Can  anyone tell me is one better than the other? Vented vs non-vented gas caps.  What came on the cars originally?  FYI - I have a 73.

Thanks,

Bret 

If your car was originally equipped with a charcoal cannister the tank venting occurs through that, so you should have a non-vented cap. If the car does not have a cannister (no vapor line to the engine bay) then it should have a vented cap.

Thank you big fish.

 

Bret

As your engine draws down the fuel in the tank, air has to replace the fuel previously there. The engine likes vacuum...the gas tank doesn't..if you put a non-vented cap on a car that requires a vented cap, the gas tank can eventually collapse with a big bang!

I believe up to late 69 some earley 70's had vented caps, late 70's and up with EEC cannisters were non vented.

its located bolted inside the drivers side inner fender in line with the LF tire, if you have the canister also known as the EEC then the non-vented cap is the correct cap, its round, about the size of a 3 lb. coffee can, plastic, black in color and has vac. hose going into it from the gas tank and out to the carb. and teeing off to the PVC valve unless GM changed it for 74.
don' mistake it for the vac. canister which operates your headlite doors which is square in shape, metal, gloss black and is bolted directley above the EEC.anips38495.5289467593

Great description, thank you very much.  I was able to dbl-check in 2 minutes and found that I did buy the right fuel cap (beginner's luck!). And of top of it, now I know what that cylinder is!