Have you ever wondered how...
Have you ever wondered how much of a difference various rocker arm designs can have on your engine's performance? We did, too, and decided to find out with a full repertoire of rockers from Comp Cams and a lot of dyno time.
For racers, the most pressing questions we ponder late at night aren't about the meaning of life, the true meaning of beauty, or other such silliness. Yes, those things have their place, but we all know the real head-scratchers are questions like "Where's the balance between outright speed and a good-handling package to get through traffic?" or "How do I make the outside line work for me?" and maybe even "Is spending the extra money on a new (insert your engine or chassis component here) worth it?"
We can't do much with meaning-of-life questions, but we can help with the motorsports-related ones. In the Mar. '06 issue of Circle Track, we debuted our Chevrolet 350 dyno mule by testing several oil pan configurations. Now we are back again, curious to find out how different rocker arm designs affect performance.
The idea behind Circle Track's dyno mule is to scientifically test different ideas to get to the real truth and not just repeat the same old magazine blather you've read year after year. The idea that press-in rocker studs on stock heads should always be replaced with screw-in studs was tested. We also used the dyno mule to find out if full-roller fulcrum rockers versus the less-expensive pivot ball rockers are worth the money.
On a previous dyno session,...
On a previous dyno session, our mule started wheezing at 4,500 rpm, which isn't nearly fast enough to stress the drivetrain as much as we wanted. To cure that ill, we switched out the stock 23-degree cylinder heads (right) with a pair of Pro Action iron heads from Racing Head Service. Bigger valves, better ports, and a more efficient combustion chamber helped move the peak power to 6,100 rpm.
To verify these notions, we devised a plan with our test partner, the NASCAR Technical Institute (NTI) in Mooresville, North Carolina. NTI is a technical training school that combines general automotive education with an advanced regimen specializing in stock car racing. In addition to fabrication, pit crew training, and other racing specific courses, NTI also offers its students an engine-building program stocked with two dyno cells (engine and chassis) and enough Nextel Cup-legal engines to start your own multicar team. We provided the engine and test components, NTI provided the dyno facilities, and the students got to see firsthand how small changes in an engine's components package-this time rocker arms-can affect overall performance. When we tested with NTI before, we were quite impressed with the knowledge of the instructors as well as the quality of the students and were eager to work with them again.
The parameters for the test are simple: Test each set of rockers on the same engine on the same dyno and compare the numbers. For consistency, we chose to work only with rockers from Competition Cams. Comp's products are of consistently high quality, so we knew that any differences would be attributed to differences in design and not quality. We also kept the rockers as close to a 1.50:1 ratio as possible. For the stud-mount rockers, ARP studs were used.
In all, we tested six different sets of rocker arms. We chose Comp's High Energy (PN 1212-16) stamped steel, Magnum Roller (PN 1412-16), Hi-Tech Stainless (PN 1104-16), Pro Magnum (PN 1304-16), aluminum (PN 1004-16), and shaft mount (PN 5412160) rockers. The High Energy and Magnum Roller used a 31/48-inch rocker stud while the Hi-Tech, Pro Magnum, and aluminum rockers used a 71/416-inch rocker stud. For the High Energy, the lash was set at 0.010 for both the intake and exhaust. After experiencing trouble, which will be discussed in detail later, the lash for all other rocker arm sets was opened up to 0.014 for the intake and 0.016 for the exhaust and not changed again.
In addition to the new heads,...
In addition to the new heads, we also added a new camshaft from Comp Cams. We asked Comp for something that would really test the stability of the rockers and studs, and they responded with a solid roller with 0.400 lobe lift and pretty strong opening ramps. With the heads and correct rockers, the combination produced an amazing 50 lb-ft increase in torque and 133 more horsepower.
For the first round of dyno runs testing oil pans, we used a standard Chevrolet crate motor with a cast crank, cast pistons, 8.50:1 compression, and four-bolt main caps. It was outfitted with a Weiand Action Plus dual-plane intake, a Holley 650-cfm four-barrel carburetor, a 1-inch carb spacer, and a CV Products water pump and pulley set. Peak power averaged 355 pounds of torque at 3,600 rpm and 275 hp at 4,400 rpm.
That was fine for an oil pan test, but for rocker arms the horsepower peak was definitely too low to find any valve instability problems. The main problem was the stock cylinder heads started starving the engine of air way too soon. To properly stress the rocker arms, we needed a test mule capable of much higher rpm levels.
By working with Comp's engineers, we spec'd in what we thought would be a much better test mule. The oil pan from Champ Pans was kept because it showed good performance on the dyno. The stock heads were ditched in favor of a set of cast-iron heads from RHS (PN 12319). These 23-degree cylinder heads feature small 64cc chambers, 2.020 intake, 1.600 exhaust valves, and 200cc intake runners. They were assembled with beehive springs that tested around 180 pounds on the seat. Intake and head gaskets from Fel-Pro sealed everything, and ARP head studs were used to lock down the heads. We also asked Comp for an aggressive camshaft that would really stress the valvetrain, much like a typical race engine. They complied with a solid roller unit with 0.400 lobe lift (when matched with the 1.5 rockers that was 0.600 lift at the valves), and some very aggressive opening ramps. Since we had a new cam, we also had to ditch the stock hydraulic flat-tappet lifters in favor of a set of solid roller lifters from Comp. The cam was installed in the engine with no advance. Finally, to make sure we had plenty of clearance for the many different styles of rocker arms we planned to bolt up, Moroso set us up with a set of high-rise fabricated aluminum valve covers (PN 68326).