Modified Mustangs & Fords
Rocker Arms and Adjustable Valvetrain Geometry
Rocker Arm Reaction
The typical four-stroke engine is made up of hundreds of parts. These parts are all designed and engineered to work together to make a prescribed amount of power, torque, engine longevity, and other parameters for the engine's design (or more accurately what management has told the engineers it needs to do for a specific cost). As long as these parameters are kept in check, the engine will provide the horsepower, torque, longevity, et al, it was designed for. However, if you begin to make changes to that internal combustion engine, be it a camshaft upgrade, new high-flow cylinder heads, or any number of hard part upgrades, you have effectively changed that engine's engineered design and steps need to be taken to ensure that the new parts are engineered to work with the other new or existing parts of the engine.
One of the internal combustion engines' greatest mysteries centers on its valvetrain components. The inner workings of an overhead-valve engine's valvetrain is often a head scratcher to the most died-in-the-wool car person. You can talk stroker engines, torque converters, or rear gear ratios, but start talking about pushrod length, rocker arm ratios, and spring seat pressures and most of your buddies with automotive knowledge will start looking at their watches and mumbling something about having to go home. Fear not, the mystery behind all those moving parts isn't black magic, but instead is simple math.
When it comes to your engine's rocker arms, their job is to transfer the camshaft's rotational movement into an up and down movement via the rocker arm's pivot point, which in turn opens the engine's valves. The rocker arm's size is expressed in a ratio, as the pivot is not in the center of the rocker (like a see-saw), but instead, offset to one side. For example, the stock small-block Ford rocker arm has a 1.6:1 ratio, and the parts are more commonly referred to as "1.6 rockers." This mechanical advantage means that the rocker arm tip moves 1.6 times the camshaft's lobe lift. For example, let's say your camshaft is a single pattern cam with 0.310-inch of lobe lift. With a 1.6 rocker mounted the cam's lift becomes 0.496-inches of lift (0.310x1.6). This is the typical measurement you see on a cam card or in a catalog with the footnote "with 1.6:1 rocker arm ratio" or sometimes stated as "with stock rocker arm ratio." This is a very effective way to increase valve lift (and duration slightly) without ever touching the camshaft in the engine. For example, the same cam specs, but with a popular aftermarket 1.7:1 ratio rocker arm would be 0.527-inch of lift (0.310x1.7). You've just increased your cam's lift 0.031-inch by simply bolting a different rocker arm on. Comp Cams tells us that the newest cam profiles are more aggressive and designed for the 1.6 rocker ratio, where as older cams and Ford Racing "alphabet" cams work well with the 1.7 rocker. You have to consider though that when you go up in lift, it's harder on the valvespring, it speeds up the cam profile (makes it more aggressive, and can be noisier), and makes the valve come off the seat faster. You also have to pay more attention to piston-to-valve clearance and coil bind.
Whether you use the stock ratio, or increase the ratio for more lift, it's imperative that the rocker arm fits the heads, clears your valve covers, and its geometry is properly set up to move the valve in the most efficient manner. If the rocker arm isn't checked and fit to the engine, you could induce several issues into your valvetrain, including excessive valve guide wear, rocker-to-valve retainer contact, piston-to-valve interference, damaged cam lobes, bent pushrods and more. While there is a lot to an engine's valvetrain to be considered, we're going to concentrate on the typical "bolt-on" setups, as in fully assembled aftermarket heads, or valvesprings being installed on existing heads that are specified/called out by the cam manufacturer's spec card or tech line. We simply don't have the room here to go through degreeing a camshaft, setting up valvesprings by opening and seat pressures, and so forth. If you're building a truly custom engine, your engine builder will deal with these measurements/adjustments, so for the gist of this story, we'll focus on the weekend wrench installing aftermarket heads or a new cam and spring kit and wanting to get their rocker arms and pushrods correct for their new bolt-on package.
There are two schools of thought when it comes to rocker arm geometry and pushrod length optimization. The easier, and more popular, method that Joe Wrench can accomplish at home during a cam swap, or even just upgrading rocker arms, is called the rollout method. The rollout of the rocker tip across the valve tip is measured and held to a minimum by changing the pushrod length. The second, more involved, method is called mid-lift method. It's also commonly referred to as half-lift centering. This method requires a dial indicator to read the valve's lift so that the midpoint of the valve's lift can be determined to allow measuring the geometry at the mid-lift point. Pushrod length is then determined to optimize this reading. Mid-lift measuring places the priority on centering the rocker tip over the valve at mid-lift. For all but the most extreme racing engines, or those who love to dabble in dial indicator measurements and check every single spec (twice) on their engine upgrades, mid-lift geometry checking is your bag. For the rest of us, you'll get perfectly acceptable results using the rollout method.
