A shorter valve won't change the compression ratio. You are not physically changing the amount of air that is compressed in the combustion chamber. To lower the compression ratio you would need to change the pistons to a dished piston. The HP and torque will go down a little if you lower the compression ratio. But it will run a tad cooler. From what I here the best thing is oil squirter's for the diesel engines. Thats the trick set up.