416ci LS3 Engine Build - Part 2 - Top End

Building up the top end of The Beast

Jeremy D. Clough Jan 1, 2014 0 Comment(s)
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In last month's issue, we began the buildup of our LS3-based 416-cube stroker by addressing the short-block. With the block machined and assembled with a bulletproof forged rotating assembly from Lunati and Wiseco, it's time to shift our attention to the top half, which comprises parts from Comp Cams, Racing Head Service (RHS), and Fuel Air Spark Technology (FAST).

We ordered aluminum Pro-Elite LS7 heads from RHS and, along with all the other engine components, had them delivered to Grimes Automotive Machine for the machining and buildup. Made from 355-T6 aluminum, these heads feature a raised intake-runner design with rectangular ports (instead of the narrower cathedral-port design found on earlier LS heads), as well as a six-bolt configuration (up from four on the stock head) for maximum clamping force. While the latter arrangement will work on a standard block, such as our LS3, to take advantage of the added bolt holes, you'll need to either use an RHS block or modify your own block to add matching bolt-mounting points.

Although the heads came CNC-ported, fully assembled, and well-deburred, the first order of business was to disassemble them and check all the critical dimensions. As with the bottom end, absolutely nothing was left to chance. Once the heads were apart, each of the valves was "lapped in" by putting a small ring of abrasive paste around its surface, then using a suction-cupped tool to spin it in its seat. This is important not only for valve seal, but also for heat transfer from the valve to the head: If there's insufficient surface area on the seating surface, the valve can't shed enough of its heat when it makes contact.

The heads were then cc'd and machined to make sure that they were true, ensuring the best mating surface possible. I'd never seen heads cc'd before, and the process was fascinating: A clear Plexiglas plate was sealed over one of the combustion chambers, leaving a small opening near the upwards side of the chamber, and fluid was slowly poured into it from a graduated cylinder. When the chamber was full, the amount emptied from the graduated cylinder was equal to the volume of the combustion chamber—a crucial measurement for calculating the engine's final compression ratio.

Once the combustion chamber was emptied and the heads cleaned, the valve-stem height was measured. The valvesprings were tested for tension when compressed to that height, and again when brought to full compression. These measurements were also used to shim the springs. To maintain consistency from cylinder to cylinder, it's important for the springs to be the same height. From the factory, the seated spring heights were all within 0.005-inch of one another, which is pretty impressive considering the possible tolerance stack of valves and both springs and valve seats.

The valves were then lubed and reinstalled with new valve seals, which completed the head work. That may only take a few paragraphs to describe, but it was a solid half-day's work. Once done, though, it was time for the heads to seal up the top end of the block.

During the head work, Merlin, who handled that part of the engine build, commented on how much air the heads would move, and how much horsepower they'd support. Without jinxing myself by repeating it here (since at this writing, we haven't dyno'd the engine yet), the rather high expected power number I'd been hearing came up again.

"Am I going to kill myself in this car?" I asked him.

"It's an option."

After sliding a multilayer steel gasket over the head studs, the heads were seated on the block and the nuts torqued down. As with the bottom end, we used ARP fasteners and studs instead of the factory-style torque-to-yield bolts.

While the Comp roller cam featured a grind that's proprietary to Grimes, I can say that it has 0.605/0.615-inch lift, a duration of 238/248 degrees, and a 112-degree lobe-separation angle. Garry Scott, who assembled the top end, slipped it carefully into place in its bearings, then installed the timing set and began the painstaking work of degreeing it. Basically, degreeing is the process of ensuring that the crank and the cam "agree" with one another about what should be done at what time.

Typically, a degree wheel is bolted onto the crank, a lifter is dropped into its place in the number-one cylinder, and its travel is measured with a dial indicator until that cam lobe's centerline can be found. Joseph Potak's book How to Build and Modify GM LS-Series Engines gives an in-depth explanation of the procedure, and reading it before starting the engine build helped me better understand what I was seeing as everything went together.

Once the cam was degreed in and the fasteners were tightened to lock it all down, the roller lifters (which had previously been soaked in solvent to remove any oil or grease) were dropped into place and secured with factory plastic lifter trays.

For the rest of the valvetrain, we used a set of shaft-mounted aluminum roller rockers from Comp. The rockers are paired together on the shaft around which they pivot, so if service is needed, they can be removed one cylinder at a time, instead of having to pull an entire bank's worth. We installed the base and one pair of rockers, checked the valve stem to see where the rocker tip would contact it, and shimmed the base to get that contact area in the optimal position. After that, we measured for correct pushrod length, then slid the rods into place and installed the rockers on top of them.

With the valvetrain in place, we moved on to induction, starting with an LSXR LS7 manifold from FAST. Made of lightweight polymer, the LSXR is designed with raised rectangular intake ports that match the ports in the head beautifully. It also has runners that are easily removable for porting, molded-in bungs for nitrous use, and an opening for a 102mm throttle body.

Speaking of throttle bodies, FAST also supplied a beautifully machined Big Mouth 102mm unit, which bolted easily into place with its included throttle-position sensor. By my math, it's about 28 percent larger than the opening on a factory 90mm throttle body. Since airflow matters, it's no surprise FAST advertises power gains of up to 26 rwhp on big-cube LS engines.

The Big Mouth also features an offset blade pivot designed to give quick throttle response, and a thicker throttle blade to eliminate the chance of blade deflection when used with a turbo or supercharger. For those wishing to retrofit the 102mm throttle body on a C6 or other drive-by-wire (DBW) car, the cable-operated Big Mouth will require the addition of a throttle cable and possibly a computer change (the factory E38 computer only works with DBW). We avoided that situation by using a FAST XFI computer, which we'll talk about next month when we dyno and tune the engine.

After lubing the O-rings on a set of 65-lb/hr FAST injectors, we plugged them into the intake and assembled the bright-red billet fuel rails, also provided by FAST. To put those together, we screwed in the male/male AN adaptors at either end of the rail (-8 AN up front for the braided steel crossover, -6 for the in/out at the rear of the rails) and screwed on the brackets that mount them to the intake. The instructions were a little hazy on the proper orientation for the brackets, so we made an educated guess after looking at the pictures, and it worked.

Once bolted up, the gleaming red rails against the gray manifold added just a touch of flash to what was rapidly becoming a very businesslike motor. For a little contrast, we added a set of tall black-crinkle-finished valve covers that were high enough to clear our valvetrain.

The finished motor—with its raw aluminum, its matte-gray-and-black top end, and its brilliant red fuel rails—is truly a thing of beauty. And when it goes on the dyno, we'll find out exactly how beautiful it is.

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