Aftermarket Rocker Arms and Valve Float

This past week we had a 2006 GTO in the shop for a dyno tune. After looking at the graphs of the initial pulls, we noticed what appeared to be valvetrain instability beginning around 5500-5600 RPM.

The customer had purchased heads, cam, and valvetrain setup from another vendor. Here’s a breakdown:

2006 GTO w/ 6.0L LS2 Engine
Trick Flow 220 cc Heads (as cast, no additional porting)
Camshaft specs: 23x/23x .61x”/.61x”
Kooks 1-7/8″ Long tube headers
K&N Cold-air induction
Magnaflow catback
Stock LS2 throttle body & intake manifold
Yella Terra Ultra-lite 1.7 Rockers

In order to resolve the valve float issue, we first verified that the Yella Terra’s were properly installed with the correct lifter preload and valvetrain geometry. Next, we swapped the Yella Terra rocker arms for a set of stock rocker arms, along with Manton 7.625” pushrods and the proper rocker arm shims. The swap was completed with the car still strapped down on the dyno, and no other changes were made to the car. The graph shows the results: a much smoother curve with increased horsepower and torque across the band, not just in areas where valve float was apparent.

Why do the stock rocker arms work better in this case?

Using aftermarket roller-tipped rocker arms in LSx applications causes the effective mass over the tip of the valve and entire rocker body to increase. Also, depending on the design of the rocker itself, acceleration and deceleration rates of the valve can be increased.

These factors, coupled with a tall, less secure design of the LSx pedestal mount, contribute to valvetrain instability. This instability causes a variety of problems, including what is seen on the graph in the area marked as “valve float.” During valve float, the valves are bouncing on the seats and not closing properly, therefore bleeding off cylinder pressure and causing a loss of power.

Changing back to the stock rocker arms corrects the valve float issue. The rocker arm shim kit that we installed establishes the proper valvetrain geometry necessary for the stock rocker arms to work with the 13.5 degree valve angle on the Trick Flow heads. The longer 7.625″ Manton pushrods are the proper length to work with stock GM type hydraulic lifters in most applications.

Does this mean that it is always bad to use aftermarket rocker arms?
No; however, it takes a broad knowledge of valvetrain geometry and measurements to ensure that you don’t run into problems such as this. Not only does valve float rob horsepower, it can be hard on your valvetrain and cause premature wear and tear, if not outright breakage.