Originally Posted by
glennsexton
There’s a lot of hype and myth surrounding 1.5 versus 1.6 rocker arms in small block Chevy engines. What a lot of folks may not realize is that the stock rockers are mass produced and the quality control allows a variance of +/- .075 which means a bone stock engine can have rockers between 1.425 and 1.575 and still be within tolerances. Stock camshafts are, for the most pretty tame and the differences in rocker arms are not perceptible. Exceptions to these tolerances have been the GM sponsored and built high performance variants of production small blocks with the ideal ratio actually being 1.52:1.
Stock rockers flex and generate heat which eventually results in warping and distortion. After 100,000 miles, it is not uncommon to find rocker arms in the 1.4:1 range. This is not a problem in most stock engines and you’d probably not notice such when driving a 350 powered Chevy Caprice.
I have personally used roller tip 1.52:1 and would attest to their and claim to reduce friction and heat. They have been a part of a total rebuild so it’s impossible to verify that the rocker added any significant horsepower to the build, but the reduction in heat and friction was reason to justify the slight increase in cost over stock replacements.
GM actually has made roller rockers out of chrome-moly and hardened steel. GM made aluminum roller versions engineered to address the heat and friction issues in certain small blocks. Aftermarket cam companies have a virtual plethora of rocker arms in ratios ranging from 1.2:1 (break in rockers) up to 1.8:1 for extreme applications that have been tightly engineered for higher ratios.
Physics tells us that larger ratios will open the valves a bit sooner and close the valves a little later. The increase is symmetrical in relationship to the cam lobe centerline so a higher ratio will lengthen the overall valve timing, i.e., as the ratio is increased the valve timing increases. Swapping from factory stock (especially if old and worn) rocker arm to a true 1.60:1 may increase your total valve duration by up to 20 degrees.
In simple terms, the rocker arm mechanically multiplies the camshaft’s lobe lift by moving the pushrod closer to the fulcrum pivot point than the valve stem tip is. If the centerline of the valve is located 0.750 inch away from the rocker fulcrum pivot centerline, a 1.5:1 rocker would have the pushrod cup located 0.500 inch from the pivot centerline (.750/1.5= .500). An increase in the ratio from 1.5:1 to 1.6:1 moves the pushrod closer to the fulcrum pivot centerline by .030 inches (.750/1.6= 0.470). This not only multiplies the cam lift; it also increases the load on the pushrod and rocker arms. As such the proper ratio is critical. If the ratio is increased too much, the valves open prematurely and “valve float” will occur at at high rpms.
As the ratio increases it multiplies the spring pressure on the cam lobes and can lead to lobe failure in a flat tappet cam. It also increases the load on rocker arm stud – making screw in or pinned studs a must with increased ratios. As rocker ratio is increased, so, too, is valve timing. Changing from the stock ratio, which may be closer to 1.46:1, to a true 1.60:1 will increase your total valve duration by almost 20 degrees.
A common issue with installation of larger ratio rocker arms is the clearance between the pushrod and the head. The hole might need to be elongated to prevent the pushrod from scraping against the head which can lead to pushrod failure and loss of the oil passage within the pushrod causing lubrication problems at the tip.
Very little knowledge is “net-new” and such can be said of this post. I have had some rocker arm and pushrod challenges and learned the hard way (read broken parts!!) I tend to pay real close attention to experts like Pat McCarthy (who make his living building engines), Jerry Clayton, Tech and others here who have been doing this stuff a long time. I’ve only been building for about 50 years and learn things from many here that are far more knowledgeable than I am. Last thing I want to do is tick these guys off ‘cause I may need a favor from them someday. And if GM says 1.52:1 is ideal for most street driven small block Chevys, I tend to agree with them as it’s based on a lot of research and years of engineering expertise that’s far beyond me.
Regards All,
Glenn
