1972 Chevrolet Corvette Scarlett Project Car - LS3 416 Installation

Down The Rabbit Hole: Inserting an LS3 416 into our '72 project car—very carefully

Jeremy D. Clough Feb 17, 2014 0 Comment(s)
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In our last installment, we stripped Scarlett, our '72 coupe project car, down to the firewall, inside and out. So began the LS conversion that will ultimately replace her conventional small-block with a 416-cubic-inch LS3 stroker, and change her Turbo-400 automatic out for a six-speed manual.

With the engine compartment vacant and detailed, it was time to get the motor in. We'll start here with prepping it for that job, as well as describing the modifications we (which in this case means Street Shop's Tray Walden and I) had to do in order to get it and its accoutrements bolted into place. The good news is that since the engine compartment of the shark was made large enough for a big-block, there's plenty of room for an LS-series mill; it's just a matter of arranging everything where it needs to go.

We began by dressing the engine, starting with the sensors. In addition to the throttle- position and mass-airflow sensors (available from most Chevy Performance Parts dealers), we had some specialty wiring that needed to go into place.

Since part of this project is trying to keep the visible part of the car nominally original in appearance, we've retained stock-style gauges, which were redone by Corvette Instrument Service in Florida. Unsurprisingly, some adjustments were required to fit the SBC sending units into the LS block. To keep the temperature-sending unit screwed deep enough into the head to give an accurate reading, we mounted it in an adapter sourced from CIS, who also sold us an electronic oil-pressure-sending unit. We screwed that into an elbow fitting that let us tuck the unit high and tight beside the block above the oil filter.

1972 Chevrolet Corvette Ls 2/30

We also needed to add knock sensors, and since the FAST XFI computer we'll be using for the fuel injection doesn't use the stock LS3 units, I made a trip to the local O'Reilly for the '80s-era GM sensors and control module that are required for the XFI. With these in hand, we opened up and re-threaded the knock sensor holes in the block, which are significantly smaller on the LS, then screwed in the sensors.

In order to make the LS block bolt up to the factory frame, we used a pair of Street Shop machined aluminum adapter plates. These let us mount a pair of standard motor mounts from Energy Suspension to the plates, which then bolt directly to the LS engine block. Because we built this motor knowing it wouldn't be going in a car that came with an LS, we already had Holley's LS conversion oil pan, and as it turned out, it cleared everything beautifully once we had the motor in place.

Although we had taken out the motor and transmission in two separate pieces, by removing the radiator and core support (which we were replacing anyway), we realized we could install the powertrain combo as a unit, and that's what we did. Since assembling the American Powertrain T56 Magnum six-speed transmission (and the modifications required to make it fit) is a pretty substantial piece of work, we'll detail that process in a separate installment. For now, suffice to say we did it before we shoehorned the engine back in.

For an accessory drive, we used a billet pulley system made by Street Shop, Inc., where we're doing the install. The Street Shop system uses a pair of belts: one to run all the critical parts such as the alternator and water pump, and a second one for the air-conditioning compressor, which it mounts low on the passenger side in the factory LS position. Since we'll also be installing a Vintage Air climate-control system (which we'll also cover in a separate article) to replace the non-functional factory air, we used that company's A/C compressor. While the pulley system we specified was polished and anodized, to match the polished compressor, other finishes are available.

Since the pulley system was designed around the aftermarket Street Shop chassis, we knew there would be some interference issues using it with a stock frame. The problems proved twofold: the water pump, whose already-modified outlets exited almost directly into the upper control arm of the passenger-side front suspension, and the compressor, which had a similar problem with the location of its inlet/outlet.

1972 Chevrolet Corvette Install 3/30

01 Before we put in the motor, we installed the flywheel (shown) and Quick Time bellhousing, and hung the American Powertrain T56 trans on the back. I’m sure it’s possible to do all this with the motor in the car, but it’s much easier on the stand.

1972 Chevrolet Corvette Install 4/30

02 The starter went on along with the other components on the rear of the engine. This’ll get complicated later, as there’s an electrical connection we’ll have to make that’s located on the block behind the starter.

1972 Chevrolet Corvette 5/30

03 Unfortunately, there’s not a hole in an LS block that’ll readily accept an old-style water-temp-sending unit, so we used an adapter from Corvette Instrument Service. While it’s possible to mount it remotely with a different type of fitting, doing so will result in less-accurate readings.

1972 Chevrolet Corvette Oil Pressure Sending 6/30

04 We also used an oil-pressure-sending unit from Corvette Instrument, since we did away with the manual gauge and the attendant risks of running a reedy plastic tube full of hot oil into the rear of the gauge.

