A standard gasoline engine relies on ignited fuel to explode and expand. This action pushs the pistons away from the head. I read that HHO fuel, when ignited, implodes. If this is true, then an engine could be designed to take advantage of the vacuum created be an HHO implosion. I also read that the speed that HHO gases implodes is much faster than gasoline vapor. This is why I think a HHO ONLY power vehicle may have to have it's timing adjusted to "Bottom Dead Center".