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.
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.
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.