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Supercharged LSX Engine - Poor Man's LS9
Using A Sturdy LSX Block And A Huge Harrop/Eaton Blower, Thomson Automotive Builds A Rival To The Zr1 Powerplant
writer: Barry Kluczyk
photographer: Barry Kluczyk
Right off the bat, let's be clear about this story's title. "Poor Man's LS9" can be interpreted many ways, because wealth is a relative term. So, with the new ZR1 stickering for $103,300 (plus dealer markup), a more appropriate-albeit less catchy-title might be "Less-Affluent Man's LS9" or "Engine for the Man Who Didn't Get on the ZR1 Wait List."
Nevertheless, what we've got here is a 427-inch engine combination that is based on GM Performance Parts' LSX cylinder block and approaches power in the same supercharged manner as the LS9, but exceeds that engine's factory-rated output. And while this mill could never be considered inexpensive, it's well within the realm of attainability for enthusiasts with the means.
The foundation for the LS9-challenging engine is GM Performance Parts' LSX block. It comes semifinished with 3.99-inch bores that were taken out to 4.125 inches for this application. The LSX also features provisions for additional cylinder-head bolts, and it accepts just about any component from the LS engine family.
This supercharged LSX is the brainchild of Brian Thomson, who runs Thomson Automotive in the industrial suburbs of Detroit. He works closely with General Motors on a variety of engine-building and dyno-testing projects, and he has about as much experience with high-power LS engines as anyone in the business. Like Tim Robbins' character in Bull Durham, Thomson envisioned the engine as a way to "announce his presence with authority" when it came to LS performance.
"Cars with 500 and 600 hp are as common as pennies," Thomson says. "We wanted to build something with basically off-the-shelf parts that pushed the 800hp barrier."
It helped, too, that Thomson had access to some of the first examples of the LSX cylinder block-the cast-iron engine foundation that was developed with assistance from legendary drag racer Warren Johnson. Because of the LSX's purported capacity to support tremendous boost, Thomson figured it was the safe bet for ensuring a scatter-free first pull on his engine dyno. Even better, it was about one-sixth the price of a C5-R block. (LS7 blocks were not readily available when the project started.)
The LSX block is made of cast iron, making it more affordable than a comparable aluminum unit. A 9.26-inch semifinished deck height is standard, but we expect a tall-deck version to arrive any day now. Like other LS blocks, the LSX uses a long-skirt design and six-bolt main bearings.
"The LSX is a really strong block," Thomson says. "Our experience suggested it would handle the power we were looking for without a problem, whereas we would have been pushing the limit on a C5-R block."
Putting the pressure on the LSX block is a new Eaton-built supercharger from Australia's Harrop Engineering. As Roots-type blowers go, it's a monster, displacing 2,300 cc (the same as the one on the LS9) and capable of more than 20 pounds of boost. Thomson figured he'd need at least 15 pounds to reach his 800hp goal.
But while the blower is not yet widely available in the United States (Harrop's U.S. arm says it will be soon), the rest of the combination is a straightforward design built with off-the-shelf components. That's not to say this was a budget build, but there's nothing here that couldn't be easily duplicated.
"The parts in this engine are either already available or will be soon," Thomson says. "That's a key part of this project: It's awesome power that is available to anyone who wants it."
On this engine, the bores were finished with torque plates installed to ensure accuracy and promote head sealing.
The crankshaft is a stock LS7 forged-steel piece providing a 4.000-inch stroke.
Forged rods from Oliver were also used. They have an I-beam configuration, which enhances strength.
Supercharged LSX Engine - Poor Man's LS9
Forged Diamond pistons feature a dished design that helps maintain a supercharger-friendly 9:1 compression ratio.
The basics of the engine combination include the iron LSX block with 4.125-inch bores, a forged crankshaft delivering a 4.000-inch stroke, and forged rods and pistons. The short-block assembly is similar to that used in GM's LS7 engine, but with forged Oliver rods and Diamond pistons, which stand up better to the heat and pressure generated by a supercharger. (The LS7 uses cast pistons and featherweight titanium rods.) The pistons feature a dished design in order to permit the lower compression ratio (9:1) needed for a supercharged setup running pump gas.
The heads are prototype LSX aluminum parts obtained through Thomson's secret pipeline to GMPP. These feature the same design as the LS7 heads but with additional, cast-in mounting holes to allow the use of extra head bolts (six versus the production-style four). The heads offer a tremendous flow rate of approximately 380 cfm on the intake side.
