Twin-Turbo 474-Cube LS Makes North Of 2,000 Horsepower With NRE

Over the past 24 years, Tom Nelson has established a reputation for building potent engines that easily qualify as automotive “jewelry.” They combine racetrack power with car show elegance. Additionally, his signature (and patented) left/right “mirror image” symmetrical turbocharger installations serve to set Nelson Racing Engines apart from the norm.

So, it was with interest that we followed a recent build of a 474 ci. LS powerplant destined to be installed in a 1971 Firebird that will primarily be street driven — with occasional drag strip duty. As such, Nelson is utilizing an “octane on demand” system that employs two fuel cells; one filled with standard 91-octane pump gas for cruising and the other with 116-octane race fuel for when boost reaches a predetermined level. More on that later.

The Basic Building Blocks

The build is based on a tall-deck LSX iron block (23-bolt) from Chevrolet Performance, to give the end-user almost half an inch of additional deck-height. The block was fitted with a Callies 4.375-inch Magnum stroker crankshaft and 6.350-inch-long Callies Ultra I-beam connecting rods — which came from the factory equipped with ARP rod bolts.

The short-block uses an LSX tall-deck block (which Nelson repaints in his favored black), Callies Magnum 4.375-inch crankshaft, a set of Callies Ultra I-beam rods, and JE Pistons. In today's abundance of awesome parts, it's easier than ever to buy off the shelf components to handle north of 2,000 horsepower.

JE Pistons forged 2618 aluminum 4.155-inch pistons with a -28cc dish and rings were employed while ARP main studs secure the bottom end. ARP head studs, made from the proprietary ARP2000 material and rated at 220,000 psi tensile strength, supplied the necessary clamping force to seal the Brodix BR7 aluminum heads and Flatout solid copper gaskets to the block. O-ring grooves were cut in the block and head with stainless steel wire employed to assure an optimum seal.

Manley stainless steel 2.125-inch intake valves let the intake charge in, and 1.600-inch high-heat Inconel alloy valves on the exit side allow the exhaust out of the CNC-massaged chamber. Valvetrain activation comes from a special NRE hydraulic-roller camshaft ground by COMP Cams. 0.135-inch wall 3/8-inch O.D. Smith Brothers pushrods sit atop COMP hydraulic roller lifters. The valvesprings are from PAC Racing Springs with 190 pounds of pressure on the seat and 380 pounds, open.

This engine uses a unique dual-fuel configuration. One fuel system uses pump-gas and handles the engine’s fueling needs up to about 1,000 horsepower. Once there, an identical fuel system filled with 116 octane race gas takes over.

The valve retainers are Manley pieces with Bead-Loc keepers providing security. A Comp Cams shaft rocker assembly in 1.8:1 ratio is the intermediary between valves and the cam, while the NRE billet aluminum valve covers fastened with ARP’s polished stainless steel hex bolts provide the finishing touch.

A Cloyes two-piece timing set is the link between the camshaft and crankshaft — to which an ATI Super Damper is fitted to control harmonics. The LSX cam cover is secured with ARP polished stainless steel bolts. An Edelbrock water pump circulates coolant, while a fabricated oil pan and windage tray are employed with Valvoline 20w-50 VR-1 engine oil filling the sump.

Oil pressure is provided by a ZL1 oil pump and purity is ensured through the use of a K&N filter. Brown & Miller braided stainless steel lines — in black, of course — and AN fittings are a Nelson Racing Engines staple.

A pair of Brodix BR7 cylinder heads were used in the combination, but not without a trip through the NRE CNC machines to put Nelson’s own touches on the ports and chamber. Massive stainless intake and Inconel exhaust valves from Manley fill the cylinder heads, allowing serious airflow.

