I have one of these, it came with my spirit coupled to an automatic. They do not make much power but thank AMC for the single barrel carb for that. They will never make a lot of power but being the only OHC engine available from AMC they offer some interesting characteristics. The 121 can rev way higher than it's OHV counterpart. When comparing one of these engine to an iron duke, i think the advantages come through. Sure there are variations of the iron duke which can yield ridiculous HP, they require extensive modification, exotic parts, and/or complete engine swaps. To take a stock duke and stock 121 and modify them both, the 121 will be stronger and produce more HP and torque but at a much higher RPM. The duke will always provide more low end torque due to its push rod configuration. The AMC 121 can also be fitted with dual weber carbs for more fuel/air capability and increased throttle response which is difficult to achieve with the duke. FYI, I own both an AMC 121 and a SD-4 version of the iron duke and can't decide which one I like better.