I just took out my 305 engine to replace it with my 350. As we did, we saw what my dad could only describe as a butterfly valve on the passenger side exhaust. Can someone tell me what this does and if it is important to leave it on or not??? Thanks.

 

It is called the EFE valve, or "Early Fuel Evaporation". It blocks off one side of the exhaust when the engine is cold, forcing the exhaust through a passage under the intake and out the other head, said passage being typically called "exhaust cross-over" passages, or cross-overs. It warms the floor of the intake plenum to help avoid fuel puddling before the engine has warmed up. As the engine warms up, the vacuum that runs the valve is cut off by a thermal vacuum switch, and the valve opens up.

Are you installing headers? Most headers eliminate the valve, Edelbrock TES being one exception. Those living in Kalifornia need it to pass emissions visual inspection. I passed a rather strict visual here without it twice - guess they either don't realize it is supposed to be there, or don't care. In Kalifornia, they care - no clue about STL, however.