Years ago I had a 340 that I had beat like a rented mule. It ran its best ET(11.98) then a few weeks later it slowed about .1-.2. I assumed it was tired and it needed to be refreshed, then it broke the crankshaft. Everything else inside the motor looked fantastic, even the bearings. The crank was just broken behind the last rod journal. Next time my car slows that much for no apparent reason it will be inspected more.