The idea behind building power is simple, remaining almost unchanged since the inception of the internal combustion engine. The quicker and more efficiently you can get the air and fuel mixture in
and the spent fumes out, the better your chances. When that’s not enough, adding more bore and stroke to the mix not only adds precious cubic inches, but creates a platform to handle even more air and fuel.
Our ZZ502ci crate motor from GM Performance Parts was in need of an update. At the time, our big-block produced some serious grunt from the factory with only 9.6:1. With 502 hp and 567 lb-ft, it made the perfect engine swap for any muscle car, instantly transforming it into a pump-gasfriendly driver. While it would have been easy to bump up the compression, add a big-lift camshaft with matching springs, and a single-plane manifold outright, we were after an engine that maintained its already docile characteristics, but increase overall output at the same time. We took each upgrade a step at a time and outlined the gains. This engine was to remain a driver with occasional trips to the track and would rarely see anything over 6,000 rpm. We wanted to showcase how much extra work is involved to bore and stroke the ZZ502. Would the extra cubic inches be worth the added labor and cost? In an effort to take a scientific approach to each portion of the build, we’ve outlined our plan with four separate dyno tests.
For our baseline numbers, we’ve utilized a freshly assembled ZZ502 using a COMP Cams roller camshaft with 0.521/0.540-inch intake/exhaust lift and 236/242 intake/exhaust duration at 0.050 inch on a 106 lobe center. We also upgraded the springs and lifters, including a Weiand oval-port manifold to the crate ZZ cylinder heads. From there, we stroked the ZZ502 to 540 ci, utilized the same valvetrain with a Weiand manifold, and compared those gains. To maximize our efforts, we took it a step further by adding larger Dart Pro 1 cylinders heads, again using the original valvetrain. Finally, we swapped out the camshafts with more lift (0.540/0.560-inch intake exhaust) and duration (242/248 intake/exhaust at 0.050 inch) to capitalize on the additional flow of the Dart Pro 1 cylinder heads.
The best part is we’ve documented the entire build by illustrating the differences between the ZZ502 and the stroked 540. With the final numbers produced, our efforts were worth 90 hp and nearly 81 lb-ft. On the plus side, the 540 is just as reliable with a healthy helping of additional low-end grunt.
Pistons: 502 vs. 540
While the freshened-up version of our 502ci received KB hypereutectic pistons, our stroked-out 540 ci got a forged SRP treatment. Both sets of 4.500-inch bore flat-top pistons came with a single valve relief and 1/16-, 1/16-, and 3/16-inch ring thickness sets. Wristpin diameters were also the same at 0.990 inch. KB’s piston set offers an affordable alternative to the forged version from SRP, yet are still 30 percent stronger than ordinary untreated hypereutectics since they are 390 alloy heat-treated pieces. They also feature 100 percent CNC crowns, a high upper compression ring location, gas accumulator groove, and drilled oil returns, and accepts spiral-lock retainers. Our inverted dome SRP pistons are made from 4032 low-expansion, heat-resistant high-silicon aluminum alloy. Their ring lands and crown thickness are specifically engineered for extra load and also include pin fitting, double Spirolocks and wristpins.
Our internally balanced rotating assembly engine kit from Northern Auto Parts (HP540K) came with all the major components we needed to stroke out our 540. Included was a Scat 4.250-inch forged crankshaft, 6.385-inch H-beam rods, with forged SRP pistons and a plasma ring set. To complete the kit, Clevite rod and main bearings were also included.
Cylinder Heads: Bow Tie 502
For baseline runs of the freshened 502ci mill and stroked 540ci engine, we utilized a set of GM ZZ502 cylinder heads (PN 12363392). They come with 290cc oval intake ports and 110cc exhaust ports with equal-sized combustion chambers. While they can be purchased complete as off-the-shelf pieces from GM, JMS Racing performed a small valve job by installing larger, 2.250/1.900-inch intake/exhaust valves and heavier-duty COMP dual valvesprings.
Cylinder Heads: Dart Pro 1
While we got our set from Dart Pro 1 cylinder heads as a bare set, Aerohead Racing Components performed the custom assembly for our 540ci stroker build. Race-proven features and affordable pricing make these Dart Pro 1 355-T6 aluminum cylinder heads one of the biggest values in racing. The Pro 1 cylinder heads feature 119cc combustion chambers and larger, 325cc intake runners with 129cc exhaust runners. Aerohead’s team added 2.300/1.880-inch intake/exhaust stainless steel valves, Aerohead spring cups, hydraulic roller 1.550-inch dual valvesprings (354 lb/in), and 10-degree split locks with Viton seals. ARP 7/16-inch rocker studs were also installed along with Dart adjustable guideplates.
Holley 950 HP
Holley’s HP-Series carburetors include updates to their traditional line of 4150-style pumpers. These updates include cast main bodies without choke horns and smoothed venture transitions for increased airflow. Since they’re aimed directly at optimizing power and performance for racing applications, other important modifications include screw-in air bleeds, stainless steel throttle plates, and notched floats. Holley also has Dominator-style fuel bowls to allow fuel line plumbing from either side. All of our 502ci and 540ci testing used the 950 HP for dyno pulls at JMS. CHP