C5 BRONZE HEADLIGHT MOTOR GEAR UPGRADE (article continues) One of our techs, Mickey Farrell, had a really bad noise coming from the engine bay of his C5 that got progressively louder. At first, he suspected that an idler belt, pulley or tensioner was going to give up the ghost shortly, but upon closer examination he determined that it was the harmonic balancer pulley that was the problem. So, being that the pulley needed to be replaced, he had to decide whether to replace it with a stock harmonic balancer pulley or upgrade to an underdrive pulley to pick up a few additional horses. After considering both options, Mickey opted to go with the underdrive unit. An underdrive harmonic balancer pulley will increase engine horsepower by reduc- ing the amount of power required to drive external engine accessories. For example, the alternator, power steering, water pump and air conditioner all rob horsepower from the engine. By replacing the factory-sized crank pulley with a smaller-sized underdrive pulley, the accessories are slowed, resulting in more horsepower available to drive the rear wheels. Typically, an underdrive pulley will increase output by 7-12 horsepower, resulting in quarter mile improvements of .1 to .2 seconds. And, as a secondary benefit, since the engine doesn’t have to work as hard, gas mileage will increase, albeit slightly, as well. He purchased the harmonic balancer/underdrive pulley and bolt from SLP and then purchased the belts they recommended. Unfortunately, SLPs belt information was all wrong, since these belts were not the correct ones for a C5 installation. Replacing the harmonic balancer pulley is a time-consuming, labor-intensive job that is best accomplished with the car on a lift. In addition to wrenches, sockets and pry bars, you’ll also need a balancer pulley puller and an installation tool. Replacing the front timing cover crankshaft crank seal is also highly recommended (when you have everything apart, why skimp and not replace a $25.00 part that may cause you grief down the road if/when it starts to leak?). Also, while you’re at it, check the A/C belt tensioner. Mickey checked his while re- placing the balancer pulley and found that his tensioner had play in it, so he replaced it as well while everything was out of the way and he could access it unencumbered. While we’ve heard of guys replacing the pulley in their driveway with the car on jackstands, we’ve never actually spoken to one of these determined (or masochistic) souls. But after documenting the procedure on a lift and seeing what’s involved, it’s hard to believe anybody would want to attempt doing this job lying on their back. All of that being said, here’s how to replace the stock harmonic balancer pulley with an aftermarket underdrive unit. © AllAboutVettes.com 2012. All rights reserved. Reproduction of any material on this website is a violation of copyright and is expressly forbidden without permission. This website has not been prepared, approved or licensed by General Motors. Some words, model names, images and designations herein are the property of the trademark holder and used for identification purposes only. VOLUME 3, ISSUE #2 - FEBRUARY, 2012