By Cam Benty
How Changing Rocker Arm Ratio is an Easy way to Increase the Effective Size of Your Camshaft
It’s a simple premise: increase the ratio of your engine’s rocker arms and you increase the size of the camshaft profile. More camshaft lift means more fuel and air into the engine and more performance, right? It’s a topic we wanted to test first hand to check the reality of what seemed to be a logical concept. But as we discovered, there were some far more interesting ramifications that occurred. No free lunch!
With a small-block Chevy engine used as our guinea pig for the test, the standard ratio is listed at 1.5:1. However, through testing at the COMP Cams research lab, the actual ratio for the standard stamped steel rocker arms are generally in the realm of a 1.46:1 ratio. To pick up the effective specifications of the camshaft, both the COMP Cams Magnum and Pro Magnum rockers utilize a 1.52:1 ratio, which helps wake up most enthusiast-level street-use and racing engines with no other required modifications. As you will see from our testing, this simple move nets a nice little lift in power and torque with the added benefit of the low friction roller rocker technology. As any non-engineering type of enthusiast can tell by looking, the roller tip contact used to depress the top of the valve stem is a significantly better system than the friction-heavy stamped factory rockers where the tip simply moves down while sliding across the valve dependent on proper oiling to operate.
Things to Watch For
Before beginning to install increased ratio rocker arms, it is important to understand the dynamics of what happens when you start messing with these parts. An increased rocker ratio will increase the effective lift of the camshaft specifications at the valve. But it also affects the duration. As noted by COMP Cams camshaft engineer Billy Godbold, a rocker ratio change of 0.05 (such as from 1.50:1 to 1.55:1) will add approximately 1 degree of duration along with that obvious enhanced lift. While that may seem inconsequential to most, think about how much of a difference a degree or two means to lobe separation dimensions. This is important information that you should know before you start the modification process.
Next, there are a number of geometric changes that occur when you change rocker arm ratios. Probably most important to you is that the rocker travel is increased. Along with this increased travel is the fact that the pushrod moves closer to the rocker mount. In many engines, that isn’t a problem. But for some engines, including the venerable small-block Chevy, the stock pushrod holes are not big enough to compensate for that movement. In many cases, the pushrod will actually contact the pushrod hole in the head. When this occurs there are several ways to handle it. First you can assess the benefit of the high ratio rockers and whether such a change is right for your needs. If you feel it is worth keeping the high ratio rockers, you can enlarge the size of the pushrod holes. Lest you believe you are breaking new technology here, there are many who have encountered the problem before you. There is even a COMP Cams available fix, the Louis tool (COMP Cams P/N 4710), which will provide adequate clearance. In most heads, especially in the case of our small-block Chevy, there are no port or water jacket concerns in the area surrounding the pushrod hole.
In addition to watching the pushrod hole contact, there can be conflict with the pushrod guide plate. Cycling the engine before firing will help note any potential contact in this area. If there is concern, changing to larger pushrod guide plate openings or machining them for additional area will avoid such power robbing interference.
While all this talk about interference and increased valve lift is great, one item has been left out of the discussion entirely to this point: valve springs. It is critical to make certain your valve springs are “adequate” to accommodate your expected new valve lift, based on the lobe lift of the cam and the calculated new valve lift. Most late-model factory valve springs are incapable of handling increased valve lift. For instance, changing a 2000 LS6 rocker from stock 1.7 to 1.85 increases valve lift to 0.600-inch–well beyond the safe capability of the factory spring. In a case like this, the high-ratio rocker swap should include a matching spring swap as well. Late-model Ford modular and Dodge Hemi engines follow suit as well. On classic engines, such as the test SBC, it’s a good idea to swap in a set of fresh (and capable) valve springs a well, although the stockers are somewhat more tolerant to increased lift than late-models. If you go over on valve lift without matching the right spring, the springs can break, float or fatigue, resulting in sure disaster. Changing your springs to a high quality, more adequate spring just makes great sense.