At the risk of earning the title “Captain Obvious”, different forms of racing require different approaches to the design of the engines powering the contestant vehicles. While that seems obvious, one of the forms of motorsport we aren’t familiar with personally, is the off-road race truck world. So when we caught wind of a build going on at Nelson Racing Engines for exactly that kind of racing, we decided to take a deeper look.
Designed for a “Pro” off-road truck class, there were very few rules surrounding the engine build. “For that particular motor, the customer told us that it was an open rulebook, with the exception of forced-induction,” says Tom Nelson, Nelson Racing Engines’ namesake.
With a blank sheet to design from, but far from a blank check to work with, Nelson set about putting together an engine with durability in mind, over squeezing out every last horsepower, for significantly less money than other entries in the class.
What he decided on was a carbureted 454 cubic-inch LSX-based engine, with a dry-sump oiling system and a significant number of components that are aimed at reliability in an extremely demanding performance environment. “The engine, besides being on and off throttle a lot, wings to the rev limiter when the tires unload,” Nelson explains. “It’s an abusive environment and [the engines] have to be built correctly to survive in that environment.”
Starting with a 9.240-inch-deck Chevrolet Performance LSX 454 short block, Nelson retained most of the LSX 454s rotating assembly, using the GM LSX forged 4.125-inch-stroke crankshaft and forged 4340 steel LSX connecting rods to handle the constant free-revving the engine sees when the tires unload.
However, hanging off the forged rods are a set of custom NRE-spec JE 4.185-inch pistons, with a mild dome, to create an 11.5:1 compression ratio. A Hellfire top ring is spec’d to stand up to the long-term abuse the engine will face, along with a reverse taper second ring and standard tension oil ring. A Cloyes timing set keeps everything in time, while Clevite H-series bearings keep everything running smooth and with as little friction as possible.
“Rod bearings are generally the first thing to go if that oiling system isn’t perfect,” says Nelson. To that end, he chose to go with a Dailey Engineering dry-sump oiling system. “The oiling system is really important, especially since everything will be bouncing around. We dry-sumped it, because that way you don’t have to worry about oil sloshing around in the pan; it’s all in the tank.”
The trick Dailey Engineering LS dry sump system incorporates a five-stage pump into the billet pol pan, for an exceptionally clean and functional package that doesn’t take a whole lot of engineering to make work correctly.
Moving to the top of the engine, the LSX-LS7 heads that came on the LSX 454 were replaced with a pair of Brodix STS BR 7 BS 273 heads. What that mouthful of a name means, is that they are Brodix’ LS7 replacement heads, with a 12-degree valve angle, 273cc intake port heads, which flow over 400 cfm through the intake ports at only .650-inch of lift, and 258 cfm through the exhaust at the same lift.
The “BS” in the cylinder heads’ name, stands for “Big Spring” and is designed to accept up to a 1.625-inch diameter valve spring. A 2.25-inch titanium intake valve with a 50-degree seat angle and 1.614-inch Inconel exhaust valve with a 45-degree seat angle meter the intake and exhaust charges, and are controlled by a set of Jesel shaft rockers, designed specifically for the BR7 big-spring heads.
“I don’t like to put a lot of spring [pressure] on a small 1.250-inch diameter LS valvespring, you can start overstressing that part,” explains Nelson. “For this, we came up with a spring that had about the same pressure–we aren’t talking massive pressures here–but we run a larger diameter and spread that force across a bigger spring. We get a taller installed height, and the spring is wider so the spring doesn’t heat up as much.”
With the durability in mind, Nelson chose a 1.550-inch PAC spring with pressure in the “maybe 475-500 pounds open” range. “The springs are all shot-peened and stress relieved, so it’s just a better package with more longevity,” says Nelson. “That spring will last a lot longer.”
Continuing with the theme of sheer reliability above all else in the valvetrain, Nelson chose a set of Crower EnduraMax solid roller lifters that do away with traditional needle bearings and instead use a bushing, which Crower calls the “Needleless Bearing Option” or NBO. Coupled with the HiPPO high-pressure pin oiling option, the lifters are about as bulletproofed as you can get. Sitting between the lifters and the rocker arms, are a set of Smith Brothers’ .145-inch wall-thickness pushrods.
The camshaft chosen for the engine isn’t easily explained through lift and duration alone, as Nelson put a significant amount of thought into the lobe shape. “We used a solid roller, but it was a gentile solid roller,” Nelson says. “The radius of the cam is very smooth so that it is very stable. It’s not like a drag race profile, but designed more for high endurance.” While the lift specs weren’t divulged, we can guess at that number based on where the flow numbers of the heads were rated. Nelson did disclose the duration numbers at .050-inch of lift, with 259 degrees on the intake and 264 degrees on the exhaust.
Moving above the heads, we get into another cool aspect of the engine – it’s induction system. First, and most obvious, externally, is the giant C.I.D. BE 4.0 two-piece cast LS7 intake manifold. “It’s got a huge plenum volume. Almost like a tunnel ram intake, but with only a single carb on top of it,” says Nelson. “Most of the class is injected, but Dennis liked the carburetor and didn’t want to deal with injection.”
On top of that unique intake manifold sits more uniqueness, in the form of a Carb Shop-modified 4150 carburetor, on a Dominator-style base. “We took a regular 4150-style carburetor, and bored it out so that it would flow 1,000 cfm,” Nelson says. But the modifications aren’t just limited to flow, because if that was the case, the team would have just gone with a 4500-series carb and been done with it. “The signal is a bit weaker to the carb in this engine, especially with all that plenum volume. I didn’t want to go to a Dominator, because I wanted a little better transient response from the carburetor,” Nelson explains.
With a modified booster in place, Bob Vrbancic also converted the carburetor to side-hung floats, and replaced the sight bowls with fittings that run into a catch can, for when things get crazy. “We ran a 180-degree vent tube over the carburetor so that if the fuel gets too high it dumps back into the carburetor, and won’t start a fire or anything like that; very similar to a marine application,” says Nelson. “Bob [Vrbancic] built us a carburetor that is really responsive.”
A set of 1-7/8-inch-primary Kooks headers get the spent gasses out of the engine, and Nelson thinks the engine could probably benefit from larger headers. To keep the candles lit, the team opted for the MSD LS Ignition Control box, with four custom timing maps for the race truck. “They go down into Mexico for some of these races and that can lead to fuel quality issues,” explains Nelson. “We put the MSD on there with four different timing maps so that he could choose between which map he wanted, and have a safe, low-octane, crap-gas tune available.”
When all was said and done, the engine spun Nelson’s engine dyno to the tune of 707 horsepower, and 610 lb-ft of torque. What is really impressive, is the air-fuel ratio numbers never fluctuate more than half a point under load during the dyno run, with a carburetor.
“710 horsepower is actually underpowered for his class from what I understand,” says Nelson, candidly. “He’s competing against things like Hilborn-stacked SB2s. This was an ‘affordable’ option–relatively speaking–against the other motors in the class, which are $50,000-$60,000 bullets.”
Remember how we mentioned that there was a conscious tradeoff for reliability over maximum power? “Taken care of correctly, this engine should last several seasons,” says Nelson. “We have relatively low compression, a mild cam, and it was built so that he wouldn’t have to be messing with it every weekend. He can go out and run in the middle of the pack, without breaking anything.” Sometimes the elegance of an engine is in the details, and this engine is a perfect example of that.