Got questions?
We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re covering rocker arm ratio and how it affects performance.
S.B. • Atlanta, GA
Q: I want to purchase a set of COMP Cams rocker arms for my 327-powered 1966 Chevy Nova. What is the difference between 1.5- and 1.6-ratio rocker arms? Does an increase in rocker arm ratio improve engine performance?
A: The lobe lift of the camshaft is multiplied by the ratio of the rocker arm to determine the valve lift. For instance, a cam with a .320-inch lobe lift and a 1.5-ratio rocker arm will have .480 inches of valve lift (.320 inches x 1.5 = .480 inches). If you were to install 1.6-ratio rocker arms, the valve lift would increase to .512 inches (.320 inches x 1.6 = .512 inches). The engine will respond as if a slightly larger lift cam had been installed.
In essence, increasing the rocker arm ratio is an easy way to improve the performance of an existing cam.
Is there any concern regarding piston clearance when using a larger ratio rocker arm? Also, is there any reason a higher ratio can not be used on exhaust or intake alone. In other words if I wanted to only increase lift on exhaust can I install on exhaust only?
If considering a change to larger ratio rockers, you need to be aware of some potential problems. The most obvious are related to the additional lift – you need to make sure you have adequate piston to valve clearance, and that there is adequate clearance at full lift between the bottom of the valve spring retainer and the top of the valve guide. Another consideration is the springs themselves – there must be proper clearance between the coils at maximum lift to prevent coil bind, and the spring must be strong enough to prevent valve float. This is more of a concern with a higher ratio rocker arm because the valve acceleration and peak speeds are increased which will make the engine more prone to float the valves. The geometry of the valvetrain must be verified – the rocker arm tip must be centered over the valve stem at 50% of maximum lift (especially critical with roller-tip rockers).
There is nothing wrong with just installing them on only the exhaust either. On one Dyno run a 5.0 Mustang actually increased by 10 hp with 1.6 on the intake valves and 1.7 rockers on the exhaust. When 1.7’s were installed on all valves,,,There was no increase in hp.
Just wondering if ’70 – ’72 Olds 350 came w/ 1.6 rocker arms from the factory etc. Thx!
Wouldnt the 1.6 lower the compression a bit
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Have camel hump sbc head all gone. Thru was running stock steel rockers 1:6 push rods summit 510 lift cam my mechanic install erson roller 1:5 rockers retaining 1:6 push rods does this combo work or do I need 1:5 push rods