It's not every day you get to hand-build a 638 HP LS9 engine that'll slip under the hood of a Corvette ZR1. Yesterday was one of those days. Here's how the magic is made.

Yesterday we drove over to GM's Performance Build Center (PBC) in Wixom, Michigan to build an LS9 engine — the most powerful production engine GM's ever built — and the 638 HP beating heart powerplant underneath the hood of the Corvette ZR1 — all while being taught the tricks of engine assembly trade by the man who's built more LS9's than anyone else.

Here's how the magic's made.

Aluminum block, aluminum heads, forged crank, Forged pistons, titanium connecting rods, valves, and valve springs, a bespoke Eaton four-lobe roots-type supercharger displacing 2.3 liters of air per rotation blowing into an air-to-water intercooler, all backed by a twin-plate clutch. It's 638 HP of smack-you-in-the-face and it'll shove the ZR1 to a 0-60MPH time in 3.4 organ-crushing seconds. But first it has to be built

Each Corvette ZR1 engine is hand assembled at GM's Performance Build Center (PBC) in Wixom, Michigan, a purpose-built 100,000 square foot facility with the express intent of building the best motors in the GM universe. It's a mix of the latest thinking in lean manufacturing paired with the old world responsibility inherent in a one-man, one-engine build philosophy. The facility also builds the monstrous huge 7.0 liter LS7 for the Z06 and the hopped-up LS3 for the new Corvette Gran Sport, but when you're offered the opportunity to build an engine of your choice, you pick the LS9.

My guide through this most unique of looks into the guts of GM's baddest engine was Mike Priest, the man who's put more of these engines together than anyone in the world and some say he assembles the fastest ones. Good hands to be in. We start the day off by getting right down to business, cracking the connecting rods. Each piston comes sub-assembled complete with a high-moly top ring, a nodular iron lower, and a 3mm oil ring.We inspect each for nicks and scratches, make sure the wrist pin turns freely, and then systematically mark the bearing cap and con-rod to make sure they match up later. The bolts are then loosened and they're separated by a light tap or a cracking fixture.

The bearing journals are then laid in and everything gets placed in a tote and wheeled to the first station on the rolling engine stand.

The line is arranged into individual stations, each being fed by pre-kitted parts and a pull-type delivery system. The stations each have at least one Atlas Copco electric torque driver which torques fasteners based on a scan of the build sheet. Each data point is recorded and tied to the individual engine, and data can be reclaimed at any time. A naked engine block is sitting on the delivery fixture awaiting assembly, it'll be engine 10ZAN 322731901. In the very first step we install studs which will hold the engine to the rolling engine stand.

Once Mike gives the engine a good once, a water passage hole is plugged, then we move on to installing the first exciting piece, the camshaft. An assembly fixture is placed on the back of the cam and then a handle screwed onto the front, the journals are given a coat of assembly oil the cam slides in easier than any cam you'll ever touch. It's nice having factory tools. Mike then uses the overhead crane to hoist the block over to the stand where we bolt it down. Finally, the cam seal and retainer is installed.

Next we loosen the crank bearing caps and use a custom pry bar to remove them without damage and set them aside.

The bearing journals and main bearing are now snapped into place and liberally coated in assembly oil.

After inspecting the crankshaft, Mike expertly drops it into place with a crane fixture. He offered to let me do it, but the idea of marring such a critical piece wasn't something I was too excited about.

The crankshaft is gently turned to make sure there's no unexpected drag.

Next we assemble the other half of the journals into the bearing caps and oil, installing them as they were delivered in the block.

The caps are torqued down using a multi-spindle driver to simultaneously drive the inner then the outer bearing bolts. Those bearing blocks also get cross-bolted to the walls of the block, which increases the assembly's overall stiffness. As a validation, the torque to turn the crank is checked, to make sure there's no undue drag.

Next we stuff pistons. Assembly oil is applied to the piston bores, the edges of the piston and the connecting rod bearing. An assembly guide is slipped over the end of the connecting rod and a ring compressor slipped over the head of the piston. I' told to run the piston up and down to make sure it slides smoothly in the compressor then place it over its cylinder. When the bottom of the piston is seated in the bore, one strong push to the top of the piston and the entire assembly slides in smooth as silk.

The connecting rod cap is lubed and installed and the process is repeated down the line, then the bolts are all torqued. The engine is rolled to the opposite side and the process repeats.

Next up the engine oil baffle is attached to the studs on the crank bearing outer bolts.

Next, the timing is set. Piston number one, on the driver's side front of the engine, is raised to top dead center. The chain tensioner is then bolted into place.

The crank gear is then pressed into place with a pneumatic ram.

The timing gear for the cam is placed on, two timing marks on both gears are are aligned and the cam gear is removed, the chain is laid over top and then the chain is attached to crank gear as the cam gear is slid into place and bolted down. The pin in the tensioner is then removed.

The dry sump oil pump is then carefully positioned with a fixture to ensure its ports are flush with the bottom of the engine block, then bolted into place.

The back engine cover compete with rear main seal and the front engine cover are then installed

In the next station, the first step is to lay down RTV sealant beads at the front and rear covers.

The pickup and oil filter-equipped oil pan is now dropped into place carefully and torqued down.