A CNC-machined NRE “Alien” intake manifold tops off the combination, with the plenum connected to the NRE Mirror-Image 88mm turbochargers via 2.5-inch tubes. A Holley EFI Dominator system controls fuel delivery with dual 70 lb/hr Siemens Deka fuel injectors per cylinder. To feed the sixteen injectors, four Walbro fuel pumps provide the required 1,800 gallons per hour of volume and 70 psi of pressure from the two-tank supply. Four Aeromotive fuel filters are employed; two 100-micron filters coming out of the tank and two 10-micron filters before the injectors. The fuel system is set up so that the 116-octane race gas kicks in when the engine power reaches the four-digit mark.

Igniting the air/fuel mixture are eight Holley Smart Coils, with the exhaust exiting through a set of stainless-steel headers made by Nelson Racing Engines and secured by ARP stainless steel studs.

NRE's Mirror-Image Turbochargers

A very early innovator of the symmetrical turbo setup, Nelson Racing Engines holds a patent on their Mirror Image turbochargers. Currently available in 72mm and 88mm compressor sizes, the turbo sets all feature one clockwise-rotating and one counter-clockwise-rotating turbocharger. The heart of NRE’s turbos are the billet compressor wheels which offer maximum flow numbers for their size. Inconel turbine wheels sit inside divided exhaust housings which are fitted with V-band outlets, machined with O-rings to ensure a leak-free seal.

The unique NRE “Alien” intake manifold allows for interesting packaging. Namely, an individual intercooler core for each of the mirrored 88mm turbochargers.

Wringing It Out

Displacement and combustion pressure work hand-in-hand to generate tremendous amounts of power. After breaking in the engine, Nelson began adding more and more boost to the equation, taking readings on his dyno with varying amounts of pressure. At a very modest 6psi of boost, the LS generated 876 horsepower at 6,200 rpm with a peak torque of 775 ft-lb reached at 5,000 rpm.

Upping the boost to 13 pounds saw 1,153 ponies at 6,400 rpm, and with the boost upped to 17 pounds, 1,300 horsepower was generated. It’s at this point the 116-octane gasoline typically takes over. Next on the agenda was 21 psi, with the dyno pegging at 1,479 horsepower at 6,200 rpm, with a peak torque of 1,315 lb-ft of torque generated at 5,200 rpm.

Nelson ran a range of boost pressures while on the dyno. From 6psi up to 30psi. Somewhere around 17psi, the unique dual-fuel system switches from pump gas to the 116-octane race fuel to carry the engine through the remaining 13-plus pounds of boost.

Given the boost-to-power progression and experience from multiple similar builds, Nelson is confident that the engine is capable of well over 2,000 horsepower when it crosses the 30 psi threshold. That said, the engine’s owner has put a moratorium on “turning up the wick” for testing and Nelson honored his wishes. However, a recent build with essentially the same combination netted 2,113 horsepower at 6,500 rpm with torque maxing out at 1,732 lb-ft at a peak of 32psi.

In the final analysis, NRE’s latest creation exhibits a strong power curve between 4,500 and 6,500 rpm and is well suited to its assigned tasks. Moreover, the overall fit and finish is such that it will enhance the Firebird’s engine bay and attract a multitude of admirers. It’s an excellent blending of “beauty” and “beast.”

Here's the dyno graphs of the 474 cubic-inch LS engine on various amounts of boost and culminating at 32 pounds of boost. That curve should make the engine relatively mild driving on the street, but have plenty of power on tap whenever you need to drop the hammer.

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About the author

Bill Holland

Bill Holland has been involved in racing and the performance aftermarket since the 1960s in the capacities of racer, speed shop proprietor, journalist, street rodder, designer and advertising/PR/marketing professional. Along the way he’s raced Top Fuel and Funny Car, been editor of NHRA’s publication, National Dragster, was involved in off-roading as publisher of SCORE News, built a variety of Featured Vehicles for the SEMA Show, as well as a Track “T” that was a Contender for the AMBR award. He currently races vintage sports cars. Bill was inducted into NHRA’s California Hot Rod Reunion Hall of Fame in 2017.
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