Destroked LS3: Build This 8000-RPM Screamer


Destroked LS3: Build This 8000-RPM Screamer


If the popularity of specific videos is any indication, enthusiasts love high-rpm motors. Look no further than Formula 1 for examples – who doesn’t love to hear the shriek of a high-rpm, small-displacement motor at full song? When sanctioning bodies give engine builders displacement limits, they make up for the lack of cubes with engine speed.

Typically, high-rpm race engines tend to combine big bores with short strokes, for a number of reasons. The first consideration is piston speed, as a shorter stroke reduces piston speed by limiting the distance traveled over a given period of time. Perhaps even more important from a power standpoint is the fact that a short stroke and big bore helps further improve head flow. Bigger bores allow bigger, unshrouded valves which in turn equates to more flow. More flow equates to more power, especially at higher engine speeds.

For NA motors, the best replacement for displacement is lots and lots of RPM.

By contrast, small bores limit head flow by shrouding the valves, so most race engine builders seek to maximize bore size for flow and then reduce stroke for rpm potential. Chevy applied this big-bore/short-stroke formula in the late 1960s when designing a production motor for the original Trans Am series, which dictated a maximum displacement of 5.0 liters. The 4.0" bore of a 327 block combined with the 3.0" stroke of a 283 crank gave us the legendary DZ302 that powered the first generation Camaro Z/28.

It was with this original formula in mind that we decided to build a big-bore, short-stroke LS motor using a factory block and crank. Unfortunately, the LS family didn’t offer the same bore and stroke combination as the original DZ302. The LS did offer a 4.0" bore 6.0L block, but there was no production 3.0" stroke crank. While duplicating the original DZ302 would have been cool, the production LS family still allowed us to combine a big-bore block with a short-stroke crank.

Looking over the myriad of LS combinations offered by the factory, the obvious choice for a crankshaft was the 4.8L LR4. Unlike the larger 5.3L, 6.0L and 6.2L, which all shared the same 3.622" stroke, the smaller 4.8L offered a shorter 3.267" stroke. Note that this stroke almost matches the 3.25" stroke length used in the original 327 Chevy.

The big-bore, short-stroke LS combination featured a 3.267" 4.8L crank stuffed inside a 4.065" LS3 aluminum block from Gandrud Chevrolet, bored to 4.070". 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.

For most enthusiasts on a budget, the cheap-date combo would be to simply build the 4.8L with a head, cam, and intake package optimized for high rpm. Unfortunately, the 4.8L was also saddled with a small 3.78" bore, which might otherwise restrict potential head flow and ultimately hurt power production. Another option would be to combine the 4.8L crank with a 4.0" bore 6.0L iron block. The 6.0L block would certainly work, but for our build, we decided to go even bigger.

For this 8,000-rpm LS, we combined the 3.267" stroke of the 4.8L with a 6.2L, (LS3) 4.065"-bore block. The new big-bore, short-stroke LS combo was sporting right at 340 cubic inches, thanks to a 4.070" overbore. Dropping the 4.8L crank into the LS3 block required use of custom JE pistons and 6.30" 4340 forged-steel connecting rods from Lunati, instead of the stock 6.275" 4.8L rods. The JE forged, asymmetrical pistons featured small, 5cc domes to increase the static compression ratio to 11.7:1 with our 58cc heads.

Running the 4.8L crank in the LS3 block required a custom piston and rod combination. This destroked – or big bored – LS was sporting a set of 6.30" Lunati forged rods and custom JE pistons with a compression height of 1.3065.

The short block was assembled using Fel Pro race bearings, timing chain and oil pump along with a Moroso oil pan, pickup and windage tray. Finishing things off was a custom cam from Brian Tooley Racing. The custom BTR cam offered a .627/.596 lift split, a 243/262-degree duration split and 110-degree (+5) 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 8,000 rpm and beyond.

The big-bore, short-stroke combo was now ready for a suitable induction system. Knowing that 8,000 rpm required plenty of head flow, we topped the short-stroke LS3 with a set of AFR LSX 230 V2 heads. As indicated by the name, the 230cc intake ports flowed 324 cfm, while the exhaust flow checked in at an equally impressive 252 cfm. Credit for the impressive flow numbers went to CNC porting combined with a 2.08/1.60 valve package. The 58cc combustion chambers helped further improve power by increasing the static compression ratio with our combination of displacement and piston design.

