So, a chamber of 3x3" square tubing roughly 18" long. This figures out to around 2.6 liters. Multiply by the energy produced by a stio mix of oxy-acetylene and again by an estimated efficiency of a typical combustion (5-15%?).
Hint: it's not going to be near 8KJ.
Oh, and could someone please enlighten the guys who posted the video rather than going on about it here? I'd do it myself but Ibno longer have access to my youtube account.