I was looking at the components to do a engine buildup and I realized that if you do it right you can take a lot of weight out of the engine. This is all rough math but a callies crank can be 5-15lbs lighter, a set of rods can be 1-2 pounds, pistons can be 1 pound. Along with a 5-10 pound lighter flywheel. Its almost conceivable to pull off 20 lbs on the moving internals. How much does that result in mpgs? I'd be doing this for power but if you had a stock internals and forged but were the same specs what would be the difference in mpg?

 

I wouldn't think it would help anything at a "steady state" cruise, a lighter rotating mass mainly aids in acceleration...

 

Why not? Any increase in efficiency and power will result in less throttle needed to keep the car going at a steady rate, thus increasing MPGs.

 

might give better mileage around town due to lower rotating inertial but the effects will likely be very small. No difference in steady state driving since loses are due to friction (bearings, rings/bore). It's possible there might be a slight reduction in piston side loading, but again, likely wouldn't show up in improved mileage.

 

i wouldn't think you would really see any better MPG

 

One thing you most likely will see is less durability.

 

I have taken about 42lb off my rotating mass, no diff in mpg.

 

I think it would help, but the results would probably be minimal.

One thing that helps for mileage is optimizing your timing tables.
I've seen many timing tables below 40 > 50 kpa flat to WOT. Yet there can be a few degrees added in the cruise regions without any detonation. Then after optimizing your timing table, you'll probably have to take out a little fuel as the fuel trims go positive cause the engine is more efficient.

 

learn to drive with a vaccuum guage

 

Just to clear it up, a steady state cruise requires a certain amount of HP (disregarding incidentals like wind, elevation changes, etc) to maintain a given speed. Messing with the rotating assembly will have zero effect on that required HP number. Hence, no change in the engine's HP output.