To ensure proper sealing for the all-aluminum, high-rpm LS, JE also supplied a Pro Seal ring package.

Naturally the AFR heads featured a valve spring package capable of supporting the desired rpm, while the BTR lobe profile offered the necessary stability. The head flow offered by the AFR LSX 230 heads was teamed with a Holley Hi-Ram intake manifold. The versatile Hi-Ram is available with a variety of different tops, so 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 8,000-rpm 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 a Holley Dominator EFI system to control ignition timing.

Before running the motor in anger, we subjected the new big-bore, short-stroke combo to the necessary break-in cycles. Unlike older flat-tappet cams, both hydraulic and solid, there was no need to break-in the billet roller cam and hydraulic roller lifters. But we did want to provide some time to allow the rings to seal properly against the freshly machined bores. After the break-in process, we changed to Lucas 5W-30 synthetic and were free to rev this baby up.

Oiling is even more critical at 8,000 rpm, so we equipped the destroked LS with a Moroso windage tray, oil pan, and dedicated pickup.

After dialing in the timing with the Dominator and making a few jet changes to the XP carbs, the little monster showed it was ready and willing to rev. After tuning and a few pulls up to 8,000 rpm, the combo posted some impressive numbers. Thanks to the breathing offered by the big-bore and the top-tier induction system, the short-stroke, 340-inch motor produced peak numbers of 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 motor to 8,000 rpm. There really is something special about hearing an engine rev for all it's worth. Sure, a bigger motor could likely do the same thing, and might even make power doing it, but that certainly won’t diminish the popularity of this destroked LS3.

“I love it when a plan comes together,” says Richard Holdener. The big-bore, short-stroke LS3 not only ripped cleanly to 8,000 rpm, but it also made impressive power when it arrived. The key to the success of any build is the right combination of components. The big bore and short stroke were just the first two pieces of the puzzle. We combined them with the right cam – high-rpm and stable – an intake equally suited for high-rpm use, and free-flowing heads with a well-matched valvetrain. Combined, this blend of parts allowed our Maverick motor to buzz the tower at eight grand. Not only did it rev, but it made power upstairs as well. The 340-inch combo eventually produced peak numbers of 607 hp at 7,900 rpm and 466 lb-ft of torque at 6,200 rpm. The sound was glorious.

A stable cam profile was a critical element in taking full advantage of the rpm potential of the LS. Brian Tooley Racing supplied this custom cam profile that offered a .627/.596 lift split, a 243/262-degree duration split and 110 +5 lsa.

The Crane hydraulic roller lifers featured advanced metallurgy that allows exacting tolerances, now measured in microns (.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.

Revving the motor was one thing, but making power up there required plenty of head flow. To ensure the necessary flow, we installed a set of AFR LSX 230 V2 heads. While you might be tempted to run big-port, LS3-syle heads – we tried that too – these cathedral-port AFR heads flowed enough to support over 650 hp on the right application.

At 8,000 rpm, valvetrain stability is every bit as important as airflow. The AFR heads feature lightweight 8mm valves and a dual spring package capable of taking full advantage of the short-stroke motor's rpm potential.

Since compression adds power, we specified AFR heads with 58cc combustion chambers to go with the 2.08/1.60 valve package. More than just chamber volume, chamber shape is a critical element in power production.

Despite the elevated rpm, this LS application relieds on a set of new stock rockers. A trunion upgrade or shafts might also 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, high-rpm combination.

A number of different top configurations are available for the Hi-Ram, but we chose the dual-quad setup with a pair of Holley 950 Ultra XP carbs. The new XP line offers a 38% decrease in weight, 20% increase in fuel capacity, with billet metering blocks and base plate. The pair ensured plenty of flow for maximum power.

Normally we would run this carbureted LS with an MSD ignition controller, but the 24X crank, LS3 combination required a Holley Dominator system (this test was run before MSD offered a controller that allowed selectable cam/crank combos).

With the Holley Dominator controlling ignition timing, all we had to do was perform a quick jet change on the XPs to dial in the air/fuel mixture.

Once tuned to perfection, we were able to really rev our destroked LS. Inching our way up in rpm with each run, we finally topped it out by exceeding 8,000 rpm. Run in anger, the destroked LS3 pumped out peak numbers of 607 hp at 7,900 rpm and 466 lb-ft of torque at 6,200 rpm.