Did you know it costs no more to buy a 540ci Rat rotating assembly than it does for a 496? It's true. The explosion of Super Pro bracket drag racers building big-inch motors has jacked the volume so much that what used to be considered exotic drag race engine parts are now available at bargain-bin street prices. Ohio Crankshaft offers a 540/555/565 stroker rotator package that includes 4340 steel 4.250-inch crank, SRP pistons, 6.385-inch, H-beam 4340 steel rods, Total Seal rings and Federal-Mogul bearings all for less than $2,000. Bolt that spinning combo into a Dart block and for less than $4,500 you have the basis of a killer big-block. Of course, Ohio Crank isn't the only game in town. Companies like Scat, Eagle, Callies, and Crower offer packages that range widely in price, depending upon quality. We found a Scat rotator assembly with a 4340 steel crank, H-beam rods, pistons, rings, and bearings for $2,030 through Summit Racing. Like these, there are dozens of combinations too numerous to list. Variables include important stuff like metallurgy where 4340 is stronger than 5140, which is, in turn, stronger and more ductile than cast iron. Small things like rolled fillet edges for the rod journals drastically increase strength, even for a cast crank. One way to compare cranks would be to evaluate weight. A lighter rotating weight requires less horsepower to accelerate. This won't show up on an engine dyno but will reveal itself on the dragstrip or an acceleration-rate-chassis dyno, where power is extrapolated by how quickly the engine accelerates the drum. Of course, a big stroke needs counterweight to offset piston and rod mass. This is why buying a complete rotating assembly is a very good idea since the company has already done the bob-weight research.
Tall Deck: 10.20
The only reason to build a stroker big-block Chevy is to make power. The best way to get maximum benefit from all this displacement is with a great set of heads. Here is where the best money is spent in terms of dollars per horsepower. High-flowing big-block heads are not cheap, so don't be misled by the siren song of budget Chinese heads. The good stuff all comes from domestic-based companies like AFR, Brodix, Dart, Edelbrock, and TFS. There are also dozens of good race heads out there, but we'll keep our discussion centered on mainstream heads. For a 540- or 572-inch Rat, we're going to focus on rectangle-port heads with an intake-port volume of between 325 and 355cc. Cam specs will determine the peak torque and horsepower points based on intake lobe duration, but let's apply Westech's Steve Brulé's formula for power that uses 1.25 torque per cubic inch (lb-ft/ci) formula for a 10.5:1 compression, pump-gas Rat. Applying this formula (1.25 x ci = peak torque) to a 540 creates 675 lb-ft of torque. Using 90 percent of that number for torque at peak horsepower at 6,500 rpm (675 x 0.90 = 607 lb-ft) delivers 751 hp (607 x 6,500 / 5252 = 751). That's amazing power for a single four-barrel pump-gas motor.
We've listed just a few of the more impressive cylinder heads on the market today so you can get an idea of what's out there. Peak flow numbers are typically in the near-400-cfm-plus range. Our chart lists the flow at 0.600-inch lift because that's a reasonable valve lift number for a street-driven camshaft. Often these heads will flow 400-plus cfm at 0.700- or even 0.800-inch lift. For best peak horsepower, don't forget to inspect the exhaust flow numbers. One way to evaluate an exhaust port is to divide the exhaust flow cfm at a given valve lift by the intake cfm; the result will be expressed as a percentage. A rule of thumb is a good exhaust port will come in at 70 percent or above. But don't condemn a head that fails to reach this 70 percent level, but it will require more exhaust duration (more time) to evacuate the cylinder at high rpm. Also keep in mind that all big rectangle-port big-block Chevy head manufacturers will raise the exhaust port between 0.250- and 0.600-inch above the stock big-block port location. This radically improves exhaust port flow but makes it difficult to fit an off-the-shelf header. This is especially problematic when using large 21⁄8- to 21⁄4-inch primary pipe diameter headers. It's best to consider that when building a 500-plus-ci big-block those custom headers should be part of the budget.
Big-Block Head Chart
||Int. cfm at 0.600*
||Exh. cfm at .600*
||E/I at 0.600*
|AFR 357 CNC
|Brodix BB-3 366cc
*The cylinder-head flow numbers were taken from manufacturer published data. There are multiple variables among these numbers from different flow benches, bore diameters, and other inconsistencies that directly affect these flow numbers, so all these numbers should be used merely as trend indicators and not as hard facts. If there are enough interested readers, we could be persuaded to do a more complete rectangle-port head test in a future issue.
Mega-inch motors will make monster power upstairs but only when valve motion is under full
One thing to watch with any good Rat head is a raised exhaust port. For example, the AFR h
Of course, if you are going to build a 540- or 572ci Rat, you are going to need a set of l