Feeding the big heads is the aforementioned 2.3L blower, which features Eaton's new four-lobe rotor design to broaden its effective range. An LS7 90mm electronically controlled throttle body was adapted to the blower's inlet, and an air-to-water intercooler was installed to enhance performance and durability. Harrop Engineering supplied all of the necessary pulleys and drivebelts for the supercharger, but the eight-rib pulleys necessitated a custom crankshaft balancer with an extra rib cut into it.
The engine uses a stock LS7 camshaft, which delivers 0.591 inch lift (intake and exhaust) and 230/231 degrees of duration; again, it's one of the many off-the-shelf parts used in the combo. Aside from a set of custom, long-tube headers fabbed in Thomson's shop, the rest of the assembly is composed of pretty straightforward stuff. Engine operation is overseen by a new GMPP controller that's based on the production-style E67 brain.
Additional details of the engine combination include:
* 62-lb/hr ACCEL fuel injectors
* LS7 ignition-coil packs
* LS7 dry-sump oiling system with two-stage internal oil pump
* LS7 oil pan
* Custom headers with 2-inch primaries and 3-inch collectors
A stock LS7 cam delivers 0.591 inch lift (intake and exhaust) and 230/231 degrees of duration. Its high-rpm performance was deemed a good match for the heavily boosted engine.
The heads are something special. They're prototype versions of GMPP's LSX heads, which are based on the LS7 units but feature two additional head-bolt provisions. The additional bolt holes permit more-secure sealing of the head, which is particularly important in supercharged or nitrous setups.
The LS7-style intake ports are huge, straight pathways to the combustion chambers. They're capable of flowing about 380 cfm on the intake side.
Neither small nor huge, the LS7-style combustion chambers of the LSX heads come stock with 70cc volume. The valves to be inserted here are huge 2.165/1.600-inch components held at a 12-degree angle.
A standard LS7 valvetrain is used, including the pushrods, springs, and offset (intake-side) rocker arms.
Ignition chores are handled by a stock LS7 ignition system, which uses individual coils for each cylinder
Supercharged LSX Engine - Poor Man's LS9
The supercharger is likely the first of its kind in North America. It came from Harrop Engineering in Australia, displaces a whopping 2.3 liters, and features the latest rotor design from Eaton. The polished appearance looks great, too.
What looks like the front of a typical supercharger is actually the rear of the Harrop unit. The rotor pulley is connected to a through-shaft with pulleys at either end, with the pulley at the front of engine driven by the conventional crankshaft-propelled serpentine-belt system.
A custom crankshaft pulley/balancer was required, because the Harrop blower uses eight-rib pulleys. It was one of the few custom engine parts required for this project.
A 90mm electronically controlled throttle body is another off-the-shelf part used on the engine. Thomson pried it off an LS7 intake-manifold assembly.
The 427-inch displacement enabled the use of GM's newly available LS7 controller, which allowed the engine to start up on the dyno with no separate tuning. The controller was simply plugged into the engine's wiring harness.
A set of 62-pound fuel injectors feeds the supercharged LSX engine, shown here mounted on Thomson Automotive's dyno. Stock-type exhaust manifolds were used on the dyno, but a set of custom headers will accompany the engine when it's installed in a Corvette.
The proof of Brian Thomson's concept was validated when the engine delivered a maximum of 850 hp and 840 lb-ft of torque on the dyno. Next up, a C6 with an emptiness to fill between its front wheels-and maybe a smug, new ZR1 owner at a lonely stoplight.
When all of the parts were stuffed inside the LSX case, the engine was strapped down on Thomson Automotive's dynamometer. It immediately made power near Thomson's 800-horse goal, but fine-tuning of the air/fuel ratio delivered best results of 850 hp and 840 lb-ft of torque. Thomson wasn't surprised by the results, but we did detect a hint of relief in his voice.
"On paper, we knew we had the parts to make the power," he says. "The 'X' factor was the LSX block and reciprocating parts, and whether they would handle the pressure. We just haven't pushed them to this level before. The block, heads, and internal parts performed flawlessly. I couldn't be happier with the results."
Color us impressed, too. The big-displacement blower doesn't whine on the dyno, it howls. It's a sound that ought to strike fear into anyone who challenges this thing at a stoplight.
We were also impressed with the apparent durability of the LSX engine combination. The head gaskets stayed put under extreme pressure, and the engine made huge numbers all across the board. This was due in part to the blower's new rotor design, which helps the engine make more power down low and sustains it higher in the rev band.
For enthusiasts who weren't lucky enough to snag one of the new ZR1s this year-or for anyone whose spouse would likely commit a capital crime over another Corvette purchase-Thomson's 850-horse supercharged engine combo is an enticing alternative.
Oh, and forget about using the "poor man's" rationale on your spouse. We've tried it. It doesn't work.
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