To Advance or Retard
Stan Wiggins, Houston, TX: I have a question regarding camshafts. What is retarding and what is advancing the camshaft? I mean, if I drill the cam sprocket and install a bushing, which way do I position the bushing to achieve the advanced position and vice versa? In layman's terms, if I advance the camshaft in increments of 3 degrees, which should increase low-end torque, how does this work, and should it affect my initial camshaft selection? Likewise, if I choose to advance or retard the camshaft, what is the maximum limit? What kind of performance numbers can I expect when advancing or retarding? These are some of the questions that cloud my constant search for better technical understanding.
Cloyes offers a slick variation on the offset bushing routine with a cam gear featuring Hex-A-Just. In this case, merely loosen the cam bolts and use an Allen wrench to advance or retard the cam up to 6 degrees in either direction. For a small-block Chevy, the Summit Racing price is $114.95.>>>
Jeff Smith: This is a great question, Stan, that is often overlooked. First, let's deal with the simple stuff. All domestic four-stroke engines rotate clockwise when viewed from the front of the engine. All camshafts are driven by a crank gear that is half the size of the cam drive gear. When installing a bushing in the cam gear, advancing the camshaft means installing the bushing so that the pin that extends through the cam gear will be moved ahead of the direction of rotation. To put this another way, advancing the camshaft would move the cam pin clockwise, while retarding the cam would move the pin counterclockwise. Generally, we drill the cam gear for these bushings from the back side of the gear and don't drill all the way through the cam gear. This captures the bushing and requires you to install it from the back side. It's also a good idea to degree the cam again after installing the bushing to verify the accuracy of the installation.
You are correct that generally speaking, advancing the camshaft improves low-speed torque while detracting slightly from peak rpm power. There are four important points that need to be addressed when considering a camshaft: intake opening (IO), intake closing (IC), exhaust opening (EO), and exhaust closing (EC). Anytime you move the position of the camshaft, it alters the location of all four of these points. To simplify the process, however, the IC is the most important of the four. Here's an easy way to remember how all this works. Advancing the camshaft opens and closes the intake valve sooner, which tends to help low-speed torque. Retarding the camshaft closes the intake valve later in relation to piston position. This is similar to using a longer-duration camshaft with a later IC point.
Each engine will react differently to changes in where the camshaft is installed. Generally, if after degreeing the cam you discover the intake lobe is 1 degree retarded from its intended location, it's not worth losing sleep over. However, if the engine is intended for mainly street use, advancing the cam 2 degrees would put it 1 degree advanced, which will help low-speed throttle response and torque. We hesitate to move a camshaft more than 4 degrees because, remember, you are moving all four valve operations this same amount. If you feel you need to place the IC 4 degrees later to make more power, it would probably be better to choose a camshaft with that IC built in. If you look at most catalog camshafts, each step of a particular family of cams is roughly in 4- to 6-degree steps.
There's also an interesting trick most street-cam grinders perform that is worth knowing. The term "straight up" can be deceiving. Let's look at a Comp XE-274-H-10 flat-tappet hydraulic cam for a small-block Chevy with a lobe separation angle of 110 degrees. If a cam is actually ground straight up, the intake centerline number and the lobe separation angle will be the same. However, most street-cam designers purposely grind a few degrees of advance into the camshaft. In the Comp Cams case, this grind features a lobe separation angle of 110 degrees yet has an intake lobe centerline of 106 degrees. Thus the cam was ground with 4 degrees of advance already built in. So advancing the cam on top of this 4 degrees is probably not a good idea.