It's the ultimate LS-engine "what if" scenario: Could the blower from the Corvette ZR1's LS9 engine be dropped on the larger-displacement LS7 found in the Z06? And wouldn't that create the ultimate "factory" combination? Enthusiasts started dreaming up this ultimate hybrid combination the second the LS9 was revealed to displace 6.2 liters, rather than the LS7's 7.0L size. That's a significant 13-percent difference in volume, but GM said it went with the smaller bores to ensure a safe, long-lasting powertrain-one that would be subjected to considerable boost pressure over its lifetime.
"On the OE side, all the considerations for 100,000 miles worth of durability must be considered," says an anonymous engineer we spoke with. "The power goals for the LS9 were reached with the smaller displacement, which offers thicker cylinder walls for greater safety margin, so there was no need to step up to the 7.0L displacement."
The standard cylinder block...
The standard cylinder block for the 7.0L LS7 is the foundation for Thomson's LS7/LS9 hybrid engine and is aavailable through GM Performance Parts under part number 19213580. It's aluminum with pressed-in steel bore liners and forged steel main bearing caps; the bores measure 4.125 inches in diameter.
That's a great textbook answer and we certainly wouldn't cast aspersions on the LS9's 638hp output, but when there's a larger-cube version of the same engine family, you just know the performance world wouldn't let the combination go untried. That would be like Dr. Frankenstein having both a spare brain and access to a graveyard, but passing on the opportunity. Brian Thomson isn't Dr. Frankenstein, but he's created more than his share of LS-based monster engines at his shop, Thomson Automotive, near Detroit. In fact, he's had great success melding the LS7 with the LS9 blower, developing basically a crate engine package-including a pre-programmed E67 controller-that delivers around 770 hp and 840 lb-ft of torque on pump gas.
"It's a sweetheart of a street engine," says Thomson. "It starts, idles, and has the low-speed drivability of a stock engine, but when you tap into it, it's amazing. With more than 800 lb-ft and the instant power delivery of the blower, it's an engine with two distinct personalities."
Machine work prior to assembly...
Machine work prior to assembly includes deck-plate honing of the cylinders and line boring of the mains. They're common high-performance engine-building procedures, with the line boring supporting the addition of ARP main studs.
OK, so maybe it's not Frankenstein's monster, but more of a Jekyll and Hyde engine. Regardless, we recently took the opportunity to sit in on the assembly and dyno-testing of one of these engines, which was destined for a Chevy S-10 project vehicle of New Yorker Joe Jones. It officially made 769 hp and 844 lb-ft, with a safe air/fuel ratio of about 11.4:1 under full boost. Thomson says that more aggressive tuning will easily yield 800 hp, but he leans on the conservative side of things for customers who will use the engine primarily for the street and only a few annual trips to the drag strip.
"The 20 or so horsepower we give up with a richer mixture are hardly missed when you've got 840 lb-ft to deal with and instant boost," says Thomson. "It's a no-brainer setup for pump gas, but you can certainly tweak it."
Of course, it was only a few years ago that even a 500hp street engine was newsworthy. The fact that Thomson is explaining why his engine "only" makes 770 horses on the street demonstrates how far we've come with LS engines and tuning.