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
Dyno Test 1
Brulé began to install each new rocker arm into place. Since this is still a hydraulic valvetrain setup we would not need to adjust the lash. With everything buttoned up, it was time to make a pull and see the results. In this instance, we found a slight gain in horsepower and torque. We also managed to clean up the stumble in the upper rpm.
|BASELINE||PRO MAGNUM ROCKERS||GAIN|
|TIMING||42 degrees||TIMING||42 degrees||~|
|AVG HP||488||AVG HP||490||+2|
|AVG TQ||540||AVG TQ||541||+1|
|PEAK HP||566 @ 5,700||PEAK HP||568 @ 5,900||+2|
|PEAK TQ||577 @ 3,700||PEAK TQ||578 @ 3,700||+1|
Dyno Test 2
Brulé began the teardown by unbolting the intake and removing it. He then removed each lifter and replaced them with the "R" series lifters. The new lifters are a simple drop-in item and didn't alter the pushrod length whatsoever. We then installed the Ultra-Pro.
|PRO MAGNUM LIFTERS + steel rockers||GAIN|
|PEAK HP||576 @ 5,900||+10|
|PEAK TQ||580 @ 3,700||+3|
Dyno Test 3
After reviewing the results, the 496ci gained torque and horsepower. Brulé still wasn't completely satisfied though, noting "The cam should have still been able to make power up to the suggested 6,100 rpm." With that he suggested that the valvesprings may be too weak. The combination of a higher tolerance lifter and higher revving cam may be overwhelming the spring. It was then Brulé decided a spring swap and a set of titanium retainers (PN 721-16) would allow more control over the valve at high rpm. Titanium is lighter and stronger than steel, which allows for additional engine speed. Sure enough, power was up again and even helped to smooth out the entire power curve.
|DUAL SPRINGS + PRO MAGNUM LIFTERS + STEEL ROCKERS||GAIN|
|PEAK HP||579 @ 6,000||+13|
|PEAK TQ||583 @ 3,700||+6|
While our build isn't a high-budget racer, the 496ci is still seeing gains with high rpm punishment. Since the camshaft is still rated to make power up to 6,100 rpm, we needed a stiffer valvespring to handle the upper rpm. The good news is, these springs are a direct swap and do not require any additional machining to the RHS cylinder heads.
|SEAT LOAD||148 lbs @ 1.900-inch|
|OPEN||456 lbs @ 1.250-inch|
|SPRING RATE||473 lb/in|
Dyno Test 4
The review of the dyno graphs showed a more linear power curve and also a substantial gain in upper rpm power with more grunt down low at the same rpm. Would it be possible to still get more power from the 496ci? Brulé had one more trick up his sleeve and re-installed the original Ultra-Gold rocker arms from our baseline configuration. Here's why; in a nutshell, we've changed the combination to a lightened valvetrain assembly and we could benefit from additional weight savings with the aluminum. Hydraulic vavletrain components benefit from lightweight pieces since it acts like an increase in the spring pressure without jeopardizing the lifter-it could collapse under too much pressure. To Brulé's credit, we were happy to find another increase in power and a smoother power curve throughout the rpm.
|VALVESPRINGS + RETAINERS + PRO MAGNUM LIFTERS + ALUMINUM ROCKERS||GAIN|
|PEAK HP||582 @ 6,100||+16|
|PEAK TQ||580 @ 3,800||+6|
What's It All Worth
To recap, we began with an excellent turnkey motor for the street. 496 ci is plenty big for hauling groceries around town, but this motor will also be spending a lot of time at the strip. It's important that the motor rev high enough and take full advantage of its valvetrain and cubic inches. Taking advantage of Comp's tech line, it was explained to us that the valvetrain components that come assembled with the RHS 320cc cylinder head will satisfy your average daily driver and weekend warrior when matched with the original hydraulic roller lifters (PN 854-16) and cam (PN 11-602-8) to its rated 6,100 rpm. Comp continued to explain that it's when you start asking more (above 6,100 rpm) from the combination that it will not suffice. Hence, the reason behind swapping to stronger springs, titanium retainers, and "R" series lifters. While it's difficult to illustrate how quickly the motor will rpm on the dyno, we are confident the driver will certainly feel it in the car. With the newer lightweight components, the engine will pull harder, faster, and stronger to its shift point-ultimately landing the motor back into its sweet spot after the shift.
1. Big-blocks need fuel to burn. We planted a Holley 950 HP on top of the Weiand dual-plane manifold. We jetted the carb with 80/80 primary/secondary jets for the all of our dyno pulls.
2. Out of the box, these 24 degree RHS 320cc cylinder heads are a bargain at just $1,020 each. They come with a 112cc combustion chamber which keeps compression low for the street and also rectangular ports. Exhaust ports are raised 0.500 inches.
3. Our cam came with 243/257 degrees of duration at 0.050 and 0.570/0.554-inch lift (intake/exhaust). This cam is good for and rated to make power from 2,500-6,100 rpm.
4. Joining our valvetrain is a set of Ultra-Gold rockers. The set only costs $273 and keeps the factory 1.7:1 ratio. These come complete with poly-locks and install on a 7/16-inch stud.
5. The foundation is a factory GM four-bolt main with steel caps and bored 0.060 inches over. SRP 4.310-inch pistons swing on an Eagle 4.250-inch stroker crank with 6.385-inch H-beam rods.
|The Shopping Cart|
|COMP||Ultra-Gold 1.7:1 roller rockers||19021-16||$273|
|Ultra Pro Magnum roller rockers||1320-16||270|
|Hydraulic roller lifters "R" series||887-16||469|
|Super valve locks||611-16||20|