Hello, I am currently looking at a 91 Firebird. Its clean and cheap but it needs an engine swap due to Rod Knock. It currently has 305 with an auto. I was thinking about just swapping another 305 in but I want slightly better gas mileage if possible. I'm sure there may be some mods but I have always liked the 3800 S/C.

Was curious if anyone ever swapped one of them in, and how hard it was. (This is a common swap in fieros which is what I am familar with)

Or if you know some good 305 mods for better mpg. Looking to make a daily driver at about 25mpg, but can get up and go when I put my foot into it.

 

Since you already have EFI that makes it a little easier. I know the 3100 and 3800 motors got pretty good mileage. It would be a big weight reduction on the car so expect to have to drop the suspension in front an inch. Anything with forced induction will generally get better gas mileage than equal HP/TRQ with more CI. I think 25mpg is about the max you could get based on weight and gearing. I can't give anymore info but I say go for it, should be a cool project. OR you could try to swap in the older style 3800 from a T-type/GN car. Turbos will provide even better mpg and more HP on equal boost.