RHS Aftermarket LS Block - Part 1 Bigger On The Inside

RHS’ LS block makes it possible to get big-block displacement in a small-block package

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There’s a sci-fi show from across the pond called Doctor Who. It’s about a time-traveling extraterrestrial that transits about in a phone booth. The gag here is that from the outside it’s just a small British-styled police call box, but once inside, it’s cavernous with the catch phrase being, “It’s bigger on the inside.” LS engines are a lot like the good Doctor’s mode of transport, even though the exterior dimensions remain diminutive, the internal spaces can be optimized for maximum displacement. Even so, you can only do so much with the factory block before you run out of room to expand. Racing Head Service (RHS) saw this limitation and decided to design an aftermarket LS block perfect for LS fanatics that want more cubes and a whole lot of race-inspired features.

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The block’s cam is raised and RHS spent considerable effort reworking the internal dimensions to accommodate spinning cranks and flying rods. They also addressed oil control and incorporated features like motor plate race mounts for the hardcore crowd and provisions to easily add a dry sump oiling system — all in a package no bigger than your garden-variety GM LS engine. Sweet! For those who don’t mind the external dimensions bumping up a bit, RHS also offers a tall-deck version that is engineered to accept up to a 4.600-inch crank. Combine that with the RHS block’s maximum bore of 4.165 and you get the potential for 501 cubes (8.2L). The cost for the tall and short deck blocks is the same, so going tall is typically the smart choice. This is especially true since RHS offers intake spacer kits (priced around $300) to accommodate the tall deck architecture. In our case we were looking to replace the 402 LS2 in our ’68 project car, so the standard-deck block made more sense since we would be able to reuse our custom headers without modifying them. After settling on a bore of 4.155 and a stroke of 4.250 (the maximum for the standard deck block) our calculator showed a displacement of 461 ci (7.6L). That’s big-block cubes in a lightweight, small-block package; just what the Doctor ordered.

Nonetheless, making power requires more than a badass block, so we hit up Lunati for some of their trinkets to fill the internal cavities of the RHS block. All those cubes require lots of air so we penciled in a set of Mast Motorsports Black Label LS3 12-degree six-bolt heads topped off with a 102mm FAST intake manifold. The rest of the build was filled with top-shelf parts from COMP, ARP, Moroso, Autokraft, Meziere, CHE, and others. Hey, when you’re droppin’ thousands of bucks on a killer block, it makes zero sense to start pinching pennies on the smaller parts. But all the high-end parts in the world are wasted if the install is sloppy, so we contacted Tom Nelson of Nelson Racing Engines to do the critical machine work and help us wrench it all together into a bulletproof package. Nelson typically builds uber-horsepower forced-induction mills, so we figured a nice and simple naturally aspirated build would almost be like a vacation for him. What we found out from Nelson is that doing it right is a lot of work, so we split the story down the middle by building the short-block in this installment and the top end with dyno testing in the next issue.

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