The short-block was built to endure rpm and a fury of nitrous induced horsepower. Last month, we showed how a Dart Big M block was filled with a Kellogg crankshaft (4.500-inch stroke), JE custom pistons (13.6:1 compression), and GRP rods. The Dart Big M block is made of iron, providing stability to keep the cylinder bores from moving around and that means better ring seal. It was modified with 4.600-inch bores and Pettis moved the lifter bores in order to straighten the pushrod angle and prevent them from intruding on the intake port. The pistons went through a barrage of modifications from tension tests with its rings and modified domes for the valve layout of these heads. The short-block and ultimately the entire long-block were assembled and disassembled several times as Pettis checked every moving part for proper clearances and fitment.
On the dyno, the Pettis 598ci didn't disappoint as it spun all the way up to 8,800 rpm without any trouble at all. The heads, intake, and valvetrain performed admirably and the final numbers are a stout 1,120 hp at 8,000 rpm. On the torque side, Pettis began the power sampling at 6,500 rpm so the only peak torque number he has is 810 lb-ft at the bottom of the pull (6,500 rpm). Peak torque probably occurs closer to 6,200 but since the engine will never run that low there is no reason to test its power output there.
Pettis dissected the dyno results, "I look at the average in a 1,200 or 1,300 rpm spread, which is the optimum range of the motor with the torque converter. This engine made 1,102 average from 7,200 to 8,500." He continued, "it doesn't fall off and will run real hard on track." Pettis did explain that this engine could have much more power if they set it up for naturally aspirated trim. But nitrous oxide injection is in the future for this beast. "It makes great power despite only having 13.6:1 compression and the pistons sitting 0.040-inch below the deck," he stated. The piston location below deck is not good for quench as it slows down the burn-rate of the fuel, which is good for use with nitrous but bad for making power in naturally aspirated trim. Au naturel engines require the fuel to burn quickly to make power. Another noteworthy item is that this engine was run on VP Fuel N20 gasoline, which Pettis says makes 35-40 hp less than VP C14 or Q16. "We were dialing it in for nitrous, so we dyno with the fuel that we run on the track to eliminate a variable," he explained.
All this talk of nitrous oxide might have you wondering about what was plumbed into this manifold. "We installed two Edelbrock E3 direct-port nozzle systems in the runners and then added a third stage of nitrous with our Punisher plenum system," commented Steve Johnson of Induction Solutions, a nitrous specialty company located in Brooksville, Florida. Johnson also flowed all three nitrous systems before shipping the intake back to Pettis Performance. The main purpose of flowing the system is to meter the pounds/hour for both the nitrous and fuel. Induction Solutions also monitors the nitrous pressure, fuel pressure, and solenoid amperage draw to make sure things are operating properly. According to Johnson, it gives them an idea of where the systems are with the nitrous/fuel ratios and gives the team a baseline. Manufacturers give a one-size-fits-all tune-up with jet sizes and fuel pressure. The flow testing establishes an application tune-up for that particular system.