Agreed on the tach for a 4-stroker. Was setting up a YS 63 this summer with a prop a bud recommended. Seemed pretty noisy, and found when we put the tach on it, it was near mfgr's redline (13K). In the air, would probably have floated the valves as the engine unloaded. Used a prop that dropped it by 3K, flew great. The tach probably saved me a repair bill.
Ed