Well, a 5L engine running at 2000 rpm, IIRC a 4-stroke replaces about 80% of the cylinder capacity in a cycle, and it runs a cycle every 2 turns of the crank. So it takes in 4000L of fuel/air charge every min. Now in a gasoline engine, almost ALL of that is air because the liquid fuel takes up very little volume. Running H2, the H2 takes up 2/3rds of your intake volume so it can only oxidize 1/3rd the air. How much force you get out of it then depends on the temp reached, which I don't know for 2*H2+O2 in an engine.
But the engine size is not the issue so much as the basic problem that it will only return 10%-20% of the energy inside H2, and the production of H2 varies but it's generally like 50%. So, for every HP you need out of the engine, your alternator needs over 13HP to turn into electricity to make it.
The HHO people keep touting a theory that small amounts of H2 make the gasoline burn much better, adding more hp than required to make the H2. It's more plausible than the "over unity" perpetual motion concept, but not all that plausible, and I have not seen any convincing scientific proof of this.