The success of any LS build is determined by the combination of the components used. You can have the best flowing heads, the wildest cam profile, or even the hottest intake, but unless all of these work together in harmony, the best parts are just a conglomeration of individuals working against each other. Truth be told, whether or not success is achieved comes even before the choosing of the parts. You must first determine the specific needs or usage for the engine. The buildup of a dedicated drag-race engine will certainly be different than that for a daily driver. A careful examination of the requirements for the build will help greatly when it comes to component selection.
Asking yourself how much power you want should actually be one of the last questions asked. By the same token, “as much as possible” should never be the answer. Who really wants (or can afford) a Top Fuel engine in their daily? Only with an honest appraisal of your needs (and financial restraints) can you start down the path to success. This 415 LS stroker was a perfect example of a successful build.
Before getting into the specifics of the stroker we should address the elephant in the room, namely our choice of cylinder heads. Among LS fans, the line in the sand is clear and distinct. The rectangular port boys are on one side and the cathedral port mafia is on the other. Which is better? The answer is, as always, it depends. Cop out, you say? Yes and no. Hear us out before you judge us. You need to understand that the question must be considerably more specific to be answered one way or the other.
In a direct stock to stock comparison, the factory rectangular port LS3 head will definitely support more power than the hottest factory cathedral port LS6/LS2 243 casting. Depending on the flow bench, the LS3 head flows 60-70 cfm more than the 243 head, so naturally it can support more power. Having tested stock LS3 heads against a set factory 317 (truck) heads (that essentially share the same intake port as the 243), we can say without hesitation that the LS3 heads made more power. We can also say that the LS3 heads lost out to the 317 truck heads below 4,000 rpm. The 243 head would fare slightly better than the 317 thanks mostly to the increase in compression, but would not catch the LS3 heads for peak power. This scenario changed dramatically when we introduced aftermarket heads into the mix. A set of AFR 230s crushed the stock LS3 heads from top to bottom. The debate rages on, but rest assured the cathedral port mafia will argue with you rectangular port fans till the cows come home.
We know the rectangular port LS3-style heads can certainly make power, but the attending port volumes are best suited to large displacement and/or high-rpm applications. Although our LS3-based, 415-inch street stroker offered ample displacement, we selected the cylinder heads based more on its intended usage. This particular engine was destined for autocross track use in the Optima Challenge series, but the owner wanted the engine to be, first and foremost, dead reliable and last forever. OK, maybe forever was a stretch, but let’s just say there was no desire to sacrifice a few additional horsepower for an early rebuild.
Among the top items that reduce engine life is rpm. Only detonation kills an engine quicker than excessive engine speed, but since we employed a Holley HP management system to dial in the air/fuel ratio and timing, a few select keystrokes easily kept detonation in check. Keeping engine speed down meant cam selection was critical. Despite the relatively mild cam timing, the 415 still managed to pump out some impressive power and torque numbers. Guys can brag online all they want about how much peak power their engine made till it exploded, but having a torquey stroker you get to drive day in and day out is better than winning some online post war. Since this engine would be used for autocrossing and road course work the owner was interested in midrange power, especially torque, and not so much in top-end horsepower. This was another reason to go the cathedral port head route. So remember, just because the “next big thing” got released it doesn’t mean last season’s parts aren’t worthy of your consideration.
1. What happens when you combine an LS3 stroker short-block with a set of AFR 230 cathedral port heads? You get a Hybrid Horsepower.
2. Our main player in this build was a new Chevrolet Performance 6.2L LS3 block from Scoggin-Dickey Parts Center (SDPC). Starting with a fresh block saved us a lot of hassle, and to save machine shop costs we kept the as-shipped 4.065-inch bore instead of massaging it out to 4.070-inch (which would have yielded a 416-inch engine).
3. The extra displacement came courtesy of a 4.000-inch, forged steel stroker crank from Scat. In fact, the entire rotating assembly came ready to rock through Summit Racing as a kit (PN 1-44207BI), which saved us from having to hunt down and match up items like bearings and rings.
4. The factory rods were not going to get the job done on this stroker so the kit from Scat included a set of 6.125-inch H-beam rods (PN 2-350-6125-2100).
5. The forged slugs from RaceTec featured a 10cc dish to help keep the static compression ratio pump gas friendly. Combined with our 62cc chamber AFR heads we were just over 11.2:1 compression.
6. We were happy to see that the kit came with high-quality Clevite main and rod bearings.
7. When we ordered our Scat rotating kit from Summit Racing we accidentally ordered one with a 58x reluctor wheel. Fortunately we had a new GM 24x wheel on hand and a local machine show was able to swap it out for us. We should have ordered kit 1-44107BI since it would have came with the right reluctor wheel to work with our harness and ECU. Moral of the story? Double check what you order before you hit the submit button.
8. After hanging the rods on the pistons, prepping the rings, and oiling down the cylinder walls, each assembly was installed with care into their respective bores.
9. Remember that in the same way the pistons have a “top,” the rods have an inside and an outside edge. We made sure the chamfered side was facing the radius on the outside of each crank journal.
