Hilborn-injected LS From Mast Motorsports Gets Happy on Nitrous

Big-bore, short-stroke engines are the way to get screaming rpm for top-end racing, such as the competition coupe class on the Bonneville salt flats, so the team at Mast Motorsports in Texas came up with an oversquare combination from which the owner is actually asking for a little restraint.

This 304ci LS-based engine features a RHS aluminum block bored to 4.155 inches and fitted with a custom 2.800-inch Callies Magnum crank. That results in a rev-happy 1.484:1 bore/stroke ratio that could easily clear 9,000 rpm on dyno.

“Actually, we’re trying to keep it down,” admits Damon Sampson of Mast Motorsports. “We made a pull yesterday, and at 8,600 the dyno graph was still vertical and climbing. The customer wants to keep it below 9,000 because he’s literally sitting on top of the driveshaft.”

The short-block also consists of R&R rods (2-inch journal), Mahle pistons( 12.7:1 compression ratio), Dailey dry-sump oiling, Calico-coating bearings, Manton pushrods and a Comp Cams solid roller in the “800 lift department.”

Aside from class rules that limit displacement to under 305ci, the large bore dimension was actually needed to support the modified Mast Motorsport PXR cylinder heads. This particular set was part of the development for the company’s new Mozez head and features raised runners, canted valve locations (4.5 degree intake, 2.0 degree exhaust). They’re assembled with Victory valves (2.200/1.600), PAC springs and Jesel rockers. The combustion chamber volume is 35 cc and the intake port volume is a rather small 285 cc to complement the modest displacement.

“Also, the injector stacks are huge at 2 3/4 inches,” says engine builder Perry Kiritsy, “and we were trying to keep the velocity up.”

While Hilborn makes a fuel-injection manifold for the LS engine, this one had to be custom made to match up with the raised runners on the PXR heads. It comes with 65-pound FAST injectors and Nitrous Express plumbing, as car owner Jack Bateman has intentions of running for both the naturally aspirated and power-added ends of the record. Fuel management is through a Mast Motorsports controller.

So far the engine has pulled 585 horsepower naturally and 817 with the squeeze, again with a disciplined hand on the dyno’s throttle control.

“There’s more potential in it,” sums up Sampson. “We’re just not pushing it that hard.”

Two views of the Hilborn-injected engine during different phases of the build. The velocity stacks were obviously shortened for the final version to increase airflow velocity in the intake ports. Note the Nitrous Express system.

About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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