1972 Chevrolet Corvette Mount Sending 7/30

05 To mount the sending unit, Tray Walden drilled and tapped a hole for a 90-degree fitting that let us tuck the unit up beside the block, just above the oil filter.

1972 Chevrolet Corvette Drill 8/30

06 While the block came ready for current-production knock sensors, the FAST XFI computer we’ll be using relies on earlier GM-style sensors that require a larger threaded hole. A little drill and tap work, and Walden had the holes ready.

1972 Chevrolet Corvette Knock 9/30

07 One of the two knock sensors mounted in place. Unlike LS sensors, which are wired directly to the computer, these earlier sensors are routed first to a control module, which then sends its signal to the computer.

1972 Chevrolet Corvette Ati 10/30

08 Unfortunately there was a bit of a miscue when the harmonic balancer was originally selected for the engine build. As a result, we had to special order an ATI balancer with a smaller inside diameter to get the proper interference fit.

1972 Chevrolet Corvette Street Shop Adapter 11/30

09 In order to make the LS block bolt up to the factory frame, we used a pair of Street Shop aluminum adapter plates. These let us mount a pair of standard motor mounts from Energy Suspension (which we modified lightly) to the plates, which then bolted directly to the block.

1972 Chevrolet Corvette Holley Ls Conversion Oil 12/30

10 Holley’s LS conversion oil pan provided excellent clearance, with plenty of room between it and all of the steering-system components.

1972 Chevrolet Corvette Street Shop Billet Pulley 13/30

11 For an accessory drive, we used a Street Shop billet pulley system. The setup uses a pair of belts—one to run the critical parts such as the alternator and water pump, and a second one for the A/C compressor.

1972 Chevrolet Corvette Pulley 14/30

12 The pulley installation began with bolting on the bracketry that will hold the pulleys. This is crucial, since it determines the spacing at which the different accessories will sit away from the block, and therefore whether or not the belt will run true.

1972 Chevrolet Corvette Power Steering 15/30

13 Once all the brackets are in place, the front of the engine may be dressed with accessories such as the alternator and power-steering pump (shown). The pulleys and tensioners (two, since there are two belts) may likewise be bolted into place.

1972 Chevrolet Corvette Install 16/30

14 All together, and ready for the belts to go on.

After pulling the motor back out, we removed the accessory drive and the water pump with it. Since the outlet fittings were screwed into the pump, I backed them out and plugged the holes, being careful to seal the threads, then drilled and tapped a new pair of holes that would locate the outlets higher up, and at an angle. While the original fittings were different sizes (one large, one small) we used a single size for replacement. A little thread sealant, and we were in business.

The compressor, however, got a little more involved. Instead of moving the outlets themselves, Walden used a longer straight fitting in the rear, effectively extending it upwards, and for the front welded up a forward-facing 90-degree fitting that would clear the suspension parts. Making clearance for the body of the compressor required even more surgery, as we learned that one of the reasons we initially had trouble getting the motor bolted in was that the compressor was resting on the frame's front crossmember.

This led to the touchy business of cutting the frame, bending the flaps back enough to allow clearance for the compressor and belt, and then re-welding it back in place, making sure the frame was properly gusseted for strength. In the process, we also had to remove some material from the lower control arm, as well as from the bracket that mounts the LCA to the frame crossmember. Once again, instead of just grinding away material, it was cut, bent, and then re-welded.

Once this was done and painted black, we started the tedious crane-and-jack work of easing the powertrain combo into a spot where we could slip the bolts through the motor mounts and tighten it all up.

Other than wiring and plumbing it, the primary physical connection that still needed to be made was between the motor and gas pedal. There was no way the factory throttle cable was going to reach all the way to the FAST throttle body at the front of the motor, so we got a Lokar LS1 cable and bracket from CBI, the local speed shop. After threading the butt end of the cable through a washer and the bushing located at the top of the gas-pedal arm, the cable was threaded through its AN-style braided stainless sheath.

The rear part of the sheath mounted to the firewall from the engine-compartment side, while the oversized forward end had to be cut to length before being fitted into its hose end. Cutting the tiny braided hose (it looked to be -2 AN, roughly 1⁄8-inch inside diameter) was as much fun as you'd expect, with its sharp little pieces of wire readily drawing blood, but the real excitement was in adjusting the throttle-cable bracket.

Unfortunately I was never able to find a bracket that would bolt up readily to our FAST LSXR intake. As a result, we had to resort to the LS1 bracket, which didn't stick out far enough to line up the cable properly with the throttle body. We have machine tools, however, and aren't afraid to use them: I used the lathe to machine a spacer that would step the mounting point for the cable out the required ¼-inch to make it line up. Walden, meanwhile, made the two nuts required to mount the bracket assembly to the intake, as standard nuts wouldn't fit in the recesses provided for that purpose.

