that all depends on how much hho you add and how much fuel you cut I don't know how much hho you would have to add to have to retard timing drastically as is we need to retard it to change the max lean out tables in the computer to achieve better fuel mileage any how. The folks on youtube running lawn mower engines on straight hho are having timing issues (back firing) but personally I feel the faster and more complete we can make the gasoline burn through introduction of hho and with this new thought of a hotter spark the more efficient we can make the ICE. It will be an experiment all of its own but as far as I can see it the worst that will happen is we will ruin a set of spark plugs and spend a few extra bucks and you have got to admit it will be a pretty cool experiment.