its bad my engine does it too. i have never taken oil cap off it just shoots out of that hole in the valve cover. It didnt do it so much when it was stock but after i put new pistons in it it did it more.