is the return fuel line necessary

on a big block mopar or it it just for smog?

I just put mine back in in stainless on my smallblock A-body on the assumption that the fuel tank needed that line for venting , I didn't see any other form of tank vent on mine..

this is the line that goes from the fuel filter
back to the gas tank

Are we talking a fuel injection system or a carb?

Mechinacal pump or Electric?

Where is the fuel filter located exactly and the year / model of vehicle would also be helpful.

It helps eliminate vapor lock.

carb --vapor lock ? i do not have the line on my
340 or 318 or 383 cars

I think they were only used on the 440's and Hemi's. At least in 1970 they were.

It's a vapor seperator, helps prevent vapor lock and sends the fuel vapors back to the gas tank. You can eliminate this piece and the return line but I would not suggest it.

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carb --vapor lock ? i do not have the line on my
340 or 318 or 383 cars

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They were used on the bigger engines that needed higher volumes of gas,they were more prone to vapor lock.
It a great thing to have with any size engine exspecially with todays gas.