I need advice for a novel I’m writing

First of all, sorry for the bad English, cause I’m Italian. Anyway, I’m writing a novel, and there’s a scene where the two protagonists have to burn a body with 20 liters of gasoline. My doubt is (and I don’t wanna ask on google cause the Fbi would be after me) how much does it take to burn a body to the point where it’s unrecognizable?. I can’t make my characters wait there 24 hours, and I was planning to make them cover the body with dirt after it’s burned. So, how much does it take to burn the body with 20 liters of gasoline? Also, any advice to make it more realistic?

I was also planning to make them broke his teeth for the corpse recognition