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850 and Counting

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by Barry Kluczyk  More from Author

Thomson Automotive’s Supercharged 427ci LS Engine.

With the last pull on his shop’s engine dynamometer, builder Brian Thomson gazes at the results on the computer screen. The results: 850 horsepower at 6,500 rpm and 850 lb-ft of torque at 4,200 rpm from a 427-cubic-inch LS engine.

He’s philosophical about the results: “That’s not bad. It would have been nice to hit 900 horses.”

This is a man who built a 2,000hp, twin-turbocharged LS engine a couple of years ago, so it’s understandable that anything under, say, 1,500 horsepower is going to seem to him like lawn mower output in comparison.

That 2,000-horsepower monster – with turbos not generally seen outside of a Caterpillar assembly line – was a one-off, custom-built creation meant to prove the viability of GM Performance Parts’ newly-introduced LSX high-performance cylinder block. In comparison, the conventionally supercharged engine on the dyno this day was based on a production cylinder block and used a surprising number of off-the-shelf parts. It ran on pump gas and was controlled by a production GM computer.

Bottom line: The blower engine is a street engine, and its 850hp would have been considered racing engine output only a couple of years ago.

“The basic LS engine responds really well to forced induction,” says Thomson. “The heads flow a lot of air, so you can really get big numbers if you know what you’re doing.”

Thomson certainly knows what he’s doing. His shop, Thomson Automotive, has been at the forefront of LS engine performance for several years. He’s worked closely with GM Performance Parts to test and develop parts and engine combinations, as well as working with racers who put those parts and engines through the ultimate validation scenarios.

The supercharged engine that is the subject of this story was custom-built for a California enthusiast who plans to install it in a classic car, but the basic combination is based one that Thomson has used on other forced-induction engines.

“We’ve got the recipe down pretty well,” he says. “We know which parts work the best together to make the most power. But more importantly, they make the power reliably. With this engine, you can run it in just about anything and use it every day.”

Indeed, with the plethora of swap kits hitting the market, installing an LS engine in most popular vintage cars, such as first- and second-gen Camaros, is easy. We think this engine would look great under the hood of a resto-mod 1969 RS or as a crate engine-style replacement for a 2010 Camaro SS.

As Thomson said, his horsepower recipe has been refined over time. The basic ingredients include:

  • GM Performance Parts LSX cylinder block
  • Moldex 4340-forged crankshaft
  • Diamond dished blower pistons and Oliver forged rods
  • Approximately 9.0:1 compression
  • GM LS7 cylinder heads
  • Harrop 2.3-liter supercharger with Eaton TVS rotors
  • Roller camshaft with proprietary specs (we couldn’t pry them out of Thomson)

“This is not an exotic combination – just the right-sized parts of the appropriate strength to support the supercharged performance,” says Thomson. “Of course, tuning is important and we’re careful about that, but the rotating assembly and other engine parts are pretty straightforward.”

The Combination

Forming the foundation of the engine is that iron LSX Bowtie block from GM Performance Parts. It’s true that the high-performance production LS engines use aluminum blocks – and the LSX block comes with an irrefutable weight penalty when compared with them – but GMPP’s iron piston house offers great strength and a low price. It is also cast with provisions for six-bolt-per-cylinder heads, which enable greater clamping strength when used with heads with matching six-bolt provisions. In fact, such a configuration is designed for high-boost forced induction systems.

The primary intention for the engine Thomson built is street driving, so boost was kept to a moderate level of about 15 pounds. That keeps the head gaskets happy, allows trouble-free use of pump gas and requires no radical ignition or tuning tricks.

As mentioned in the list above, the device delivering that boost is a Harrop-supplied Roots-type supercharger that’s filled with Eaton’s ground-breaking, four-lobe TVS rotors. In addition to the additional lobe per rotor, when compared with conventional Roots blowers, the lobes have a higher helix design (greater rotor angle). It all adds up to a blower with a greater range of performance, extending into the low and high rpm ranges – the areas where Roots blowers tend to fall down. This one displaces a whopping 2.3 liters, too – the same as the TVS blower on the Corvette ZR1’s LS9 engine.

“It’s a great supercharger and flows a tremendous amount of air,” says Thomson. “Its big displacement really fills up the engine, so we didn’t have to spin it at a crazy speed to get a lot of boost out of it.”

The blower feeds a set of LS7 cylinder heads that Thomson uses more or less out of the box. They’re filled with stock titanium intake valves and Ferrea inconel exhaust valves, PAC valve springs and production-style 1.6-ratio rocker arms.

“We could have spent time and the customer’s money porting and polishing the heads, but the payoff isn’t really worth it,” says Thomson. “In their stock form, the heads flow exceptionally well, so we used them as-is.”

