There is a lot to love about crate engines, including the fact that they come assembled and ready to rock. Even crate short-blocks that require finish assembly, like our boost/nitrous-ready stroker version, make life so much easier than configuring a DIY assembly. There is just something satisfying about grabbing a tested combination that will withstand almost anything you plan to throw at it (within reason, of course). Having someone take the guesswork out of the components and completed combination provides peace of mind, especially when (like us) you plan to apply nitrous, boost, or even both to your stroker. Since we were searching for something we could upgrade—then hit hard with power-adders—we went to the short-block experts at ATK. The ATK short-block was stuffed with all manner of boost-ready goodness, including a forged crank, forged rods, and matching forged pistons. With all those forged internals stuffed inside an iron 6.0L block, we knew the ATK short-block was more than up to the task of withstanding the power-adders we had planned.
Before we get to the initial testing, it is important to know that the ATK short-block came assembled and ready for finishing. The seasoned 6.0L iron LS (technically LQ) block was align-honed, bored 0.030 over, and then treated to a 4340 forged steel crank from Manley. The 4.000-inch stroke combined with the 4.030-inch bores produce a finished displacement of 408 cubic inches. Strokers add power the easy way, since bigger is better when it comes to power production. The forged crank was combined with a set of forged steel H-beam rods measuring 6.125 inches. ATK offers their LS 408 stroker assemblies with either 24x or 58x trigger wheels, as well as with a number of different piston configurations, or more specifically compression ratios. For our boosted application, we chose big (-29cc) dished pistons. Supplied by Manley, the forged slugs ensured plenty of strength while offering a power-adder–friendly compression ratio of 8.8:1 when combined with our 69cc LS3 heads. ATK also offers short-blocks with -10cc or -4cc pistons to increase the static compression ratio. The short-block was supplied painted, assembled, and ready to be topped off with heads, cam, and intake of our choosing.
Though we plan on running boost and nitrous, we decided to start things off with some naturally aspirated upgrades, which would give us a baseline to work off of. To that end, we needed some kind of starting point for the stoker, so we installed a factory LS3 cam to go with our GM Performance LS3 heads. Sure, there is no shortage of headgear available for an LS application, but the factory LS3 heads offered an impressive combination of price and performance. Equipped with 69cc chambers, a 2.165/1.590 valve combination, and massive rectangular ports, the as-cast LS3 heads flowed a tad over 315 cfm. Having produced nearly 700 hp with these stock heads on a wild stroker, we knew they had plenty of potential and would work well under boost. The stock LS3 cam was hardly a performance grind, but we needed to start somewhere and if you’re on a budget you can find one for nearly nothing. The LS3 head/cam package was combined with a dual-plane, carbureted intake from Holley and a 750 XP carburetor. Ignition chores were handled by an MSD ignition controller, while the stroker exhaled through a set of 1 7/8-inch Hooker headers. Equipped as such, the low-compression stroker produced 446 hp at 5,600 rpm and 478 lb-ft of torque at 3,600 rpm. Now it was time for some mods!
Being an LS, and a stroked one at that, the first upgrade we selected was a camshaft to replace the mild LS3 grind. Given the cubic inches, we selected a healthy stick from Comp Cams. The cam offered a 0.621/0.624-inch lift split, a 235/251-degree duration split, and a 113-degree LSA. Knowing the cam was plenty powerful, we combined it with a single-plane Victor Jr. intake. The single-plane offered increased flow, but more importantly rpm potential to work with the revised cam specs. The Comp cam was matched with a dual valvespring upgrade to replace the factory LS3 springs supplied on the GM heads. In addition to the cam and springs, Comp also supplied a set of their new Max-Lift BSR shaft rockers for the LS3 heads. Designed to minimize deflection, the new rockers featured a trunnion upgrade, common shaft mounting, and hardened mounting hardware. This gives the stability of a shaft system at an affordable price.
Equipped with an 850 XP Holley (worth 4-6 hp over the smaller 750 XP), the cammed stroker produced 554 hp at 6,300 rpm and 519 lb-ft of torque at 4,900 rpm. The cam and induction upgrades netted over 100 hp, but more importantly made the ATK stroker ready for either nitrous or boost. Check back with us to see how the ATK responds to a NOS CrossHair nitrous system followed by some boost from a ProCharger F-1A supercharger.
As supplied by ATK, the 408 stroker (PN SP40-B) featured an iron 6.0L block stuffed to the gills with a Manley 4340 forged steel crank, matching Manley forged H-beam rods (with ARP 2000 bolts), forged Manley pistons (-29cc, dished) with moly coated skirts, and Total Seal rings.
