I think there is a hint of a real eco-mod in
brent777's fuel cooling idea.
Reason I say this is because I know that returnless fuel systems act differently than return-style fuel systems. For instance, the returnless system on my truck goes to a horseshoe shaped fuel rail assembly. The fuel inlet is on the left bank, between the injectors for cylinders 3 and 5. The left fuel rail is connected to the right fuel rail via a hard line. So, at one end of the fuel rail assembly, fuel only has to travel about 8 inches or so to reach the fuel injector at the one end of the assembly, to reach cylinder 7. However, fuel has to travel something like 30 inches to travel to the other end of the assembly, at cylinder 8.
What does that mean? Well, at low speeds, the fuel absorbs heat from the engine as it travels through the fuel rail assembly. There is little air cooling to be had, since the fuel rail is somewhat buried under other engine components. As the fuel travels down toward cylinder 8, it absorbs more and more heat. And... as fuel absorbs heat, and its temperature increases, its density decreases. That makes it nearly impossible for the engine computer to deliver the same amount of fuel to each cylinder. You end up with some cylinders leaning out, and other cylinders going rich. This is not efficient.
Now, some later cars might have bandaged over the problem by putting a Y-fitting that feeds two rails from the center of the fuel rail assembly, or by using a pre-cat O2 sensor on each bank of a V- engine (as opposed to using just one pre-cat O2 sensor, period), but the problem's still there.
This leads to an early eco-mod of mine, before I even knew of this board.
This other truck board has my write-up here, and I saw a very noticeable increase in fuel economy immediately after installing the mod. Basically, I replaced the horseshoe with a branched setup. There's a Y that splits fuel equally to both sides' fuel rails at the center. This equalized temperatures across the fuel injectors tremendously.
It's not even my idea. Somebody who goes by the handle of
duner first came up with the idea almost a decade ago, for the exact same engine I have. However, he equalized his fuel rail temperatures by using chilled liquid water instead of Y-branching. He did a dyno run of before-and-after. Here's his chart.
As can plainly be seen,
duner was able to gain about 20 ft-lbf of torque throughout his torque curve.
I was able to go from about 14 MPG in mostly city driving to around 17 MPG, using my implementation.
One final thought - the idea isn't so much to cool off the fuel, but to ensure that all of the fuel injectors receive fuel at the same temperature, as much as possible. Temperature differentials between injectors are something to be avoided.