[some_other_dave]

Originally Posted by bp944

1-Does lightening internals actually better gas mileage (about 10 lbs lighter)?

It should, but it wouldn't surprise me if the difference is too small to notice.

2-Should I go with a 5.3, 5.7, or 6.0L engine?

Small for fuel efficiency, large for max power and/or torque.

3-I assume higher compression is a must?

I would expect so.

4-Higher flow heads give one a better gas mileage or not?

Heads that have good port velocity at your freeway engine speed will help fuel economy. Heads that flow tons at WOT and high RPMs usually don't have good velocity at low RPMs and throttle settings, which where you are on the freeway.

5-Larger fuel injectors use more fuel so they must weaken mpg?

Not really--to a point. If the injectors are so large that mixture control at low throttle and RPM is poor, the fuel economy will likely be worse. If, for instance, your injectors are at 40% duty cycle at WOT at the red line, they may produce uneven mixtures at cruise or idle.

6-Does one want to shorten the stroke or lengthen it?

Depends on the displacement you want and what shape you want the torque curve to be. In general, the longer stroke motors will make better low-end, while the shorter stroke motors will rev a little more easily. Short stroke and long rod is an interesting combination, as it will have lower side-loads on the pistons, so less friction. Might be worth a look, might be too small a difference to care about.

7-Does the piston shape (dome, flattop) have any effect on mpg?

Yes, it should have some effect. It will also allow other mods that also have an effect. The piston crown is effectively the bottom of the combustion chamber, so it has to work with chamber shape. Flame front movement, "squish", mixture quality, and lots of other factors all play off of each other here.

8-What exactly is the best hp curve to aim for with a cam? Should it be a very smooth curve or peaky? Does one want a low hp at low rpm so it uses less fuel?

For the highest power, spin the motor fast. In order for it to breathe well at high RPMs, it will usually lose low-end torque, so the curve will be fairly peaky. In general the lower-RPM cams will be less peaky.

If you want freeway MPG, then you want an engine that makes good torque at a fairly low RPM (<2000, possibly much less for a big V8) and a transmission geared so that freeway speed is right there. Let's see, what does the C6 Vette motor turn on the freeway? I think I read 1600 RPM, so that sounds like a good figure to shoot for. Even lower revs might be good.

That may be tough to do with the 944 trans. I'm not sure they made alternative gear sets for it like they did the 901-based boxes (e.g., early 911). Those have several dozen ratios to choose from, and the drive gears and driven gears can be swapped to invert the ratios in many cases! Not sure about the 944, though.

If you can't get the revs down on the freeway, your MPGs will suffer significantly. So check into gearing choices. You may also find that 400+ HP will be more than the 944 box can deal with for very long.

Sounds like a fun project, good luck with it!

-soD