i have an 84 z28 with a 283 with 041 heads a cam with .422/.444 lift and 204/214 duration at .050, edelbrock 600 cfm carb and edelbrock performer intake, and short tube headers. i want an engine with more torque but i don't want it to use a ton of gas. if i put a 400 in built about the same would it use a lot more gas driving in the city? i know it would use more highway because the engine isn't under much of a load and that much more air is being pumped through. but with city driving wouldn't it be close to the same because of the torque and the engine dosn't have to work as hard?

also, i have a th2004r with a .67 OD and a 3.23 rear. the 283 has a hard time pulling it in OD lower in the rpms so i mostly run it in drive when driving in the city.

 

My 400 used to get 24 on the highway, and on one trip to LA, from Park City, I got 25.x! This was with EFI(see sig for combo) so don't expect to do that well, but my point is that the milage didn't suffer at all from the original 305, to the 350 I had and then to the 400. it remained relatively constant for all three engines.

On a side note, I did drop to 12 mph around town with the 400, but that was due to mods to the CFI intake that caused pooling when cold (start it cold drive to work, shut it off), and the fact that I was always romping on it.

 

the size of an engine plays a very small part in the big milage picture. gearing, how you drive, tire preasure, and any number of other factors all play into the equation

 

Put some gear in the car.

Put in 4.56's if your car has 26" rear tires.

Put in 4.10's if your car has 24"/25" tall tires.

Now your 283 will feel like a 400 and still be better on gas.

And your cruising rpm while on the Hi way while in OD will be just right.

It's too low right now.

 

That 400 engine with similiar specs to the 283 isn't going to kill mileage. Mileage is all about efficiency IMO. Then using the correct gears to keep the engine at peak efficiency. Then lots of other things like a light right foot, tire pressure, etc... The cam can be a huge mileage killer if it's too big. That cool rough idle is a gas eater. Look at it this way, how much mileage difference exists between a stock 305 and 350--if driven the same not much if any. I would build the 400 mild and enjoy the torque. Oh and a bigger engine(400) at the same rpm holding a constant speed down the highway isn't making any more power than a smaller engine(283) at the rear wheels. So 'about' the same about of fuel and air are used to create the same about of power. Some more is lost due to friction in the 400, and of course the mere fact you have more power means you are tempted to use it.