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416ci LS3 Engine Build - Part 2 - Top End

Building up the top end of The Beast

Jeremy D. Clough Jan 1, 2014
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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.

LS3 Top End Assembly LS7 Pro Elite Heads 2/27

1. For the top end, we selected LS7 Pro Elite heads from RHS. These 355-T6 aluminum units come fully assembled with lightweight Manley valves.

LS3 Top End Assembly Valve Stem Lock 3/27

4. Note the rounded mounting surface for the valve stem lock. While I haven’t seen it specified, it makes sense that the radius avoids creating a stress riser better than a square-edged cut would.

LS3 Top End Assembly CNC Ports 4/27

2. The RHS heads are also CNC-ported. There’s still a lot to be said for the line-of-sight approach to flow, and there’s clearly plenty of that here.

LS3 Top End Assembly Valve Stem Install 5/27

7. Since removing the valves damages the seals, new ones were installed when the valves were put back in. The numbers denote the spring heights with the valves installed.

LS3 Top End Assembly Valve Mating Surface 6/27

5. Here’s the mating surface of the valve after lapping. The frosted grey area is the area that’s been lapped.

LS3 Top End Assembly Valve Lapping 7/27

3. The first step in our head work was to disassemble the heads and “lap” the valves into their seats in the combustion chamber. The heads were then surfaced, to ensure a true surface where they mate to the block.

LS3 Top End Assembly Valve Stem Compressing 8/27

8. A pneumatic tool is used to compress the valve stems and slip the retainers into place.

LS3 Top End Assembly Bolt Anchor Point 9/27

11. Unlike factory LS heads, the RHS units feature a six-bolt configuration. Mated to an RHS (or LSX) block with the anchor points for those extra bolts, this arrangement offers significantly increased clamping force, a plus on boosted engines.

LS3 Top End Assembly Dual Spring 10/27

6. Good springs are crucial, especially at high rpm. The dual-spring feature is added insurance: If one of the two breaks, there’s still another to keep the valve from dropping.

LS3 Top End Assembly Comp Hydraulic Roller Lifters 11/27

14. With the cam in place, we dropped in the Comp hydraulic roller lifters and installed a factory lifter tray to keep them in place. The lifters had previously been soaked in solvent to prepare them for installation.

LS3 Top End Assembly Comp Cam Cam Shaft 12/27

12. Comp Cams provided the cam, which features 0.605-/0.615-inch lift, 238-/248-degrees duration, and a 112-degree LSA. After being lubed, it was slipped carefully into its bearings in the block.

LS3 Top End Assembly Combustion Chamber Size Check 13/27

9. After machining, the heads were cc’d to check the size of the combustion chamber. In our case, we wanted to stick with the factory 10.7:1 ratio.

LS3 Top End Assembly Comp Shaft Rockers 14/27

16. In addition to providing more stability than traditional stud-mounted units, the aluminum Comp shaft rockers come drilled with oiling holes to keep everything lubricated.

LS3 Top End Assembly Roller Rocker Base 15/27

15. The next order of business was to install the base for our Comp 1.8-ratio shaft-mounted roller rockers. Part of the installation process was measuring for the optimum mounting height, then shimming the base.

LS3 Top End Assembly Cam Degreeing 16/27

13. After the Comp Cams timing set was installed, it was adjusted to degree in the cam. The cam gear on the Comp set uses a screw on an eccentric to give some adjustment without having to disassemble everything.

LS3 Top End Assembly FAST LSXR Big Mouth Throttle Body 17/27

19. We paired the LSXR with one of FAST’s beautifully machined 102mm Big Mouth throttle bodies. It’s every bit as big as it looks it the picture.

LS3 Top End Assembly Combustion Multilayer Steel Head Gasket 18/27

10. After putting a multilayer steel head gasket in place, the heads slipped onto their ARP studs. The fasteners were then torqued down, followed by the top cover.

LS3 Top End Assembly Pushrod And Rocker Shaft Install 19/27

17. Once the bases were shimmed and torqued in place, we could install the pushrods and bolt down the rocker shafts.

LS3 Top End Assembly FAST Fuel Rail Install 20/27

23. To install the assembled rails, we aligned the tops of the injectors with their respective holes in the underside of each rail, then pushed the rails down, essentially “plugging in” the injectors. After that, we indexed the fuel-rail mounting brackets with their holes in the intake, screwed them down, and put the crossover in place.

LS3 Top End Assembly Pedal Assembly 21/27

20. While the factory Corvette throttle body uses drive-by-wire actuation, the FAST unit is cable-operated. That means you’ll need to add a cable to your pedal assembly and, more than likely, change out your engine computer.

LS3 Top End Assembly FAST Injectors 22/27

21. With their O-rings lubricated, the eight FAST injectors slipped easily into place in the intake. Thanks to a healthy flow rating of 65-lb/hr, they should provide plenty of fuel to feed our big-inch stroker.

LS3 Top End Assembly FAST LSXR Intake 23/27

18. FAST’s LSXR intake is made of lightweight polymer. The LS7 version we used features raised ports, removable runners, and a 102mm throttle-body opening.

LS3 Top End Assembly Valve Cover 24/27

24. The height of our valvetrain kept us from using stock valve covers, so we used this set of cast-aluminum covers instead. They came in a black-crinkle finish, with oil-control baffles already installed.

LS3 Top End Assembly FAST Fuel Rails 25/27

22. Along with the injectors, FAST supplied a set of red-anodized fuel rails and an -8 AN fuel crossover. We used an aluminum AN wrench to ensure we didn’t scratch up the anodized fittings.

LS3 Top End Assembly Valve Cover Install 26/27

25. Here’s a shot of one of the covers as installed. As you can see, they come with mounting studs that retain the factory centerbolt mounting arrangement.

LS3 Top End Assembly 27/27

26. Done. Our LS3 stroker is now officially together, and ready to hit the dyno.


Comp Cams
Memphis, TN 38118
Racing Head Service
Memphis, TN 38118
Fuel Air Spark Technology
Memphis, TN 38118
Grimes Automotive Machine



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