Since the invention of the overhead valve internal combustion engine, auto manufacturers have been faced with the added complexity of the rocker arm becoming a necessary part of the valvetrain in the engine. Previous "flathead" engine designs incorporated a valve whose stem was most often located in the engine block, and pushed upward by the camshaft or tappet to open the valve at the appropriate time. In the more powerful overhead valve engine design, the upward motion of the cam lobe needs to be transferred to a downward motion of the valve itself, which is located in the cylinder head, and that transfer of motion is the function of the rocker arm.
Several rocker arm systems were incorporated by early automotive engineers, leading to two basic styles of rocker arms used in modern engines, pedestal (or stud) mounted rocker arms, and shaft mounted rocker arms. Pedestal mounted rocker arms mount individually and can be found in the Chrysler Magnum small-block engines. The vast majority of Mopar rocker arms, however, are shaft mounted rockers, which, as the name implies, ride on a shaft that bolts to the cylinder head. As Mopar owners, most of us are fortunate in that our small-blocks, big-blocks, and Hemis have shaft mounted rocker arms right from the factory, as this design has proven to be more stable and less prone to wear or failure than individually mounted rockers.
But while our factory shaft mounted rocker arms are arguably a very good system, there are limitations to the stock rocker arms found in most Mopar engines. The first, and biggest, drawback of factory shaft mounted rocker arms is that there is no real provision for adjustment of the valve lash, other than placing shims between the rocker shaft and cylinder head casting. So although factory stamped steel rocker arms can be adequate for small to moderate lift hydraulic camshafts, they really aren't practical at all for solid lifter cams. Additionally, the stamped steel factory rockers are prone to have a wide variance in fit, finish, and ratio, which is what we'll discuss next.
Rocker arm ratio is the length of the valve side of the rocker arm to the center (pivot point or fulcrum) of the rocker arm divided by the length of the cam or pushrod side of the rocker arm to the center of the rocker arm. Since the valve side of the rocker arm is typically longer than the pushrod side, the rocker arm will multiply its motion from the pushrod side to the valve side, as a function of the ratio. For example: If the cam lobe lifts the pushrod side of the rocker .350 inch, and the rocker ratio is 1.5, the valve lift will be 1.5 times the cam lobe lift which is .525 inch (.350 x 1.5 = .525). The same camshaft with a 1.6 ratio rocker arm would lift the valve .560 inch (.350 x 1.6 = .560), considerably more than the 1.5 rocker. Common rocker arm ratios are from 1.45 to 1.7 or even higher, but most Mopars utilize a 1.5 ratio rocker arm from the factory.
There are a couple of simple ways to figure rocker arm ratio, and the first is to measure each end of the rocker arm from the center of the pivot point to the contact area of the pushrod and valve, and divide the two numbers. For example, if the valve side of the rocker measures 1.5 inches and the pushrod side measures one inch, the ratio would be 1.5 (1.5/1 = 1.5). In basic terms, the longer the valve side of the rocker arm is compared to the pushrod side, the higher the ratio of the rocker arm.
As you can see, installing rocker arms with a ratio higher than stock will lift the valve higher off of the seat, which can have a positive effect on engine performance. So if you want a higher lift camshaft without having to replace the cam, bolting on a set of rocker arms with higher ratio can give you the lift you want. Remember, however, that many Mopar cylinder head castings need to be machined so that the valve retainer doesn't interfere with the top of the valveguide, so always consider that factor before swapping rocker arms. The valvespring needs to be considered as well when installing higher ratio rocker arms, as more valve lift will bring the spring closer to coil bind.
While higher valve lift can mean more power, we consider duration a more important factor when it comes to performance, and higher ratio rocker arms do not increase the duration of the cam. The cam should always be matched well to the rocker ratio, giving the proper valve lift and duration for the application the engine will be used for, and for this we recommend speaking with the cam manufacturer directly through resources such as Comp's cam help line. Speaking to one of the professionals will ensure that your combination of cam, pushrods, rockers, and valvesprings is compatible and optimizes the capabilities of your engine.
Choosing the proper rocker arm can be a matter of preference to an extent, but has more to do with the camshaft and valvespring than it does with the engine as a whole. The higher the lift of the cam, the higher the valvespring pressure needs to be, in order to keep the valves in control at high rpm, which equates to more load on the rocker arm. In fact, as the pushrod applies pressure to one side of the rocker and the spring applies pressure to the other side, the cumulative effect tries to break the rocker arm in half, kind of like you'd break a stick in half over your leg.
Stud mounted rocker arms like found on the Magnum series of small-blocks suffer the additional strain of deflection caused by the forces of the valvespring and pushrod. Guide plates for the pushrods and a stud girdle for the rockers can help alleviate this issue, but the best way to solve the problem is to convert to shaft mounted rocker arms. Fortunately, Magnum engine enthusiasts can benefit from RHS's line of cylinder heads which not only offer the option of shaft mounted rockers, but also have either the Magnum or LA intake bolt pattern. Luckily, all other popular Mopar engines have the stronger, more stable shaft mounted rocker systems.
Engines with mild, hydraulic, flat-tappet camshafts up to around .500-inch lift can survive with factory stamped steel shaft mounted rocker arms, but can always benefit from the higher accuracy of quality aftermarket rockers. And if you have a solid lifter camshaft or roller cam in your engine, you'd better consider a rocker arm upgrade as mandatory. Additionally, if your engine is topped with aftermarket cylinder heads, the offset of the rocker arm needs to be matched appropriately as many high-performance cylinder heads have wider ports than factory heads, and sometimes the valve location is modified.
Increasing cam lift significantly over .500 inch, or installing a roller cam in your engine, means you'll need an aftermarket set of rockers. There are a good variety of moderately priced aftermarket rockers for Mopar engines on the market today, some made from aluminum, others from steel, and nearly all feature a roller tip for improved valvetrain geometry. The roller tip only marginally reduces friction and doesn't improve performance so much as it allows the rocker to be made from lighter material. We've had good luck with moderately priced aftermarket rocker arms from many companies including Comp Cams and Crane, but for engines that are designed for maximum power and endurance, you'll want the best quality rocker arms available.
Premium shaft mounted rocker arms not only contain a roller tip where the rocker engages the valve, but also needle bearings where the rocker rotates around the shaft. These bearings keep the rocker arm securely in place, and can take a higher load than rocker arms without needle bearings. Consider Harland-Sharp, Comp, or T & D Machine for premium rocker arms, and for the ultimate valvetrain stability, Jesel makes rocker arms for most Mopar applications that utilize individual shafts for each pair of rockers. Designed for maximum endurance and stability, the Jesel rockers are nice, but come at a price. The cylinder head must have some special machine work performed to accommodate the Jesel rockers, which can be cost prohibitive for the average engine build.
So, remember, aftermarket rocker arms can be a great upgrade to your Mopar engine and should be considered mandatory when utilizing a solid lifter camshaft, roller camshaft, or even a hydraulic cam if over .500 inch lift. We encourage you to shop around, as many of our advertisers manufacture great rocker arm systems for Mopar engines, and all are easy to install in a day or weekend. The right rockers can not only make more power, but add reliability to your engine as well. So keep your Mopar rockin' with a new set of roller rockers!