So, concentrating on rocker arms once again, why would you want to upgrade them in the first place? For starters, Ford, like many OE companies, used a basic stamped steel rocker arm on the majority of their engines. These stamped steel rockers do not have accurate rocker ratios due to the stamping process. Furthermore, the stamped rockers can flex under higher cam lift specs, effectively reducing the potential of that shiny new bumpstick you just installed. Lastly, the stock stamped steel rockers, often called sled or sliding rockers, have no bearings or roller parts to them to reduce friction, just a ball-type fulcrum to pivot on. The friction robs the engine of power and increases oil temperatures. If you're taking the time, effort, and money to install decent aftermarket cylinder heads, converting to a roller cam and lifter setup, or just want to bump up your stock cam lift a bit with increased rocker ratios, you'll be best served with a quality aftermarket roller rocker design. Most importantly, you need to set the rocker arm geometry and pushrod length accordingly.
Important Valvetrain Terms
Camshaft Degreeing—Measuring the exact position of the camshaft in degrees of rotation relative to the engine's combustion cycle. Doing so allows timing correction to adjust the performance of the engine.
Coil Bind—The point at which a valvespring can't be compressed any further and the coils touch each other. Valvetrain damage can occur when this happens.
Duration—The amount of time the cam lobe is lifting the valve, measured in degrees of camshaft rotation.
Hot vs. Cold Valve Lash Setting—Lash is the setting for a mechanical camshaft that allows for a measurable distance between the rocker arm tip and valve tip. The cold setting is set before the engine has been run and is larger than the hot lash setting, which is completed after the engine has achieved operating temperature.
Intake Centerline—The midpoint of the cam's lobe, however this may or may not be maximum lift as some cams have an asymmetrical design.
Lift—The amount of travel the lifter moves from the base circle of the cam up to the nose of the cam lobe. That measurement, when multiplied by the rocker arm ratio, generates the total valve lift figure.
Lifter Preload—The initial pressure applied to the lifter through the pushrod and rocker arm when adjusted.
Lobe Separation Angle—The number of degrees between the intake and exhaust lobe centerlines.
Piston to Valve Clearance—The distance between the valve and the piston when the two are at their closest point.
Opening Ramp—The portion of the cam lobe between the base circle and the lobe peak.
Open Pressure—Spring pressure that is created when the valve is open. There needs to be enough pressure at this point to maintain control of the valvetrain.
Overlap—The time, measured in crankshaft degrees, when the exhaust valve and intake valve are both open on an engine's cylinders.
Ramp Speed—The ramp angle of a cam lobe. The more aggressive the lobe's ramp, the faster the valve opens and closes.
Rocker Arm Ratio—The difference between the pushrod end and valve tip end of a rocker arm from the pivot point, expressed as a ratio.
Seat Pressure—The pressure exerted on the spring seat, most commonly stated in pounds per square inch.
Shim—A thin, flat disc available in varying thicknesses used to adjust (compress) the valvespring's installed height.
Single and Dual Pattern Camshafts—A single-pattern cam will have a profile with identical intake and exhaust lift and duration. When the intake and exhaust lift and duration are different, it's considered a dual pattern cam.
Symmetrical vs. Asymmetrical lobes—When the opening and closing halves of a camshaft lobe are different, it's considered an asymmetrical lobe.
Split Duration Camshaft—A cam that has different intake and exhaust duration specifications.
Valve Float—The point at which the intake lifter and its cam lobe are not following the same track/path. In extreme conditions, this will cause valve contact with the piston.
Valve Lash—A measurement taken on solid lifter cams between the tip of the valve and tip of the rocker arm. Valve lash is always measured with the lifter positioned on the base circle of the cam.
Valve Overlap—Overlap is a function of both a cam's duration and lobe separation angle. If the lobe separation angle remains the same but duration is increased, the amount of overlap will also increase.
Valvetrain—Valvetrain is an all-encompassing term used to describe the parts of the engine that operate the engine's intake and exhaust valves.