Steve: thanks for the emissions info, however my 57 has the regular pipe vent system that arcs up in side the quarter panel, & exits by the filler cap. ( I blew air thru it to ensure it was clear) So, it shouldn't need a vented cap. Considering current emission regulations, it just struck me odd that most of the gas caps available at the parts outfit, are vented...??  

Paul

I don't think it will be in the best interest of your fuel pump (among other things) to be trying to pull a vacuum on a closed tank. If it's desirable to create a more modern "semi-closed system" like the seventies cars had - in that era the traditional "vent line" was run back up under the hood to a charcoal canister - which was then vented by a small line into the air cleaner. This does a pretty fair job of decreasing the raw fuel fumes that the EPA ultimately determined was responsible for most of the visible smog. 

In states like California where visible yellow smog was a big part of the "emissions problem", they are so sensitive to this raw fuel fume situation that the gas pumps have a vacuum "seal" on the nozzle. This is a flexible tube that fits over the tank entry while the nozzle is in place and prevents gas vapors from escaping to atmosphere as the fuel fills the tank.

Too bad they couldn't just deal with the visible smog problem and leave the engine emissions alone. Simple, well tuned, efficent engines don't pollute very much and they use a lot less fuel (with a lot less expensive and complicated equipment fitted to our cars).

 
 Steve Metzger       Tucson, Arizona

Recently repaired the tank vent on my 57. As the gas cap is vented & the gasket isn't good, I decided to replace it with a non vented cap. However, am I labouring under a misconception? I thought that most modern cars had semi closed fuel systems & non vented gas caps. When I went to buy a cap from our local parts retailer, most of the caps they have on hand, are vented. What gives?   

Paul