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?

[MO]

Only one way to find out!!!!!

[NC_Steve]

If the car is running lean to begin with it might have some benefit.

[351ciofgrunt]

If the car was tuned correctly in the first place then extra fuel vapour would hurt rather than help...as said above, would only help if the car was running lean

[xdclevo]

It would be true for sure on an old carbied car. I recently fitted a larger breather to my tank recently, just to make sure. It would be easy to try on the dyno, by doing a run with the cap on, then take it off.
If it is effeciant allready, it wont help, but if it is super restrictive, you won't be getting the propper fuel delivery, especially at WOT, as this is when it will move the most fuel.

[Mike Gayner]

Just slightly off topic, I read a year or two back in Street Machine in an article about Top Fuelers that their breathers are set up facing towards the front of the vehicle to create a ram-air effect into the fuel tank, and without it the fuel pump is so powerful is would crush the fuel tank with the vacuum.

[LTDHO]

This will explain why cars may be less effiecent when close to emtpy. I might run a 1/2 inch breather line! ha ha, seriously, trhough I might lookk at the breather as the Vitara needs every HP it can get!