While on paper our Boulevard Bruiser ("Ground & Pound," October '09) seemed well at home in the upper rpm. A closer look revealed a less-than satisfactory issue with valve float and subsequently a loss in power. It's been known that the more cubic inches the more difficult it is to spin that reciprocating mass at high rpm. Yet, with the availability of off-the-shelf items such as lightweight and higher tolerance valvetrain components, the ability to wring out even more power is also available with the right combination.
By 5,700 rpm the 496ci big-block was still making great street power but by 5,800 rpm we saw a dip and by 6,000 rpm it had all but fallen on its face. On the surface, you may think that's not too shabby for 496 inches. However, our Comp hydraulic roller 243/257 duration 'stick with 0.570/0.554-inch lift (intake/exhaust) was rated for operation at 2,500-6,100 rpm and should still have been making power, especially with large 320cc RHS cylinder heads. As we examined the dyno graphs, a number of contributing factors were realized. The 496ci was not only suffering from valve float but also from a lifter that couldn't quite keep up with the combination. The culprit? We determined that a stiffer spring and higher tolerance lifter should cure what was occurring in the upper rpm band along with a possibly more rigid rocker arm. Valvesprings are one of the most critical and overlooked components in your engine. They must be matched to your cam in order for the engine to reach its full potential. It does absolutely no good to install a cam that will perform up to 8,000 rpm if you do not have the correct springs. Ultimately, a stiffer spring and lifter should allow the big-block to extend its power curve and once in the car, allow a higher rpm shift that would enable more time in its sweet spot. While on the dyno we were only expecting to clean up the valve float issue, we also found an extra 40 hp in the process and extended our reach by 600 rpm.
In order to make sure we weren't chasing any ghosts, we placed the 496ci back on the dyno, only this time we headed over to Westech Performance. We started by running the motor through the rpm and backed up each run for our baseline numbers. Since our original intention was to eliminate the valve float condition, our first line of defense would be to replace the rockers along with the other major valvetrain components. From there, we tested each individual modification and not only did we completely resolve the valve float condition, we extended our upper rpm capabilities by 600 rpm. In the end, we installed a new set of springs with titanium retainers and locks, including a fresh set of roller rockers and lifters. Read on as we document our findings and learn how the minor changes completely altered the characteristics of this big-block powerplant.
What We Did
Installed a new set of stiffer springs and higher tolerance lifters to create a higher revving 496
Pull the rpm longer to shift back into the powerband