The best part of a crate engine is that all those cool parts come professionally installed in a fully inspected and ready-to-go block. This engine is built for serious boost, so ARP main studs (PN 234-5608) are part of the mix.
Put it all together and it makes for a solid foundation for whatever power-adder you want to go with.
The forged pistons reside in 4.030-inch bores and when combined with the 4.000-inch forged crank pencils out to 408 inches. The forged pistons are -29cc, which works out to 9.3:1 compression with 64cc heads, 9.0:1 with 68cc heads, and 8.7:1 with 72cc heads.
Our block was a Gen III version, but ATK can provide either 58x or 24x reluctor wheels.
The short-block arrived without a cam, so to start the party off we decided to toss in a stock LS3 camshaft we had at Westech. To ensure plenty of oil flow for the stroker, we installed a high-volume Melling oil pump from Summit Racing.
The new short-block also received a set of Chevrolet Performance hydraulic roller lifters (standard travel) from Summit (PN NAL-12499225).
To complete our short-block we also hit up Summit for lifter trays, front cover, rear cover, and a set of Gen III ARP head studs. We had a Holley valley cover, but it was for a later Gen IV, so we “borrowed” a Gen III valley cover from another engine.
Both the factory LS3 and Comp cam upgrade were secured using this cam retaining plate from Comp Cams (PN 5463-KIT).
The increased displacement offered by the stroker required plenty of airflow, so we installed these Chevrolet Performance LS3 heads from Scoggin-Dickey Parts Center (PN 12629063). The 69cc chambers will put our engine at right around 8.8:1 compression.
The GM LS3 heads featured generous rectangular ports and a 2.165/1.590-inch valve package that produced flow numbers over 315 cfm.
The GM heads were originally supplied with factory LS3 valvesprings, but our cam upgrade required both increased pressure and coil bind clearance.
We upgraded the factory LS3 springs with a dual-spring combo from Comp Cams. The Comp LS spring upgrade was designed to allow up to 0.650-inch lift, more than enough for our planned cam upgrade.
To ensure adequate sealing under boost, we installed a set of Fel-Pro MLS head gaskets (PN 1161L-041, 1161R-041). Note from the part number on the gaskets that there is a left and right head gasket. You can also see our ARP head studs (PN 234-4316) in place and ready to go.
With a cam swap in the cards, we took the liberty of installing a set of 7.35-inch, hardened pushrods from Comp Cams.
Comp Cams also supplied this trick set of Max-Lift BSR shaft rockers (PN 1982-16). The LS rocker set combined a common shaft, trunnion upgrade, and hardened mounting bolts to minimize deflection. This gives you the stability of a shaft rocker system at an affordable price. It also allowed us to run the lightweight OE-style rockers. The BSR rockers bolted in just like stockers using a GM rocker rail we sourced from Gandrud Chevrolet.
Because we’re planning on running a ProCharger supercharger on this engine, we installed an ATI Super Damper rather than a stock unit.
For our baseline test, we equipped with ATK 408 with a dual-plane, carbureted intake from Holley (PN 300-129). These are a common choice for street cars that are looking for low-end grunt rather than top-end power.
The Holley intake was teamed with a 750 Ultra XP carburetor, but we also tried a larger 850 version that offered 4-6 more hp.
Run on the dyno with the stock LS3 cam and dual-plane intake, the 8.8:1-compression 408 produced peak numbers of 446 hp at 5,600 rpm and 478 lb-ft of torque at 3,600 rpm. Not bad for a NA, low-compression mill with a mild cam that could run all day on 87-octane gas. But, we knew there was room for improvement.
With our baseline complete, the first upgrade was this Comp camshaft (PN 54-470-11) The Comp cam offered a 0.621/0.624-inch lift split, a 235/251-degree duration split, and a 113-degree LSA.
To properly install the three-bolt cam, Comp Cams also supplied this adjustable LS timing set (PN 3172KT).
It was then time to retire the stock LS3 cam and slide in our more aggressive Comp cam and the new three-bolt timing gear. Note we installed the cam with the adjustable cam gear lined up at 0, meaning no advance or retard.
To take advantage of our new cam we moved to a single-plane Victor Jr intake.
Though boost and nitrous are planned for our power-adder ATK stroker, we initially equipped the 408 with a dual-plane intake and stock LS3 cam. Equipped with these humble components, the low-compression 408 produced 446 hp and 478 lb-ft of torque. If you are thinking this engine was just begging for a cam and boost, you would be right. After installation of the Comp cam and single-plane intake, the power output jumped to 554 hp and 519 lb-ft of torque. The upgrades increased the peak power output by over 100 hp, but this stroker is still in dire need of a power-adder.
Photos by Richard Holdener and Steven Rupp