Lets talk "Power Adders"

[oil pan 4]

With a gas engine you are always fighting the isobaric expansion (throttling losses) that starts off the otto cycle.
The only way I know to beat isobaric losses on a gas engine is to put a tiny effieient little engine in a car, like a twin cylinder 15hp briggs and straton and run it wide open on the highway your top speed will be about 60mph until you encounter a hill.
There is no way around it. If you get more air through the gasoline engine no matter if you use a free flowing exhaust or intake, you have to add more fuel to burn it properly.

See:
Cold Air and Ram Air - EcoModder
Increased compression ratio - EcoModder (click the popular hot rodding link at the end of the page)
Turbo charging - EcoModder
Water injection - EcoModder

Most things that give a gas engine more power typically lowers fuel economy, but on a diesel engine you get more power and more fuel economy most of the time.

[wmjinman]

Originally Posted by oil pan 4

Most things that give a gas engine more power typically lowers fuel economy. . . .

Unless it's strictly efficiency things that have more power and fuel economy as side effects of lower losses, right? Things like lighter/slicker oil(s), better combustion efficiency (higher compression with effective knock sensors/suppression), super-precise fuel injection, variable timing, etc....

Then I'd expect to see *slight* increases in power at the dyno, and also *slight* increases in fuel economy. But I think basically, if the engine's halfway efficient already (which also means running well and properly tuned), engine improvements are probably not where to find much in the way of increased MPG (short of completely changing the engine). Rather, aero for highway speeds, weight for stop & go, and driving technique ("hypermiling") for both.

Do I have it right?

[Viturro]

Aero is maybe the best thing to do... but diesel is a good choice too, if you want to get to the best.
In clear, don't spend money on your gasoline engine, spend that money on the body, now if with the body isn't enough for you, you can change your engine to a diesel one, but spend money on a gasoline engine is maybe the money worst spent.
Intake and exhaust will help, because oem engine isn't on the limit, but with closer you get to limit you get close to the detonation ...that is a bad dude haha.

[wmjinman]

That's why I threw in the thing about "unless you completely change engines". That's what I did with my Swift - the engine was so bad that I completely replaced it. Now a small diesel, or even the smaller 3-cylinder engine, if I had found one, would have probably helped MPG even more, but I was in kind of a time crunch and a budget crunch, and I knew this identical replacement engine with super-low miles on it would be an easy and painless swap. And of course, I knew that a properly operating Swift, even with the "big" 4-cylinder engine, would give me great gas mileage - - maybe not world record type mileage, but something I'd be content with.

[t vago]

Originally Posted by wmjinman

Do I have it right?

Looks like it.

About the only things that can improve both power and economy of a gasoline engine are things that allow it to pass the existing air more easily, or that more precisely balance the amount of fuel going into each cylinder, or that allow fuel to burn more completely during the actual combustion. Any other mods will add power, but will sacrifice fuel efficiency in the process.