Fuel rails see a lot of exposure to radiated heat since they sit atop the engine, plus smaller engine compartments mean less airflow to help move heat out. Lower-profile hood lines and factory engine covers on modern cars can also trap heat, despite composite intakes, aluminum heads and aluminum blocks that dissipate heat faster. And hot gasoline hurts performance.
As the temperature of fuel increases, it becomes more vaporous, and in a closed system it can build pressure that will cause fuel-flow irregularities, especially within the fuel rails. Hotter fuel is also less dense, so not as much fuel charge can be delivered to the cylinders, causing a potential lean issue that leads to detonation, making the computer to pull timing out of the ignition system to eliminate the detonation, which reduces horsepower. Want to know what reduced ignition timing does to power output? Just one degree removed from the engine’s normal programmed timing can equal a loss of 10-12 horse