A top fuel racer is very loud because they have short, per-cylinder exhaust pipes and are powered by nitromethanol ("top fuel"), which burns so slowly that the fuel is still exploding when exiting the exhaust pipes.

They are also ridiculously inefficient.

They're very efficient if you measure efficiency in terms of HP for engine weight, which is about all that matters in drag racing :)

Well sure, that's why they use nitromethanol (which is worse in kJ/kg than gasoline, but it contains an oxidizer) and ridiculous engines.

But "efficiency" in the context of engines usually means energy conversion efficiency. And power-to-weight ratio is what you mean.

Sure, but power and efficiency are different things!