For those of you keeping score, the ATK 408 stroker short-block had yet to receive any power-adders. This despite being designed specifically for such an occasion, but we are about to remedy that situation here and now. If you just joined the party, we’ll take a slight detour here to catch you up. Some time back, we received an assembled short-block from the good folks over at ATK. This particular short-block had a lot to offer, including increased displacement, low compression, and being chock-full of forged internals. The trio made the short-block primed and ready for boost and/or nitrous. The and/or portion of the equation being important here as we subjected the stroker to what can be considered a healthy dose of nitrous. The stroker short-block featured an iron block teamed with a forged crank, rods, and pistons, all from Manley. The -29cc-dish pistons helped create a very boost-friendly static compression ratio, while the extra displacement ensured plenty of off-boost power.
As good as the boost-ready short-block was, it needed a few other parts before it could even be called an engine, let alone swallow all that glorious nitrous. To complete the 408 for testing, we added a Comp cam, a set of as-cast GM LS3 heads, and a single-plane Victor Jr. intake. Other niceties included an adjustable timing set, Holley oil pan, and Comp shaft rockers. The single-plane intake was fed by a Holley 850 Ultra XP carb, while an MSD controller designed specifically for the 58x crank sensor handled the ignition chores. The -29cc-dish pistons combined with the 69cc combustion chambers and Fel-Pro MLS head gaskets (0.041-inch thick) to produce a static compression ratio of 8.8:1. ATK also offers the 408 stroker with piston configurations to increase the static compression, including -4cc flat-tops. To complete the stroker, we relied on a stack of parts from Summit, as well as ATI for one of their Super Dampers, which will come in handy on the next test when we introduce boost from a ProCharger centrifugal supercharger. Run in naturally aspirated trim, the low-compression 408 produced 557 hp at 6,300 rpm and 514 lb-ft of torque at 4,900 rpm. Now it is time for our first power-adder.
Nothing adds power as easily or inexpensively as nitrous. Originally used by both the British and Germans on their World War II aircraft, it eventually trickled down to hot rodders. Street racers have been using this amazing substance for decades, but its popularity surged when The Fast and the Furious franchise brought it to the big screen, to say nothing of the successful Street Outlaws TV series. Nitrous oxide works by introducing extra oxygen molecules to the engine. Since power production is primarily a function of the amount of oxygen present, the extra oxygen molecules provide a substantial power boost. It should be pointed out that nitrous oxide is not technically a fuel; in fact, it is not even flammable. Only after the oxygen molecule has been released from its nitrogen counterpart can it be successfully combined with the requisite fuel (gas), then properly converted to horsepower. This release takes place when the compound is heated to 572 degrees, a temperature that is easily reached during the combustion process. As another plus, the nitrous oxide is stored as a liquid and converted to a gas (a process called boiling), but this happens at a very low -129 degrees, meaning the injection also provides a substantial cooling effect. We call that a win/win.
To demonstrate the power offered by nitrous oxide, we installed a NOS CrossHair plate kit. The CrossHair kit employs the usual nitrous plate, but rather than the standard array of two bars, one each for nitrous and fuel, the CrossHair system adds another set, running perpendicular to the first. This cross pattern allows sizable power gains since it is essentially a pair of nitrous kits all wrapped up in a single (taller) plate. To ensure adequate nitrous flow, the CrossHair kit features high-flow Cheater Race solenoids. The power supplied by the system is adjustable through the provided jetting, up to 350 hp. We ran a pair of the smallest (52) nitrous jets designed to add a solid 200 hp. The nitrous jetting was combined with 1,000 psi of bottle pressure, 47 fuel jets, and 60 psi of fuel pressure. Though we ran on 112-octane Rockett Brand race fuel, we still retarded the ignition timing by 8 degrees. The result was an increase in power from 557 hp and 514 lb-ft of torque to a peak of 814 hp and 763 lb-ft of torque. The activation spike number looked impressive, but the system settled in to a more realistic peak number of 784 hp. The NOS nitrous kit added a solid 227 hp to our 408 stroker, more than we expected, but when is that ever a bad thing?
Photography by Richard Holdener; Steven Rupp