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Given the following post, respond as if you're commenting on Reddit:
human
What's the noise a formula 1 makes when it changes gears?

I've just came back from the F1 Belgian GP and was thinking about what makes the noise (kind of a small explosion) when a F1 (also GP2 and GP3) car changes gears. What is the reason and why it sometimes happens and sometimes doesn't?

EDIT: Thanks for the responses, guys!
gpt
It's most likely a backfire. When the car is accelerating its at full throttle/load, and the engine runs out of power, so it's time to change gears. imagine going from full throttle to no throttle (changing gears) then back to full throttle. 
The bang u hear is unburnt fuel exploding in the exhaust after its left the combustion chamber, which is after engine has gone off full throttle to change gears. 
It's excess fuel that was needed to sustain full power, but is no longer needed when off throttle.