Engine Rebuild.. Fuel Economy?

[brucey]

So I just remembered my engine isn't stock, I had it rebuilt a few years ago. Bored .40 over stock, so its technically a larger engine. Would this be hurting or helping my fuel mileage? I'm getting pretty close to 35 mpg with no real mods, so 40 mpg with some work seems possible for me still.

Just curious what you guys think it would do. I am thinking more displacement = more gas, but I'm not an automotive engineer.

[brucey]

Thats what I was thinking too, but it came up and I wanted to make sure. Best case theres no difference, worse case theres a minor difference for the worse from an OEM engine.

[ttoyoda]

What did you have done to the camshafts.. If you had more overlap put in (is the idle kind of lopey or rough) then I would expect power to go up and mileage to go down.

[brucey]

The power did go up, it was actually quite noticeable increase in pep.

[ttoyoda]

I mean, did you ask them to put a new grind on the camshafts? Or did they or you index the camshafts differently relative to each other relative to what is stock spec for the car? Did you put the motor together yourself after the machine work was done?

[Bror Jace]

In the 60s, when representatives from Chevrolet were asked why they put a 454 in the 'Vette instead of the 427, they said they bored it out to "save weight."

Obviously, the weight savings is insignificant ... and the extra displacement (I think you bored it .040"?) is just a few cubic inches ... but if that is 1% then you will probably be close to 1% more power and 1% more fuel consumption.

The increase in pep was probably better compression, not increased displacement.

[Duffman]

Bror, I think you have your engines mixed up because 427 to 454 was a pure stroke increase.

http://en.wikipedia.org/wiki/Chevrolet_Big-Block_engine

I am sure somebody said in in relation to something though.

[wagonman76]

Im not sure what a larger bore would do. I know that I have had 2 examples of the same car which gets better mileage with a larger engine. The 3.1 is a stroked 2.8 and in my case gets 2-3 mpg better in the same car driving the same roads.

90 Pontiac 6000 wagon with 3.1 got 30 highway
89 Pontiac 6000 wagon with 2.8 got 28 highway
Both have 4 speed automatic with 3.33 final drive
Both have same options and are basically the same car

89 Celebrity with 2.8 got 29 highway
Same 89 Celebrity with 3.1 got 32 highway
Same 4 speed automatic with 3.33 final drive

A stroked engine gets the benefit of a longer lever arm in addition to more displacement for extra torque.

These are just my experiences. Other cars may differ greatly.

[Blue07CivicEX]

Final Drive? I am only moderately car literate, when it comes to trannies (car part not crossdressers) do you want a high final drive (my civic is 4.294) or a low final drive?

[bhazard]

A higher numerical final drive (its actually 'lower' or 'shorter' gearing) means higher rpms at any speed. You'll have better acceleration and a lower top speed. City mpg could even go up, but highway mpg may go down.

A lower numerical final drive (which is 'higher' or 'taller' gearing) gives you lower rpms at any speed. Pretty much the opposite of shorter gearing. Highway mpg should go up, city mpg depends, you may end up lugging your engine more.