We took an LS3 block from Summit Racing, added a 3.267-inch stroke crankshaft from a 4.8L, spun it to 8,000, and made 607 hp. The secret to this small-block screamer is the short stroke and good head flow. As a general rule, the reduced piston speed provided by short-stroke engines allows for increased overall engine speed: engine speed multiplied by torque, then divided by 5,252 equals horsepower. If you combine big rpm with big bores, you get a combo that makes plenty of power. Why? Small bores limit head flow by shrouding the valves, large bores unshroud the valves and allow more air to get into the engine. More air allows more fuel to be burned. More fuel makes more power.
We decided to apply this formula to the modern LS engine family. Given the availability of aftermarket blocks as well as custom cranks, rods, and pistons, it is possible to build any combination your heart desires, but we decided to do it with as many factory components as possible. Not that this was a low-buck, junkyard build by any means, but the effort produced a engine that was every bit as cool as an original L76 or DZ302, those high-winding race motors from the muscle-car era of V8 racing.
Looking over the myriad LS combinations offered by the factory, one stood out from the rest: the 4.8L LR4. One thing the majority of the LS engines shared from the factory was the stroke of the crankshaft. The 3.622-inch stroke was used in everything from the 5.3L up to the supercharged 6.2L. This list included every variant of the 5.3L, 5.7L, 6.0L, and 6.2L, with the extra displacement coming from changes in bore size. Other than the limited-production 7.0L LS7 (with an even larger 4.0-inch stroke), only the 4.8L differed from the rest of the family. Sporting a 3.267-inch stroke, the 4.8L nearly matched that offered by the original (high-winding) L76 327. If you are thinking the cheap-date combo would be to simply build the 4.8L with a head, cam, and intake package optimized for high rpm, you’d be wrong. Unfortunately, the 4.8L was also saddled with a small 3.78-inch bore restricting head flow and power potential. In this way, the 4.8L can be likened to the original 283, though at 3.87 x 3.00 inch, the 283 had a slightly larger bore and shorter stroke.
To cure the small-bore dilemma, we combined the 3.267-inch stroke of the 4.8L with a 6.2L, (LS3) 4.065-inch bore block. The new big-bore, short-stroke LS combo was sporting 339 ci (5.56L). Dropping the 4.8L crank into the LS3 block required use of custom JE pistons and 6.30-inch, 4340 forged-steel connecting rods from Lunati. The JE forged pistons featured 5cc domes to increase the static compression ratio to 11.7:1 using 58cc heads. The short-block was assembled using Fel Pro race bearings, timing chain, and oil pump, along with a Moroso oil pan, pick up, and windage tray.
Finishing things off was a custom Comp cam from Brian Tooley Racing (BTR). The cam offers 0.627/0.596 lift, 243/262-degree duration, and a 110-degree LSA. The cam was teamed with a set of Crane hydraulic roller lifters that featured heat-treated and carburized 8620 steel with precision machined bodies and internal components that allowed them to run safely at, and beyond, 8,000 rpm.
With the big-bore, short-stroke short-block cammed and ready to go, it was time for the remainder of the power producers, namely the heads and induction system. Topping the LS3 hybrid was a set of AFR LSX 230 V2 heads. As indicated by the name, the AFR heads have 230cc intake ports that flow 324 cfm, while the exhaust flow checked in at an equally impressive 252 cfm. Credit for the high flow numbers goes to CNC porting combined with a 2.08/1.60-inch valve package. The 58cc combustion chambers helped further improve power by increasing the static compression ratio.
The AFR heads were teamed with a Holley Hi-Ram intake manifold. Available with a variety of different tops, we configured the tunnel-ram style Hi-Ram with a dual-quad top designed to accept a pair of Holley 950 Ultra XP carburetors. Additional components used in this build included an ATI Super Damper (a critical upgrade at this engine speed), a set of 1-7/8-inch headers from American Racing Headers and Holley Dominator EFI system to control the ignition timing.
Before running the engine, we subjected it to a couple of break-in cycles using Lucas break-in oil. After the break-in process, we changed to Lucas 5W-30 synthetic and were free to run it in anger. After dialing in the timing with the Dominator and a few jet changes to the XP carbs, the combo spun all the way to 8,000 rpm, posting some impressive power numbers along the way. Thanks to the big bore and short stroke, the 339-inch combination produced 607 hp at 7,900 rpm and 466 lb-ft of torque at 6,200 rpm. More than a few Westech customers walked over to the dyno room when they heard us zing this thing to 8,000 rpm. The ultimate compliment came from the owner of a 1969 Z/28 being testing on the chassis dyno. Said the small-block owner, "Man, I thought my DZ sounded good—that thing is awesome!" You know you’ve done something right when your engine gets the thumbs up from a DZ owner! Now we just need a Camaro to put it in.
