like everyone else has been saying a 4barrel is only more fuel efficient if you can keep you foot off of it but speaking from experience it is extremely hard. when i got my 81 it had a holley 850cfm double pumper which could dump gas quick. never actually drove it on the road with that 850 on there but i am glad someone gave me a 570cfm holley. on average i only get about 10mpg with it on there. i got a carter carb sitting in the basement that i am thinking about trying next.

 

I get about 10 mpg now on my 318 with a broken carter that drips gas. When I get my new carter and have it tuned I'll let you know the kind of mpg I get. If I'm not mistaken you have a 318 from a 1977 diplomat too right?

 

i have a 318 out of a 85 and a intake manifold off a 77. i am not exactly sure what they came out of since i didn't do the swap.

 

I'm not 100% sure myself on mine either but when I looked up the block numbers diplomats came up. Did you put on the intake from a 77 360 to fit the 4bbl or did you use an adapter?

 

the truck came with the intake but from the numbers on it i think it might have came off of a 360. since the engine is from a 85 i was not too worried about the ports on the heads being smaller than the ones on the intake. i think in the 80's they started using the same heads but different size valves between the 318's and 360's. not too sure where a 77 318 falls. there is an adapter on my intake to go from carter 4barrel to Holley 4 barrel but that is it.

 

I was doing a lot of research on 360 intakes and couldn't find a clear answer on if the edelbrock 1404 would bolt on without an adapter. I didn't know these 318s could run on a carb as big as 850. I've only ever read guys run up to 600.

 

i think anything bigger than a 600 and you would just be dumping fuel out the tailpipe.