Teddy Roosevelt said it best: Walk softly, and carry a big stick. In the world of high-performance Bowtie muscle cars, there's no bigger stick than a blown big-block Chevy, and since we're all muscle car fans we can forget about the whole "walk softly" thing.
You've seen plenty of big-block buildups before. Slap a centrifugal supercharger on a 502 crate motor or an HP-series Holley on a high-compression 427 and it's easy to make 500-plus horsepower. Here's the catch: horsepower doesn't mean squat on the street. In fact, horsepower isn't even a real number, it's a calculation based on torque and rpm. What you feel in the seat of your pants, that super duty grunt that makes your tires bark and your front wheels reach for the sky, is good old-fashioned torque. The trick to building a reliable pump gas big-block that will live for years and rocket your ride into the 10s is to keep the redline below six grand and the torque curve as flat as possible. That's what we set out to accomplish with our latest engine, the Long Rod Rat.
It's Hip To Be SquareOur original plan was to build a huge displacement mill with relatively high compression. However, since the engine will eventually end up in a street-bound Camaro that sees occasional track duty, our main goal was to build the engine to last. With that in mind, we decided to keep the compression low and keep the torque curve as flat and low as possible.
We started with a standard-bore four-bolt main 454 truck block, which was punched out .060-inch (up to .100-inch is safe with a solid block, but sonic check the walls to be safe). Rather than mixing and matching parts from different companies, we turned to the Lunati Custom Shop for a turn-key internal rotating assembly. Since a "square" engine is ideal for obtaining maximum torque, horsepower, and longevity, we ordered a 1/4-inch stroker forged stroker crank for the project.
A square engine is where the bore and stroke are roughly the same. In this case, that meant using a 4.250-inch stroke (stock is 4.00-inch), and a 4.31-inch bore (stock is 4.250"), for a total displacement of 496 cubic inches. Rather than using stock connecting rods, we ordered a set of 4340 forged steel 6.385-inch I-beam connecting rods, which are a quarter inch longer than stock (6.135-inch) Chevy rods. The longer rod changes wrist pin angle for improved operation, and flattens the torque curve for improved street performance. It also lengthens piston dwell time at top dead center, improving the combustion process. Finally, in order to compensate for the longer connecting rods, we had to order special forged pistons that have the wrist pin hole moved up higher to compensate. Luckily, Lunati offers these pistons right off the shelf, and since the wrist pin bore actually interrupts the bottom ring groove, they supply a special ring land seal that keeps cylinder pressure from leaking past the pin.
The Lunati Pro Series Engine Kit provided everything we need for the internals, from the forged crank, rods, and pistons, to King bearings and moly rings. Everything is fully internally balanced and blueprinted for maximum longevity performance.
Rather than running high compression pistons that require expensive race gas and put a huge load on the motor-whether you're on the throttle or not-we decided to combine a moderate compression ratio and a small supercharger. The combination of bore, stroke, and a stock thickness Cometic steel laminate head gasket with a flat-top piston worked out to 8.4:1. That number is low enough to be streetable and pump-gas friendly when the motor is in vacuum, and when the boost comes on it's still low enough to keep detonation at bay and keep things reliable.
Our first thought was to run a centrifugal supercharger with a blow-through carburetor, but since the eventual home for the engine will be a Pro Street-style Camaro, we decided to go with a wilder look and bolt on a Roots-type blower. Rather than using a 6-71, we went with a Weiand 250 fitted with Teflon-tipped rotors for maximum efficiency. Topped with two Holley 750 blower carbs (specifically engineered to work with the altered vacuum signal from a Roots blower), the package is relatively compact, powerful, and impressive looking.
Selecting the right cylinder heads is a key step in a big-block Chevrolet buildup. Several companies make impressive oval port aluminum heads that make big top-end power like a square-port and solid bottom end torque like old-fashioned round-port heads. However, since we're dealing with increased displacement and the added airflow of a supercharger, we decided to use Holley's aluminum rectangle port heads. They come from the factory fitted with Manley heavy-duty stainless 2.25-inch intake and 1.88-inch exhaust valves, screw-in 7/16-inch studs and guide plates, and heavy duty springs. These heads can support up to 1,000 horsepower, so we knew they'd be more than appropriate in this application.
Finally, the cam choice was a big decision. Rather than stepping up to a huge lift solid roller that would push the powerband into the stratosphere, we went with a very conservative Lunati flat-tappet hydraulic blower cam specifically designed for street use with Weiand superchargers. With 224/234 duration at .050-inch, and .498-/.520-inch gross valve lift and 112-degree lobe centers, the cam is quite mild for a 500-inch motor. However, it kept the peak horsepower numbers below 5,500 rpm and the torque curve was as flat as the Nebraska plains. Perfect for the ultimate street/strip brawler.
Our first pulls on the Jim Grubbs Motorsports dyno were intended to squeeze maximum yield from the long rod 496. We locked the timing in at 30-degrees, put the small pulley on the blower, and filled the fuel tank with 110-octane race gas. After a few pulls, we found that 80 jets in the primaries and 86 jets in the secondaries worked best, and the boost gauge was peaking at 12.5 pounds. That's a lot of boost, but the numbers made our jaw drop. The torque curve was nearly flat, running over 600 lb-ft from 3,000 rpm to redline, with a peak of 836.7 lb-ft at a relatively high 4,800 rpm. Power doubled from 3,500 to 5,300 rpm, where it peaked at 855.2 hp.
Our second setup of the day was a more conservative setup. The big blower pulley was bolted on, which produced a significantly more conservative 5.5 pounds of boost. We swapped in 78 and 82 jets for optimal air/fuel, and dialed the timing back to 28 degrees. The fuel tank was drained and filled with 91-octane gas from the corner station. While not as awe-inspiring as the race-gas figures, the motor still made well in excess of 500 lb-ft of torque throughout nearly the entire rpm range, with a peak of 688 lb-ft at 4,900 rpm. Horsepower peaked at nearly 700 at a relatively low 5,500 rpm. Overall, running the engine in pump gas trim lost 157 horsepower and 148 lb-ft of torque, but with average numbers in the high 500-range, this mill could still propel a well-prepped street car into the 10s on pump gas.