The replacement for displacement? RPM and lots of it!
The big-bore, short-stroke combination featured a 3.267-inch 4.8L crank stuffed inside a 4.065-inch, LS3 aluminum block from Gandrud Chevrolet. To make sure to use the proper reluctor wheel on the crank, we chose a 24X-tooth wheel, but a late-model version will require a 58X wheel.
Running the 4.8L crank in the LS3 block required a custom piston from JE with a compression height of 1.3065 and 6.30-inch Lunati forged rods.
To ensure sealing for the high-rpm hybrid, JE also supplied a Pro Seal ring package.
Since oiling would be critical at the elevated engine speeds, we installed a Moroso windage tray, oil pan, and dedicated pick up.
The cam profile was a critical element in taking full advantage of the rpm potential of this engine. Brian Tooley Racing supplied this custom Comp cam profile that offered a 0.627/0.596 lift split, a 243/262-degree duration split, and 110 +5 LSA.
The Crane hydraulic roller lifers featured advanced metallurgy that allowed exacting tolerances, now measured in microns (0.000000393 inches—yes, six zeros!). The combination all but eliminated the two most common ailments associated with hydraulic roller lifters: lifter pump up and bleed down.
Knowing the high-rpm stroker needed plenty of airflow, we installed a set of AFR LSX 230 V2 heads. While you might be tempted to run big-port, LS3-syle heads, these cathedral-port AFR heads flowed enough to support more than 650 hp on the right application.
Every bit as important as the airflow was the fact that the AFR heads featured lightweight (8mm) valves and a dual-spring package capable of taking full advantage of the rpm potential of the short-stroke motor.
To raise the static compression and further improve power production, we specified 58cc combustion chambers to go with the 2.08/1.60 valve package. Like airflow, chamber shape is a critical element in power production.
Despite the elevated rpm, this LS application relied on stock rockers, though a trunion upgrade would be a good idea at these engine speeds.
Making power at high rpm required a dedicated, short-runner, induction system. We selected the Holley Hi Ram intake to feed the short-stroke hybrid.
Though a number of tops were available for the Hi Ram, we teamed the dual-quad setup with a pair of Holley 950 Ultra XP carbs. The new XP line offered a 38 percent decrease in weight, 20 percent increase in fuel capacity, and billet metering blocks and baseplate.
Normally we would run a carbureted LS with an MSD ignition controller, but the 24X crank, LS3 hybrid, required a Holley Dominator system.
With the Holley controlling the ignition timing, all we had to do was perform a quick jet change on the XPs to dial in the air/fuel mixture.
Before the day was done, the LS wailed all the way to 8,000 rpm, pumping out peak numbers of 607 hp at 7,900 rpm and 466 lb-ft of torque at 6,200 rpm.
Big-Bore LS Stroker
Not only did the big-bore, short-stroke LS3 rip cleanly to 8,000 rpm but it made impressive power doing so. The key to the success was the conglomeration of components. The big bore and short stroke combined with the right cam, intake, heads, and valvetrain, allowed this motor to buzz the tower at eight grand! Not only that, the peak power occurred at 7,900 rpm, with peak torque coming at a lofty 6,200 rpm.
Parts List DescriptionSourcePrice 4.8L core motorNA$500 LS3 blockSummit Racing1,502 Lunati 6.30-inch RodsSummit Racing568 JE PistonsSummit Racing856 JE premium ringsSummit Racing147 Coated race bearingsBrian Tooley Racing210 Balancing and machiningL&R Auto325 Comp Cams hydraulic rollerBrian Tooley Racing399 Crane roller liftersSummit Racing696 AFR LSX 230 V2 cylinder headsSummit Racing2,520 Stock LS rockerscore motorN/A Comp 7.35-inch pushrodsSummit Racing102 GM LS9 head gasketsBrian Tooley Racing110 ARP head boltsBrain Tooley Racing109 Holley Hi Ram intakeSummit Racing634 Holley 950 Ultra XP CarbsHolley1,606 pair Weiand dual quad linkageSummit Racing222 Holley Dominator EFISummit Racing1907 Moroso LS oil panSummit Racing248 Moroso LS windage traySummit Racing121 Moroso LS pick upSummit Racing137 Total: $12,419
