The ratio of a rocker arm is determined by the distance between the
centerline of the pivot point to the centerline of the roller tip
(or area of contact with the valve stem), divided by the centerline
of the pushrod to the centerline of the pivot point (X). Most aftermarket
roller rockers have the ratio stamped on them.
To determine the change in lift when changing rocker arm ratios, divide
the lift of the cam by the original rocker ratio (which gives you
the lobe lift), then multiply this number by the new rocker ratio.
For example, the stock Mustang 5.0L HO cam has .444 at the valve.
Divide this number by the stock rocker ratio of 1.6 to get the lobe
lift, .2775 and multiply by the new rocker ratio 1.72, to get
the new lift of .477.
should I do next? It is the proverbial question, pondered by every automotive
enthusiast at least once a day. Most of us use the same universal formula
for deciding upon the next performance modification -the mod which will
provides the most gain for the least amount of money and effort.
One of the modifications that seems to meet this criteria are higher ratio
roller rockers. The increased ratio in effect is like installing a slightly
larger cam, and the roller bearing fulcrum and tip provide slight gains
in power and longevity by reducing frictional losses.
In this article Tech Editor Jim Langley replaces the stock sled style
rocker arms on a stock 5.0L engine, and tests whether this modification
is worth anything at the dragstrip.
get into the installation and results, let's review in detail the theory
behind increasing the rocker arm ratio.
The rocker arm is the link between the camshaft and the valves - functioning
as a lever arm to multiply the cams lobe lift to the valve lift. Stock
rocker arm ratios vary between manufacturers and engine types, but generally
all small block Fords come with a 1.6:1 ratio rocker arm.
Virtually all the aftermarket camshaft manufacturers offer high-quality
aluminum roller rockers in ratios greater than stock. For small block
Fords you can easily round up a set of 1.7:1 or 1.72:1 rockers from Crane,
Comp Cams, Harland Sharp or Omega. For the Cleveland and 429/460 motors
most companies offer a 1.8:1 ratio rocker arm over the 1.73:1 production
ratio. Winston Cup engines can be setup with as much as 2.0:1 rocker arm
Are there any cons to higher ratio rockers? The answer is yes. The increased
leverage can contribute to faster valve guide wear, especially if pushrod
length is not properly checked and adjusted. The increased lift may also
necessitate a stiffer set of valve springs, or springs with more clearance
before bind. Finally be sure to check for sufficient clearance under the
valve covers, as the beefier rocker arms can hit baffles or other parts
of the cover.
Lift and Duration
We all know that stepping up to a cam with a little more lift and duration
can yield a nice gain in performance. Unfortunately cam swaps are time
consuming, and when you factor in the cost of the cam, lifters, gaskets,
and fluids, it quickly becomes a project not worth tackling -especially
on a stock motor.
The idea behind increasing the rocker arm ratio is to essentially give
the motor a slightly larger cam, without all the hassle. Naturally, decreasing
the rocker arm ratio provides the opposite effect -making the cam events
slightly smaller. The side bar above shows how we can calculate the valve
lift as a result of changing the rocker arm ratio.
Many people incorrectly assume that a change in rocker arm ratio has no
effect on the duration the valves are open. In actuality there is some
change to the duration at the valve depending on the lobe profile of the
camshaft. Comp Cams explains that by lifting the valve an additional 7-8%,
but beginning and ending the lift at the same point, the opening and closing
rates become steeper. This increases duration, measured at the valve at
0.050" lift, by 1-2 degrees. You can measure the actual duration
change by using a degree wheel and dial indicator set on the valve retainer.
Coincidentally the principle of a steeper rate of opening and closing
is what Comp Cams has engineered into their Extreme energy camshafts to
enable more valve duration for a given lobe duration.