Fuel tank under vacuum?

[*rayman*]

I was having a few drinks with some mates the other night, and one of them was telling me about an article street machine magazine did recently covering this topic.

Basically he said they took an old commodore, and ran an extra breather on the fuel tank. Apparently gained 3-5hp or something... then ran another breather, and picked up a total of 8hp or so. I think from memory thats what he said.

So is this rubbish, or a real possibility? And does this only affect cars with a mechanical fuel pump?

My eb v8 has a very obvious sucking sound as the fuel cap is released, is this normal?