knocking sound when car accelarates with A/C running

[rollin-in-a-rolla]

hi all,
my 95 JDM rolla is a warehouse of problems. when its running without A/c, the accerataion is just fine. not great, fine. but i turn the A/C on and wow, it barely moves. the engine revs high, heats up, makes great noise, but dont go nowhere. and it makes a funny dull metallic knocking noise when i push the gas pedal...(with the A/C on of course). why the hell is the A/C robbing so much of the so little power i have? oh and the noise is not piston slap. thats for sure.
so, any explanations anyone? thanx in advance.