While I try not to argue, let me restate. Hp is a function of harnessing expanding gases due to a controled explosion. The greater the explosion the more hp; the greater the bang the more heat that must be dissipated or engine failure will occur. Small explosions yield lower heat than big explosions. Aftercoolers, cold air packages, nitrous lower the temp of air COMING IN to the cylinder, this increases the mass of air or density which allows more fuel to be mixed which makes the potenial of more hp due to a bigger bang. Keep in mind all this is a matter of effiency of controlling PCP (peak cylinder pressure) which must occur at or around 10-15 degreee ATDC and continue as long as possible until dissapating just BBDC. As to nitrous oxide it is an oxygen bearing neo-catylist which if injected into a gas engine without extra fuel (gasoline_alcohol_nitromethane) would kill the engine as it will not burn itself but supports combustion. Added to diesel it of course provides free O2 just as added boost. NOS is easier to handle than pure O2 because under pressure it forms a liquid. To your comment on intercooler; if one takes a modern, non charge cooled diesel engine which is not turned up and is set at factory specs and then add a intercooler, it will not affect the hp. If the fuel is turned up then the intercooler will lower the air temp and allow more O2 for cumbustion with the added fuel. On a gas turbocharged computer controlled engine adding a intercooler will 'fool' the computer to allow the fuel injection to add more fuel as it senses more air to maintain the A_F ratio approx 14:1, but this is stuff books are written about. Besides most automotive supercharged engines and truck engines are now factory charge cooled. Another history thing. Diesel 4 strokes all started out naturally asperated, then as HP races and requirements went upward the turbo was added which allowed_forced more air into the combustion chamber. More air and fuel meant larger radiators and stronger blocks, rods,etc. As the boost increased so did the heat generated as anything is compressed it gets hot. Intercoolers were not introduced as performence improvers at first but were necessary to keep the engine life from decreasing at the new high HP from the same old naturally asp engine that they started with. Finally, (thanks for the comment of my youth) I had the pleasure of building a small block Chevy 400 to the max back in the 70's along with a diesel tractor pulling motor or two.. Engine building for HP is a science that requires quite a bit of math, theory, and physics over trial and error and rumor to be successful and economical in lieu of broken parts and frustrations.