Boost is a stat that represents the effect a boost will have on a machine. A high boost will go faster but will consume more power (Only in certain games)
Boost fires are only available in F-Zero Climax and the anime series. (Sort by weakest to strongest)
Boost Fire (Game version)
There are two techniques that are considered Boost Fire by the race commentator..
First, Boost Fire is when the player further boosts, which makes them go even faster. Each boost fire will make the glow yellow, and will cost one reactor point in Climax (One point gained per lap). You can use this to extend boost times to go twice as far in boost mode.
Second, Boost Fire occurs when the player activates a booster while the player is perfroming a spin attack. This technique is called Spin Booster by Bart Lemming, as he teaches you in a tutorial. The perk of this ability is that the player's attacking ability will increase as well as speed. In addition, the commentator will pronounce "Boost Fire!" slightly differently than using regular Boost Fire's reactors..
Regular Boost fire (Anime)
In the anime series, boost fire is boosting and spinning at the same time. There is no colour change to the vehicle's aura. This is possible to stimulate in Climax by spinning then boosting, but it doesn't really do much than a normal boost.
Black Fire (Anime)
Only available to the Death Anchor, Black Bull or any other machine with a purple aura, this requires 2 reactors in a single machine. It is slightly faster than the original and lasts aprox. 2 seconds longer. Again, this can be stimulated in climax, but will only prove stylish.
Full Power Boost Fire (Anime)
Only available to the Dragon Bird in the anime (Not confirmed for Blue Falcon, this is when Ryu literally pulls the lever to the back till it breaks. This is much faster than then the black fire and this can be activated by one reactor only. This is applied only once during Lap 51 of the anime series..