my 318 has a 2bbl but i'm wanting to go to a 4bbl carb because mine needs rebuilt and i thought about the swap since i'll have it all apart any how

what should i expect as far as fuel consumption and performance?

 

putting a 4 barrel carb on your truck will most likely hurt your fuel mileage unless you can keep you foot off the gas. currently i have 570cfm Holley street avenger carb on my 81 and it works good and seems to have a fair bit of power and it is definitely way more fuel efficient than the 850cfm Holley that came on the truck when i bought it.

 

So we now know that CW is a lead foot. Welcome to the club. CW is correct that a lead footed person will experience poorer mileage with a 4 barrel while grandma will experience better mileage with a 4 barrel. (which are you)

Here is why a 4 barrel can be more efficient. The primaries on a 4 barrel are smaller than in a 2 barrel and this uses less gas when you are idling, slowly accelerating, or driving slowly because the secondaries are closed. The primaries and secondaries put together on a 4 barrel can flow more CFM than a properly sized 2 barrel and this gives more power. A properly sized 4 barrel will give more power and a better idle than a 2 barrel because you basically have two separate carbs in one unit. The 2 barrel must compromise between power and idle while a 4 barrel is designed to optimize at both ends.

 

i'd say i'll be ok 80% of the time. i'm kinda worried about projected fuel prices later in the year, $5 a gallon is gonna kill me with the 2bbl