Increasing performance
The single most effective (and cost-effective) way to increase horsepower is to increase compression ratio. If you are overhauling an engine anywa,y it costs very little to put in higher compression pistons. You can then use the extra power when you need it and throttle back when you don't to save wear and tear on the engine. If you have one of those 4-tubes-into-a-big-can exhaust systems I would dump that in favor of a system by Power Flow, or Vetterman if experimental, or a similar type exhaust. Next I would work on the cylinder head ports, because it is not terribly expensive if done at overhaul time. Only then would I consider things like cams, cold air induction systems, etc.
Another thing you can do with an O-320 is change the crank to an ECi stroker and bump the displacement up to 340 inches. This is not a cheap mod, but as the racers say, there is no substitute for cubic inches.
The problem with looking to the cam for increased horsepower is that any "improvements" tend to hold the valves open longer and thereby decrease the effective compression ratio. You can compensate fro this by increasing the compression ratio, but gains tend to come at the top end of the power band at the expense of the bottom. In a car you can just spin the engine up faster and adjust the gearing, but not so in an airplane. The prop controls your maximum rpm.
The trick is to make as much torque as possible at 2700 rpm and hang onto as much of that torque as possible down to about 2300 rpm where you are going to begin your takeoff roll (assuming fixed prop). If you increase torque at 2700 rpm but lose it at 2300 rpm you are a net loser with takeoff performance. Since the factory cam is already pretty well optimized for this rpm range it is not easy to make usable power gains with modified cams.
If you really want to talk to a cam expert when it comes to Lycoming engines call Ken Tunnell at Ly-Con. (559-651-1070) He has forgotten more about these engines that I will ever know.
