“A new generation of 800-cubic-inch engines has turned those old Mountain Motors into molehills.”
Back in the early days of Pro Stock match racing, any engine with more than 500 cubic inches of displacement was labeled a “Mountain Motor.” Now a new generation of 800-cubic-inch engines has turned those old Mountain Motors into molehills.
Displacement is king in the Land of the Giants. When I look at the engines we’re now building at Reher-Morrison Racing Engines, I’m amazed at the number of big-inch motors that we’re producing for classes ranging from Top Sportsman and Top Dragster to Pro Mod. While we still assemble dozens of 500-600ci Super Series big-blocks for bracket racers and Super eliminator cars, we’ve seen phenomenal growth in the market for 700-800ci motors.
The availability of aftermarket cast-aluminum and billet aluminum blocks has made it possible to build these massive motors. One of the key dimensions in a block is its bore centerline – the distance between adjacent cylinders. This dimension determines the maximum diameter of the cylinders, which in turn dictates the maximum valve sizes.
The bore centerline in a production Chevrolet big-block V-8 is 4.840 inches, and the maximum allowed in NHRA Pro Stock is 4.900 inches. But racers in classes that aren’t restricted by Pro Stock rules recognize that a bigger bore is even better. The hot rod industry has met this demand by creating cast-aluminum blocks with 5.00-inch bore centerlines. When engine builders began to push the limits of these cast blocks, aftermarket manufacturers responded by machining blocks from a solid billet of aluminum.
With 5.200-inch bore centerlines, these billet blocks are more than an inch longer than a production block to accommodate the huge cylinder bores and to provide room in the crankcase for the massive counterweights that a long-stroke crankshaft requires. Combine 4.875-inch diameter cylinders with a 5.350-inch stroke crankshaft and the impressive result is 800 cubic inches of piston displacement. Add a four-stage nitrous oxide injection system and the payoff is a 2,400-horsepower Pro Mod engine.
Regular readers of this column know that my engine building mantra is to use the right parts for the intended application. In keeping with that long-running theme, I’ll state right now that not every racer needs a billet block. If the goal is to build a 1,500-horsepower engine for a Powerglide-equipped dragster, then a cast-aluminum block will almost always be suitable. But if the plan is to assemble a nitrous-injected firebreather to power a six-second Pro Mod, then a billet block is a necessity.
In applications where a cast-aluminum block is a logical and affordable choice, the quality of the casting can be significantly improved by having the block ”HIPped”. HIP is an acronym for Hot Isostatic Pressing, a process that subjects a component to high temperature and high gas pressure simultaneously. The part is placed inside a pressure vessel that resembles a bathysphere used for deep sea exploration. The heated chamber is then pressurized with gas (usually argon) to more than 15,000 psi for a period of time. Treating a cast-aluminum block or cylinder head with this process actually compresses the aluminum, reducing its porosity and improving its mechanical properties. Cast aluminum that has been through the HIP process has a higher tensile strength and higher yield strength than untreated aluminum.
I’m not a metallurgist, but I can definitely see a difference in a block or cylinder head that’s been through the HIP process when we machine it. The metal is much denser and more uniform than an untreated casting.
Even after the HIP treatment, a cast-aluminum block still has limitations. A casting is less elastic than a block made from a billet of 6061 aluminum, so it is more prone to cracking when subjected to stress. While repairing a cracked cast-aluminum block is not as difficult as welding cast iron, it still requires considerable skill to do it properly.
The popularity of billet blocks in high-end applications is growing exponentially. Just as Top Fuel and Funny Car racers exceeded the capabilities of cast blocks several years ago, Pro Mod and Mountain Motor Pro Stock racers are now making more power than a cast block can endure for an extended period of time. The 6061 aluminum used in forged billet blocks is stronger and denser than the 356 alloy typically used in a casting – 6061 has a tensile strength of 60,000-70,000 psi, compared to 13,000-14,000 psi for typical cast aluminum. As a result, billet blocks are almost indestructible. I’ve seen billet blocks survive blowups that would have destroyed a cast-aluminum block. Although more expensive than a cast block, a billet block is a good investment over the long run from the standpoint of longevity and ease of repair.
The other big advantage of a billet block is its versatility. A billet block is CNC machined from a 700-pound chunk of solid aluminum, so the only design limitations are the programmer’s imagination and the equipment and software that is used to machine it. A casting, in contrast, is restricted to whatever the patterns and molds can accommodate.
It’s this freedom of design that makes billet blocks so desirable for big-inch drag racing engines. When we order a billet block from Dart, it’s not a problem to spread the bore centers, raise the deck height, relocate the camshaft higher in the block, or specify a special head bolt or oil pan pattern. Dart can also machine full water jackets that have cooling capacity that rivals a production casting, with CNC-machined plates and O-rings to seal the water passages. Depending on the complexity of the block design, the machining procedure takes from 25 to 40 hours and turns approximately 575 pounds of metal into aluminum chips!
It’s amazing to me that 25 years ago we used a cast-iron block as the starting point to build a championship-winning 615ci Mountain Motor. Now we build dozens of sportsman racing engines every year that are bigger and far more sophisticated than those engines. With the advent of billet aluminum blocks and 800+ cubic-inch engines, the mountain looks much bigger in 2008.