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Rocker Arm Ratio
What modification should I do next?
It is the proverbial question, pondered by automotive enthusiasts at least
once a day. Most of us use the same universal formula for deciding upon the next
performance modification which will provide 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.
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 Chevy's come with a 1.5:1 ratio rocker arm.
Virtually all the aftermarket camshaft manufacturers offer high-quality
aluminum roller rockers in ratios greater than stock. For small block
Chevy's you can easily round up a set of 1.6:1 rockers from Crane, Comp
Cams, Harland Sharp or Omega.
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 Math
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. See picture above.
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,
a popular small block chevy cam has .468 lift at the valve. Divide this number
by the stock rocker ratio of 1.5 to get the lobe lift of .312. Multiply .312
by the new rocker ratio 1.6, to get the new lift of .499.
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