All that torque?
You didn't mention any turbo................
So what are we talking about here?
IMO; you'll get more mpgs, out of your engine at cruise-rpm, simply by giving it the cruise-timing it craves. The factory D is not likely to be able to supply enough cruise-timing, no matter how you modify it. But more cylinder pressure is almost always a good thing. The thing is, at cruising speed, you can adjust the Dynamic cylinder pressure with the gas-pedal.
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Warning; opinion coming;
You may already know all what's coming. If so, I'm sorry to waste your time.
Lets suppose your combo in it's current state, takes 30hp to maintain 65mph. At .45 pound of fuel per hp per hour, that would be about 13.5 pounds of fuel or 2.25 gallons. In one hour, the car will have traveled 65 miles, so the simple math is 65/2.25=29mpgs. It is the tuners job to get the combo working as best as he can to achieve that.
With a given powertrain, about the best you can do is;
eliminate as much parasitic drag, wind-resistance, and mass as possible, and
try to get all the fuel burned up and transferring as much of the expanding gas pressure to the flywheel, as is possible, at exactly the right time................ This is the KEY.
CCP at cranking rpm is not part of this equation because in steady-state cruising, the gas pedal will adjust the pressure to whatever the combo needs. And whatever the throttle-position ends up being, the tuners job is to try and put the right amount of fuel into the intake-air, to make the 30hp.
If you increase the CCP by say 30 percent, at cranking speed, that just means that it will take less throttle to make the required pressure. The car will still need 30hp to go 65mph.
Now, if the transfers cannot make 30 hp, and she has to get up on the mains, what then? Your mains have to be a certain size to satisfy the engine's WOT needs. What if at 65mph, that leads to the combination of; mains plus transfers plus mixture screws, being too fat? What is the solution to that?
I mean IDK.
Well I have an idea, but you'll figure it out; maybe you already have ........
The point is this;
If you make more than one change atta time, and get an improvement in mpgs;
how will you know which change made the improvement?
Maybe you get no improvement; what then?
Suppose one change made an improvement, but a second negated it with an equal and opposite loss of improvement. You may never know it.
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But here's a thought;
Suppose your combo requires 30 hp to go 65. What size engine is required to do that?
I mean; could you do it with a 318/340/360/400 ?
Well of course you could; but each bigger sized engine is gonna bring more parasitic drag, and combustion inefficiencies to the game. If you can run the bigger engine at a lower rpm, suppose you break even? A lotta people will say no you cannot break even. Well they might be right. But then again, they might not.
What if your combo takes 30 hp, but only because 7 of it is due to parasitic drag inside the engine? That's 7 hp for 6 cylinders. You might assume then, that an equally sized 8 cylinder ( now; 225/6 x8=300 cubes), would lose 7/6 x 8= 9.33hp to friction. So now you need 9.33 -7 +30= 32.33hp which will make the mpgs to be 26.8. Ouch that hurt right,lol. But hang on, say yur combo is running 3.23s, so 65=3000rpm@4% slip with 24.5" tires.
What if you regear your little 300V8 with 2.76s and run a taller tire, to get 65=2350? Would you get your mpgs back?
What if you install a 367/ manual trans/ with overdrive, and drop the rpm to 65=1600? Maybe you get the parasitic down enough to make 32mpgs?
I mean, I'm just crazy spitballing here......................
