So I read somewhere that insulating the fuel lines in the engine bay may contribute to increased horsepower when the engine is hot (or decreased loss of horsepower). What are your thoughts on this? I doubt this would have anything close to the same effect as adding intake spacers, but would it be worth it? Just askin' is all

 

Nissan already took care of that for you my friend. They installed the fuel pressure regulator in the tank instead of putting it a the end of the fuel rails. Therfore you dont return hot fuel to the tank and you are not warming up the fuel that will eventually get to the engine.

The idea behind this is to cool the mixture entering the combustion chamber, just like the spacer would avoid warming the air due to the intake to getting too hot.

A cooler mixture means you will better resist to detonation, so more timing can be added and leaner you can run. Same effect than if you run **** gas VS high octane.

 

Let's take a trip in the "way back machine"......

Way back in the day, a fella I used to know, had a mid/late-90's I guess it was, Mazda MX-6, and he was farting around with it, knew it wasn't terribly fast, but he had money to burn at the time, and went to playing around with it.

One of his more creative dragstrip tricks was he lengthened his fuel inlet line using just some old copper tubing, coiled it up like a snake, and dropped it into one of those big ole Folgers coffee cans he'd stuffed into his engine bay. When he got to the track, he'd fill that can up with crushed ice. I lol'd, until I saw the results:

plain as night and day. That damn car would consistently run 3-4 tenths faster with the fuel super-cooled by that ice bath, than with the can removed and the fuel heated by the engine bay.

Not something easily achieved in everyday driving, and I'm still trying to figure out exactly why that little trick made such a dramatic difference, I was expecting sublte, maybe a tenth or two, but a whole half second? I say the car wasn't terribly fast, but he'd run low 14's compared to high 14's when he was toying around at the track. It was that apparent.

FWIW....just sayin....

 

Either the ECM was seeing less detonation due to the cooler mixture and was adding timing, or your friend was tuning it consequently.

Dont forget that cooler mixture means also more dense. Therefore more fuel/oxygen could enter the chamber at every cycle.