A protective cover on the side of the pan is removed and set aside to be sent back to the supplier and the oil cooler is bolted in its place.

After a quick look we drop in one side of the cam followers living in their cam carriers.

Now it's time to for the aluminum heads. Here's a bit of information that'll be useful in garage trivia: The aluminum heads on the LS9 are exactly identical, there is no left side or right side, they are the same part number and on the engine one gets a screw in temperature sensor and the other gets a screw in plug, that's the only difference.

After tapping in guide studs and laying on a crazy seven leaf gasket you just drop them on without fanfare.

Fourteen stainless steel bolts go into each head and get tightened down simultaneously with a multispindle driver.

Next up is the valley cover which seals up the 'V' between the cylinder banks.

The final process in this station is putting together the valve train. First, the pushrods are slid in place. (Aren't those old fashioned?)

A cam rail is placed across the inside of the head and the rockers are put in and the bolts are hand started. A pair of installation guides are then placed over both heads. These indicate the torquing order for the rockers, begin on the green number one and travel around the engine in order driving the bolts in, when you run out of greens, stop, manually turn the crankshaft 180 degrees to realign the pistons and then pick up the tightening order for the yellow numbers.

Once installed, a generous application of oil on the rockers finishes the station.

At this point, the line turns around on the other side and starts back the other direction. Here a set of guards are temporarily put in place to protect the lower intake as rivet-like cup plugs are popped into small holes in the water cavity.

Next, the valve covers are put in place and torqued down followed by the coil packs that aren't delivered preassembled for the sake of assembly access.

The water pump is installed along with a couple of hard vacuum lines and the alternator bracket.

The crankshaft damper is placed on the crank and a pneumatic ram presses it into place.

The next step is where happiness is made.

The supercharger comes largely preassembled and is lifted from a tote by crane and inspecting for the hoses, wiring, and gaskets.

It's gently placed into the valley of the engine. The entire unit is bolted into place and the supercharger gets a good spin to make sure it turns freely.

You can hear it pulling a lot of vacuum against the closed throttle.

Next we take a quick look at the intercooler, which incidentally is the only class-A surface underhood in any GM product, which means it must be protected from damage just like a fender or a hood. The intercooler gets a gasket, debris and paint finish inspection.

It's then dropped onto the supercharger, torqued down, then covered with a cardboard guard.

Next, Mike puts a lot of faith in me as he applies his name plate to this engine which I've had a dangerous amount of involvement in building.

Next up is an all-too-unsexy but entirely necessary step, a complete leakdown test of both the coolant and oil passages. All the ports and still open areas are sealed off with plugs and covers, then a controlled amount of air pressure is applied and the rate which the pressure drops is measured. The less leakage the better, this one passed with flying colors.

Once all the testing equipment is removed, spark plugs are put in place and the exhaust manifolds are installed as well as the manifold studs, the engine mounts are put in place and two temporary engine hoist loops are put on the front and back for transport.

In the final station, coil wires are installed and then the engine number decal is put onto the head.

The engine is then taken off the rolling engine stand and dropped into the flywheel and clutch installation fixture.

The holes in the flywheel are then numbered to make balancing engine easier later downstream,

The flywheel is installed and torqued down.

Next the clutch housing is picked up with an assembly reaction arm.

A mark on the twin clutch pack is then aligned with a mark on the housing to balance the assembled parts.

The clutch is then installed. And with that, the assembly process is complete.

The engine is weighed (530.6 lbs dry) and placed on a palette and an engine audit is completed another employee.

The palette is taken over to the final step in our process which would be running the engine through cold test.

In the cold test process, 2.5 quarts of oil is put into the engine after it's affixed to a test stand.

The stand is rolled into the test cell and hooked up to electric couplings, various sampling sensors, like noise vibration and harshness sensors, pressure sensors, etc. as well as a big electric motor. When the doors are closed, the motor spins up and turns the engine over after building up oil pressure. There are an array of tests done in the 90 second cycle time, but in the end all the lights were green and it passed everything very convincingly.

Since it was a shutdown week, we didn't proceed to the next step, but normally the engine would go to the balancing chamber. In the balancing chamber, the engine is hooked up to an isolation fixture which freely allows multi-axis vibration. It's then hooked up to a special ECM and a natual gas fuel source and fired. That's right, a Corvette ZR1 motors first breaths come from a clean fuel. During the engine's operation, sensors detect vibrations and calculate where to add weights on either the flywheel or the crankshaft damper to bring the engine into balance. The weights are installed and the process is repeated until it passes the test. The final step in the process is a second, intense inspection, from the assumed position of the customer at the Bowling Green Corvette factory. After it's gone over with a fine toothed comb it's off to storage awaiting the truck to Kentucky.

It wasn't really my engine, I did a lot of the installing and bolting down, but I was just a monkey following directions. It was, and is Mike's engine, and I have to pay him my respects on his level of care and attention to detail over even the smallest potential flaws in any part or element of the process. He's like a parent watching over a child. And yet, at the end of the day, it was gratifying to see the engine turning over, pumping air and passing all of its tests. Someday in the recent future, that motor will find its home in a ZR1 and it'll be rocketing some happy new buyer to delirious speeds. Kind of a surreal day if you ask me.