theres a balance shaft assembly that sits in the oil pan, basically a set of counterweighted gears that spins opposite the direction of the crank, to compensate for the imbalance of a 4 cylinder engine with a huge stroke. not only does it suck power by running off the crank, its spinning gears in the oil pan froths the shit out of your oil, potentially leading to some nasty oil pressure related issues.
jim wolf tech has a removal kit that replaces it with a baffle in the pan. the only downside to removing the assembly is increased vibration, which ive heard is no worse than having stiff motor mounts.
ive heard it suggested that you will gain 5-8 whp with the balance shafts removed. ive also heard that its probably a good idea to ziptie all loose wiring in your engine to prevent harness failure due to the extra vibration.