Your argument handily ignores the two highest production auto conversions ever, the VW and the Corvair. I'd hardly call Viking "high volume." The VW alone probably outnumbers every other auto conversion combined, and I'd gamble that more people are/have built more Corvairs than Hondas. You can buy off the shelf every single part to build a half or whole VW., and the few builders out there building them professionally are swamped. Google experts said you can't put a VW on a low and slow airplane. "No torque!" "Too heavy!" Good thing Leonard Mulholland and Morrey Hummel didn't have Google to stop them. They produced a couple of the most popular LEGAL ultralights being homebuilt today. They also spawned an industry that keeps a few engine builders busy full time, and some parts suppliers hopping as well. Huh. Also the cheapest option out there, which is why they remain popular.Sorry Tom, but in fact there are now very few direct drive options left anymore. Those that exist are very expensive and only produced a few hundred per year....The high volume auto engine conversions like Viking, have a reduction gearbox.