Well in theory about 15-20 hp or so. 65mm pulley gives about 17-18 psi vs the 62mm pulley at 20-22 pending on your altitude. You would think it gives than just 20hp but the 62mm produces more chance of heat soak which actually in some cases can decrease performance. If you run a 62mm pulley or a 185mm crank pulley you HAVE to have at the very least a supercooler and fluid isovation kit to prevent heat soak. I currently run a 185 with a supercooler, isolated kit, and a killer chiller kit and It still heat soaks at times. All this advice is coming from an owner who ran a 65mm pulley for years.. stay tuned in the next 2 weeks, I'm gonna dyno mine with the 185mm pulley which produces about the same boost as the 62 does. We will see how much of a true difference it made. It for feels faster than a 65mm