Holy heaps of lb-ft, Batman, this 572 Rat is wicked. Although Robin has a reputation for getting excited far too easily, there’s good reason for his enthusiasm this time around. The Smeding Performance 572ci crate big-block presented before us makes 692 lb-ft of torque, but that’s not the impressive part. That 692 figure isn’t peak torque, but rather the motor’s average torque throughout its entire 2,600- to 6,000-rpm rev band. Yikes. Peak torque checks in at 740 lb-ft at 4,400 rpm, and better yet, the massive Rat kicks out 700-plus lb-ft from 3,900 to 5,000 rpm. Now that’s what you call kicking some serious heinie throughout a very broad rpm range. That said, man does not live by lb-ft alone, and torque without horsepower is nothing more than a glorified dump truck motor. Fear not, for the Smeding big-block backs up its impressive torque tally with 737 hp. Can you say block-long burnouts with 2.73:1 gears? Now this is our kind of motor.
The Smeding 572 Extreme crate motor certainly has all the right stuff. It’s based on a Dart Big M block, and fitted with rugged forged internals. Up top, Airflow Research 325cc aluminum cylinder heads, an Edelbrock Victor Jr. intake manifold, and a Quick Fuel Technology 850-cfm carb provide the air supply while a mild COMP 252/262-at-0.050 cam kicks the valves open. Nevertheless, anyone can bust out their checkbook and order up a pile of nice parts. It’s the ability to select the right components and engineer a fundamentally proficient combination that separates the fly-by-night hacks from the experienced pros.
As Ben Smeding explains, big peak numbers aren’t everything. “It’s only natural to look at the peak horsepower and torque numbers that an engine makes, but we build our combinations for the widest powerband possible. “We try to maximize torque as low as 2,500 rpm because when you mash the gas at cruising speed, you want the motor to respond,” he says. This approach also means that the 572 produces peak power at 5,700 rpm, and is ready to shift by 6,000, reducing stress on the bottom end and valvetrain while increasing durability. Quite frankly, we were shocked to see the 572 big-block crank out 737 hp considering that it’s rated at 690 hp. “We dyno our motors using regular pump gas we buy down the street, and day-to-day variations in fuel quality and ambient air conditions can impact power output. As such, we underrate our motors to leave a safety margin for these variables. The bottom line is that we want to make sure that our customers are happy, and they’re getting all the horsepower that they’re paying for.”
By virtue of underrating one of its crate engine packages by 47 hp, Smeding Performance might just be one of the most honest engine shops in the business. As you might expect from such an outfit, the price of admission for this 572 is pretty darn reasonable as well. Complete from carb to oil pan—including the distributor and plug wires—the Smeding 572 Extreme lists for $12,995. We’re all for getting your hands dirty and building an engine yourself, but matching this kind of performance for the dollar is no small feat. DIY builds don’t come with three-year, unlimited-mile warranties, either.
Most engine shops only tune for peak performance at WOT, but the problem is that street motors operate at part throttle most of the time. As such, Smeding takes exhaustive measures to tune each of its crate engines for maximum driveability. Unlike many dynos that can only control throttle, Smeding’s DTS unit can place load on an engine independent of throttle input. In other words, the DTS dyno has one handle for throttle control and a separate handle for load control. “After we break in a motor, we set the idle mixture and ignition timing. Next, we increase rpm to 2,000, put 100 pounds of load against the motor, then check the air/fuel ratio again,” Ben Smeding says. “We want the air/fuel ratio to be in the mid 13s during this part of the tuning process. Next, we increase the load to 150 pounds, with a target air/fuel ratio of low 13s. We repeat the procedure at 2,300 rpm, and vary the load between 100 and 150 pounds, tweaking the air/fuel ratio as necessary with the air bleeds, jets, and mixture screws. Optimizing the fuel mixture in this range is what makes an engine driveable. If a motor is too rich or too lean at cruising speeds, it will run dirty and hesitate. The squirters don’t do much when you gently roll into the throttle, so they can’t cover up a carb that’s out of tune. After the tuning for driveability is complete, we never touch the primary side of the carb again, and only alter the secondaries when tuning for max power.”