thats just the thing, i have been researching, and can find no difinitive answer on what the "pressure pulse" does, or what effect it has on an engine with no pipes, or short pipes? or how it will "burn valves" a lot of info about it doing these things, but no hard data on why.. remember, forget 2 strokes, i know they need the -ve pressure pulses to scavange and help drag through the next fuel/oil/air charge into the chamber, but nothing on a conventional 4 stroke engine other than, its just bad and will cause this and this etc.. if it reduces cyclinder pressures, it would produce less power, but how would it result in burnt valves? what changes are made to engines with straight short pipes, to stop this supposed change in cylinder pressures??