On the Dyno

Using a GM E67 controller sourced from GM Performance Parts, the engine’s operating parameters were carefully dialed in. That got the engine up and running, and a little more fine tuning optimized the fuel and spark, enabling the peak 850-horsepower/850-torque numbers. More importantly, the engine delivered repeated performance at this peak, making great low-rpm torque – 500 lb-ft at only 2000 rpm – and horsepower trailing off only slightly past the 6,500-rpm level.

During our visit to the dyno, we noticed the engine started, idled and pulled extremely easily and smoothly. It didn’t stumble at idle when cold and, in fact, the idle quality was remarkably smooth. There were no hiccups when the engine was flogged, either. It simply pulled through the rpm range.

“It’s a great street engine,” says Thomson. “It has the drivability qualities you’d expect in a daily driver, but this one happens to make 850 horsepower.”

On top of it all, the big, 2.3-liter blower emits an almost supernatural wail. If you’re used to the shrill whine or whistle of other blowers, the lower-range, guttural sound that comes as the revs increase will send a shiver down your spine. And, in full disclosure, so will the price tag. This one was in the neighborhood of $25,000. That’s certainly pricier than your basic, 250-horse rebuilt small-block, but there’s a lot of technology packed into this engine.

If you’ve got the means and a project car with an unfilled hole between its front fenders, this engine combo has a lot to offer – whether that project is a 1970, 1990 or 2010 Camaro.

Thomson Automotive

The starting point for the engine assembly is GM Performance Parts’ LSX Bowtie block. Its iron construction makes it admirably strong and surprisingly inexpensive. It can also be generously overbored, which is something that can’t be done with production aluminum blocks. Thomson started with a standard-deck block and machined the cylinder bores to 4.125 inches.

A Moldex 4340-forged steel crankshaft was laid in place next. It delivers a 4.000-inch stroke, which combines with the 4.125-inch bores to give the engine a 427ci displacement. Thomson specified that a keyway was machined into the crankshaft nose, because production LS engines don’t have one – the damper/crankshaft pulley is press-fit onto the crankshaft. With the high loads and quick spin-up implemented by the supercharger, a keyway is necessary to prevent slippage.

The rod-and-piston combination includes Oliver forged rods, measuring 6.200 inches in length, and Diamond forged aluminum pistons. The pistons are designed for forced induction, with a thick crown. The rods and pistons are married with heavy-duty, full-floating wrist pins.

The pistons have a slight dish to keep the compression ratio at a detonation-avoided 9.0:1. That’s because the cylinder heads have relatively small combustion chambers, which would promote a higher compression with flat-top or slightly domed pistons.

One of the few sets of specs we couldn’t pry out of Thomson Automotive are those for the camshaft. We can report that it’s a hydraulic roller and delivers a surprisingly docile idle quality. But as for lift, duration and lobe separation angle numbers, they remain mysteries.

The middle men between the blower and the rotating assembly are LS7 cylinder heads. They’re fitted with 2.20-inch titanium intake valves and 1.61-inch Ferrea inconel exhaust valves and have excellent airflow characteristics, with a nearly straight path from the top of the intake port to the valve face.

Valvetrain components include PAC springs and LS production stamped steel rocker arms that deliver a 1.6:1 ratio. Nothing exotic here – just carefully matched, very functional components.

The aluminum lung for this project engine is Harrop’s 2.3-liter, Eaton-filled supercharger. It uses the latest TVS four-lobe, high-helix rotors to extend the effective performance range of the blower, building power sooner at low rpm and sustaining it at higher rpm.

A view from the other end of the blower assembly shows the integrated bypass valve, located in the lower right corner of the compressor case.

Here, the supercharger is carefully lowered onto the prepped long block. Traditionally, the big, fat blower belt drive signified the front of the supercharger, but to accommodate the necessary throttle body for the fuel injected engine, the primary blower drive is at the rear of the case, with a satellite-style driveshaft connecting the primary drive and the crank drive at the front of the engine. The driveshaft runs beneath the blower case.

The supercharger pulley is a big factor in the maximum boost the blower produces. Thomson used a stock 2.750-inch pulley from Harrop to produce approximately 15 pounds of boost.

The effectiveness of the supercharger is also determined by the amount of air it can draw. A 90mm electronically-controlled unit was employed on the project engine. A larger-diameter opening would have been helpful, too.

Fully dressed on Thomson Automotive’s dynamometer – with high-flow LS7 exhaust manifolds – the engine was tested with accessories including a power steering pump to simulate the real-world load on the engine. Its impressive peak output was 850 horsepower and 850 lb-ft of torque on premium pump gas.

Thomson’s supercharged LS engine is right at home under the hood of a classic or the newest Camaros.


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