10. Once we had some rods in place we turned then engine over by hand and found that some of the rod bolts were just barely hitting the bottom of the cylinder bores. There are two ways to fix this. The first is to make a mark where the bolts are hititng and then notch the bottom of the cylinder sleeve for clearance. The other way, which we opted to do, was to torque the bolt to spec, mark the edge where it was hitting the bore, and then remove the bolt and shave a tiny bit off. Once retorqued to spec, the shaved edge easily cleared the edge of the cylinder bore.
11. To balance maximum engine speed, power, and torque production, we installed a Comp hydraulic roller cam that served up a 0.601/0.613-inch lift split, a 227/241-degree duration split, and 114-degree LSA. The result should be excellent street manners and great mid-range torque and power. This was a custom cam using a cam core 54-000-11 and the grind number 1377R/13782R HR 114.0 +2.
12. Running a front cam sensor required the use of a 1x cam sprocket on our Comp timing chain set (PN 3158KT).
13. Rather than rely on drop-in replacement lifters, we installed these Comp link bar lifters (PN 8957-16). The link bar design eliminated the need to run the factory lifter trays and retaining bolts.
14. What made our LS3 stroker a hybrid was the use of cathedral port 230cc Mongoose large-bore heads from AFR. The Mongoose line of cathedral port heads from AFR come in sizes ranging from 210 cc all the way up to 245 cc. The large bore version requires a minimum of 4.000-inch bore and can be had with either 62cc or 72cc chamber volumes. AFR also offers a small-bore version (3.900-inch +) which has a 65cc chamber volume. Thanks to flow rates reaching 328 cfm (at 0.600 lift), these LSX Mongoose heads were capable of easily supporting our power goal.
15. The AFR heads featured full five-axis CNC porting, a 2.165/1.600 valve package, and a dual PAC spring package with titanium retainers, which allowed for plenty of rpm potential and ample coil-bind clearance for our 0.617-inch-lift cam.
16. The AFR 230 heads were installed using Fel-Pro MLS head gaskets (PN 1161R/L053) and ARP head studs (PN 234-4313).
17. Here is a shot of the all-aluminum, long-block nearly completed. The valley and rear covers were in place, but the oil pump had yet to be installed on the crank. Keep in mind that the new blocks from Chevrolet ship bare so small items will be need to found. Used is fine since covers don’t wear out, but new parts are sold, often in kit form. One nice kit is the Chevrolet Performance (PN 192999099) one we found over at Summit Racing. It included a front cover, valley cover (both with bolts and gaskets), as well as head dowels, cam sensor, chain damper, and a crank sensor. Summit also sells a cool block kit (PN SUM-G1584) with all of the needed LS threaded block plugs and the sometimes overlooked, but necessary, oil-restrictor (also referred to a dog bone or barbell).
18. The AFR heads received a set of factory rockers and hardened Comp 4.000-inch pushrods. Don’t trust the stock pushrods with elevated valvespring pressures and rpm.
19. Before installation of the front cover and damper we installed a new Chevrolet oil pump we picked up from Summit Racing (PN 12678151). Make sure to pre-lube the pump with assembly lube or engine oil prior to installation. This will make priming much easier prior to startup.
20. The MSD Atomic intake relied on O-ring sealing to mate the intake to the AFR heads. The long-runner intake offered an impressive balance of peak power and torque production.
21. MSD provides several spots on the Atomic AirForce intake to mount the MAP sensor, but whatever spot you chose (top rear, top front, or rear) you’ll need to drill open the passage. We opted for the rear mounting position.
22. The Hybrid Horsepower stroker was ready to fire up on the dyno. For testing, we used a 102mm cable actuated throttle body along with Hooker 1 7/8-inch long-tube headers, a Meziere electric water pump, and a few quarts of Lucas 30w break-in oil. For tuning, we used the Holley ECU that was grafted to the Superflow 902 dyno.
23. As you can see from the results, the AFR 230 cathedral port heads were right at home on the LS3 stroker. Despite the mild (street engine) cam timing, the combination produced an impressive and usable torque curve that peaked at 563 lb-ft at both 4,500 rpm and 5,100 rpm. The curve was more of a plateau, as torque production varied by just 2 lb-ft from 4,400 rpm to 5,200 rpm. The small cam limited peak power, but thanks to a combination of ample displacement, efficient AFR heads, and MSD intake, the 415 still managed to pump out 597 hp at 6,200 rpm and didn’t fall off much, even at 6,500 rpm. On the plus side, the smaller cam will have excellent street manners and more than enough vacuum to run power brakes.
24. Once the 415 LS3 was in the car we were able to swap over to the 103mm MSD billet throttle body (PN MSD-2945) that didn’t arrive from Summit Racing until after our dyno test day. The throttle body uses an extra-strong stainless steel shaft, which equates to less twist and better TPS signal stability. At our power level we could have ran the 90mm version (PN MSD-2940), but the larger unit was actually less money and will give us more flexibility to add more power down the road.
25. To make life easier we ran a factory LS2 fuel rail and FAST 65 lb/hr LS3-style fuel injectors (PN 30657-8). The problem was that the harness in the car was set up to utilize LS1 injectors. The quick and easy fix was a set of adapter harnesses (PN 170604-8) from FAST that would bridge the gap from the USCAR injectors and the Minitimer plugs on the harness. For the dyno we ran a GM oil pan from a newer Camaro, for the Chevy it was being installed into we ran a Holley oil pan (PN 302-2) and their trick trap-door baffle system (PN 302-11).
Photography by Steven Rupp