All in all, we machined six parts to make the throttle cable work, five of them from scratch, and the whole shebang took the better part of a couple of work days. The part we didn't have to make ex nihilo was the adjustable brass stud that holds the end of the cable and slips into a hole in the throttle, giving the cable something to pull against. Unfortunately it was well oversized for its hole, so to the lathe it went.

Once we had everything made, modified, and assembled, I picked up the Corbeau driver's seat that was sitting next to the car and set it back in the passenger's compartment, then sat down. Pushing my right foot down hard on the gas pedal, I watched the cable do its work and rotate the throttle all the way back.

Considering the fierce power of that 635hp LS3, it occurred to me that I'd never again be that cavalier about bringing it to wide open throttle.

1972 Chevrolet Corvette Engine 17/30

15 The LS3 with the trans already mounted, ready to be installed. While you can do this with a cherry-picker engine hoist, the crane made it much easier.

1972 Chevrolet Corvette Install 18/30

16 Installing the powertrain required a careful balance of location and angle, the trick being to slip the trans down its tunnel underneath the car without punching the front of the motor through the fiberglass front clip. We used a crane and a pair of floor jacks, and as many hands as were available.

1972 Chevrolet Corvette A C 19/30

17 In addition to the body of the A/C compressor itself, the red-capped fittings where the hoses connect to it caused major clearance problems. The water pump presented a similar issue.

1972 Chevrolet Corvette Cut 20/30

18 Since the compressor originally rested on the front crossmember, we modified that part of the frame by cutting it, bending back the flaps for clearance, and re-welding it in place. A little grinding, a little black paint, and we were back in business.

1972 Chevrolet Corvette Replace Factory 21/30

19 To make room to attach the hoses to the compressor, Walden replaced the factory fittings with an extended straight fitting at the rear, then welded up a 90-degree elbow for the front fitting.

1972 Chevrolet Corvette Water Pump 22/30

20 To make the water-pump fittings clear the suspension mounting points, I unscrewed them from the pump and plugged those holes. Next, I drilled and tapped two more holes, relocating the fittings above the originals, at an angle.

1972 Chevrolet Corvette Modified 23/30

21 The modified fittings for the Vintage Air A/C compressor, capped in blue and red. While it’s still close, the modified fittings make it possible to route the hoses around the suspension mounting points.

1972 Chevrolet Corvette Compressor 24/30

22 By the time we were done with the frame mods, there was sufficient clearance between the compressor and the frame/control arm/control-arm mount. Running clearance for the compressor belt was adequate as well.

1972 Chevrolet Corvette Install Throttle 25/30

23 Installing the throttle cable starts from the rear. After threading its butt end through a washer and the bushing located at the top of the gas-pedal arm, the cable was slipped into its AN-style braided-stainless sheath, and the pedal was bolted back to the firewall.

1972 Chevrolet Corvette Rear 26/30

24 The rear part of the cable mounted to the firewall in the factory position. We routed it in a gentle S-curve behind the rear of the intake and up beside the passenger-side fuel rail.

1972 Chevrolet Corvette Ls1 Throttle 27/30

25 We started with an LS1 throttle cable from Lokar, machining a spacer to get it to sit at the proper offset for our FAST intake and throttle body. We also had to make the nuts to bolt it to the intake.

1972 Chevrolet Corvette Braided Steel 28/30

26 Once the bracket was bolted onto the intake, we were able to measure the length of the braided steel hose that the cable passes through, and cut it to length. Word to the wise: Tape the hose, and keep it taped while you twist it into its fitting.

1972 Chevrolet Corvette Throttle Cable 29/30

27 This little fella connects the throttle cable to the throttle body itself, and locks onto the end of the cable. Unfortunately, it was too large to fit the throttle body, so I turned it down in the lathe, narrowing the hex head to make it sit deeper and reducing the diameter of the body.

1972 Chevrolet Corvette Installation 30/30

28 Success at last. With all the extra parts made and fitted, now something actually happens when you push down on the gas pedal…and it’ll happen a lot more dramatically once we get the motor wired and plumbed.

Sources

Energy Suspension
San Clemente, 92673
888-913-6374
http://www.energysuspension.com
Vintage Air
San Antonio, TX 78266
800-862-6658
www.vintageair.com
Holley EFI
Bowling Green, KY 42101
270-782-2900
http://www.holleyefi.com
Fuel Air Spark Technology
Memphis, TN 38118
877-344-8355
http://www.fuelairspark.com
American Powertrain
(931) 646-4836
www.americanpowertrain.com
Street Shop, Inc.
Athens, AL 35611
256-233-5809
www.streetshopinc.com
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