Most of you hard-core gearheads know all about World Products’ Merlin heavy-duty Chevy Rat-motor blocks and cylinder heads, which combine performance and durability without driving the working man to the poorhouse. But what you may not know is that for the last two years World has been building some of the most kick-ass crate motors in the business. The World big-block crate motors are built up from all-new parts, then run on the engine dyno for one hour—a 45-minute initial break-in on the outer valvesprings only, then at least six full-throttle, full-power runs. And World Products covers every Merlin with a $4,000, two-year/24,000-mile parts-and-labor warranty.
The best-selling package is an iron-headed 509ci stroker engine. With its massive 4.5-inch bore and 4-inch stroke, the 9:1 motor carries an official 540hp rating on 92-octane pump gas. Over the last two years World has built 480 of them—a quantity that virtually approaches mass-production status for an aftermarket specialty engine. The obvious learning curve generated from so many engines has resulted in dozens of incremental running improvements in honing procedure, carb calibration, piston ring end-gaps, and balancing—so much so that the actual power of current assemblies has crept up to the 580-600hp range (although the official rating remains unchanged).
The real-world power gain is now so high that World will build future Merlin engines with a hydraulic flat-tappet cam in place of the previously used solid flat-tappet cam. According to World Products President Bill Mitchell, the change is by popular demand. Apparently too lazy to run the lash, today’s spoiled Car Crafters are willing to trade off 21 hp and 20 lb-ft of torque for the hydraulic cam’s greater convenience. But that should still bring the engine in at 560-580 hp, well in excess of its “official” rating.
What You Get
In the meantime Car Craft got a chance to inspect and wring-out one of the solid-cam 509s on Westech Performance Group’s SuperFlow engine dyno. The 509 came complete and assembled with virtually every part you need to make it run, less the water pump, flywheel, headers, and—for some odd reason—an oil pan dipstick. Mitchell says World is looking into supplying dipsticks on future builds. The block was painted bright blue, but the heads were delivered unpainted—apparently due to local emission laws at the cylinder-head foundry in Michigan. World is in the process of installing a spray booth for painting the heads.
We were impressed with the stout four-bolt–main Merlin II block, which sports extra-thick cylinder walls (4.560 max allowable bore size), blind-tapped head-bolt holes, thick main webs, reinforced bulkheads, and priority main oiling system. The blocks can accommodate up to a 4.250-inch–stroke crank without clearancing (clearancing allows up to a 4.750-inch stroke). The short-block is packed with premium components, including a Scat forged crank, Manley rods, Speed-Pro rings, Wiseco pistons, Clevite bearings, and Fel-Pro gaskets.
Cast-iron Merlin rectangular intake-port heads are no lightweights—both from a weight as well as a performance standpoint. With ports practically big enough to shove a fist through, the high-flow castings are designed to support engines up to 540 ci. For those with hefty wallets (or weak backs), Merlin aluminum heads are a $1,000 extra-cost option.
Induction is handled by Merlin’s proprietary single-plane intake, which is actually version 2.0 in the manifold’s design evolution. World has taken 1,200 cfm out of the intake runner volumes. In the real world, the big-block’s heavy reciprocating mass and overly large cylinder heads make the engine slow to accelerate when installed in a vehicle—the reduced runner volume crutches the problem, improving average drag e.t.’s by at least 0.2 second. Mitchell says the improved single-plane is also up 3-4 hp at 2,500 rpm and 15 hp at 4,000 over Merlin’s dual-plane offering.
Rated at 870 cfm, the carb is a hybrid Holley double-pumper built up from a machined 750 main-body and 850 baseplate. A pure 850’s air-speed is too slow; a 750’s is too restrictive for so many cubes. The carb retains choke provisions if the owner cares to add one. Variations in the carb castings remain one of the biggest tuning variables; and even though each carb is individually tweaked and tuned when the motor is dyno’d, they are primarily responsible for the up to 20hp real-world output differences in these otherwise identical engines.
What It Makes
True to World Products’ claims, the engine came out of the crate ready to run—just add oil. No break-in or timing adjustments are necessary. World Products recommends 36 degrees of ignition lead, and that’s where the timing marks lined up. No carb adjustments are required, not even idle mixture. Just slam it on the dyno (or in your car) and yank the throttle (or mash the pedal). So that’s what Westech did. The only deviation was that Westech used its standard headers with 2-inch primaries x 3-½-inch collectors instead of World’s recommended 2-1/8-inch x 3-½-inch headers.
Would you believe 580.9 hp at 5,700 rpm, and 586.9 lb-ft of torque at 4,300? How ’bout over 500 lb-ft throughout the entire 3,100-6,100 rpm test range, and over 500 hp from 4,500 rpm on up? And all this for $7,495, not including shipping and handling (and, for New York state residents, sales tax).
Want more? There are Merlin 540ci “600 hp” crate motors that make in excess of 650 hp. Then there are custom builds that go as large 632ci; these churn out at least 775 certified dyno horsepower and 810 lb-ft with just 9:1 compression. When it comes to Rats, Merlin spawns a nasty breed.
