The L98 350 engine that Chevrolet factory installed in the 1991 C4 Corvette was a pretty potent package. Producing a respectable 245 horses and 345 pound-feet of torque, it was fun to drive, especially when coupled with the 700R4 transmission and the lightweight platform of the C4.
The worst thing one could say about this combination was that it didn’t get very good gas mileage. The problem with poor fuel economy numbers is one reason GM and Chevrolet developed the LS-series of engines and as many owners have discovered, they’re also pretty fun. Rafael DeMoya recently found this out when he installed an LS engine in his 1991 C4 Corvette that he calls “Number 5.”
Taking A Look At the LS1 Powerplant
Rafael isn’t one to leave an engine stock before he installs it in his car. He started out with a vanilla LS1 out of a 1999 Camaro Z28.
Controlling the flow of the air-fuel mixture and exhaust gasses is a Magic Stick 3 camshaft from Texas Speed & Performance. This cam is pretty gnarly, with a lift specification of .600 inches for both intake and exhaust, duration of 238 and 242 degrees, intake and exhaust, and a lobe separation angle of 113 degrees. Keeping the valves from floating is a set of high-quality double valve springs.
Topping the block is a set of 241 heads. These babies have cathedral-type heads for cleaner and more complete combustion inside their 66cc combustion chambers. Feeding the air and fuel into those chambers is a stock LS6 intake. Rafael used an SLP Claw cold air intake that simply looks wicked to help boost power output.
Transmitting Torque To The Rear Wheels
The ’91 C4s originally came with a 700R4 automatic transmission. However, Rafael considers it too tame for a serious ‘Vette and the 4L60E was easier to wire up for the LS engine than the 700R4 would have been, so he swapped that out for a 4L60E he had built up to handle the extra torque. Converter specialist Greg Slack provided a 3500 stall speed converter, PN CDS 3500.
Getting the torque to the wheels is a venerable Dana 36 differential. Rafael picked up a low mileage used unit with 3.07 gears installed, a near ideal gear for lightning launches. He then stuck an aftermarket transmission cooler between the radiator and condenser and fitted both ends of the cooling lines with AN fittings.
Prepping For The Engine Swap
The first step in the swap was to carefully remove the original L98 engine. This involved draining and properly disposing of all the fluids. Next, he carefully labeled both ends of all wires, vacuum lines, and hoses prior to removing them. Then, he removed the radiator and the serpentine belt and accessories.
Next up, disconnecting the high and low side power steering lines at the pump and rack. A chain was bolted to the front of one head and the rear of the other, then looped over the hook on a cherry-picker and the slack in the chain was taken up. The driveshaft was then disconnected from the differential and the transmission crossmember C-frame removed. Finally, the motor mount bolts were removed and the engine carefully lifted out of the car.
Rafael then thoroughly pressure washed the engine bay with Simple Green. Although it was now almost clean enough to eat out of, he didn’t like how it looked, so he picked up a small can of POR-15 black heat resistant engine enamel. He started out using a foam paint brush for the task but switched to an old toothbrush-style cleaning brush and had excellent results. Rafael said the paint covered quite a bit better than he expected it to.
For areas that were too tight to reach with brushes, he soaked a rag and used a small screwdriver. He then painted the valve covers, coil brackets, and the fuel rail a bright red.
Installing The Engine-Multiple Times
In order to install the LS1 engine and 4L60E in the ‘Vette, an LS1-swap motor mount adapter kit from Speed Hound Racing and Performance of Georgia was needed. He also picked up a 4L60E to 700R4 six bolt to four bolt tail shaft adapter from Advanced Adapters so he could continue using the ‘Vette’s torque arm with minor adjustments.
Rafael took the part to a local transmission shop and had the VSS reluctor ring out of a ’96 ‘Vette specially customized to fit because the tailshaft adapter he used put it in the wrong place. The transmission specialists had to do a little careful grinding on the side of the housing to keep things from rubbing against each other.
The LS was actually installed in the ‘Vette few times. The first time was to test the fitment before Rafael cleaned everything up and painted the engine bay. The second time was to test the fit of the wiring harness and see where he needed to extend the wires. During this test, he noticed that the driver side rear coil bracket was hitting the bulkhead/firewall, so he shaved a bit of it off with a cutoff tool/die grinder.
If his 100225 Powerbond balancer Street Legal Parts had come with the pin kit that it was supposed to have been shipped with, that very well might have been the last installation, but the pin kit was missing. Once that came in, he was able to pin (see image above) and install the balancer. He says you can use the stock balancer, but he chose the Powerbond underdrive balancer because his plans for the car include racing it and the underdrive decreases the amount of parasitic drag caused by the accessories.
Preparation Of The LS1 Wiring Harness
Rafael decided that reworking the Corvette’s wiring harness for the LS1 engine and ’99 Camaro Z28 PCM would be too much work, so he also picked up the Camaro’s harness with the PCM, then took the harness apart sensor by sensor and labeled everything on both ends, including the pins on the PCM side.
Corvette accessories fit best and using an aftermarket adapter will get your A/C back in place. – Rafael DeMoya
Once the engine was remounted in the car, Rafael laid the harness out and mocked it up so he could see which wires would need to be extended, and by how much. This is where he started tracing out the common 12-volt supplies. The ignition and coil are on the same circuit — INJ1 and INJ2 — so he decided to wire these to a secondary fuse box he installed behind the battery and supplied with a fusible link.
Offering advice to others tackling this project, Rafael said, “Leave your TPI harness intact and just remove it as a whole unit and sell it.” He also said that EFI Connection is an excellent one-stop shop for all your wire needs, while Mouser Electronics had every 18- and 20-gauge connector he needed to complete the project. With the engine installed the third and final time, Rafael realized that the wiring bulkhead/pass-thru needed to be modified by routing the wires out the top instead of the front because of how far back the engine sits.
Swapping F-Body For Corvette Accessories
The engine Rafael bought came with the requisite accessories-alternator, power steering pump, and A/C compressor. However, he soon discovered that the C4’s engine bay wouldn’t accommodate them, so he picked up a new Corvette power steering pump and bracket, Sanden-type A/C compressor from Vintage Air with high mount bracket, and an alternator.
Installing these items with the engine in the car took most of an afternoon, with more than two hours invested in tweaking the power steering high-side line to get it to fit on the pump side correctly. Since Rafael wasn’t able to reach a tube bender into the space he had, he used the box end of a 17 mm wrench to carefully get it to the right shape and position to thread cleanly. The following bolts were used in mounting the power steering bracket:
- Three 10×1.5×70 mm flange head bolts
- One 10×1.5×60 mm bolt in the bottom hole of the bracket
- The alternator required two 10×1.5×100 mm bolts. All brackets received both shoulder and lock washers as well.
Getting Properly Exhausted
Rafael had originally thought that he was going to use a set of stock C5 manifolds on this LS swap. He painted them with POR-15 manifold gray and they looked great. However, they were too restrictive, so out they came and in went the biggest set of Melrose Corvette headers that fit in the tight Corvette engine bay. Both of these were mounted using Grade 8 8mm x 1.25×30 bolts with 10 mm heads he picked up at Ace Hardware.
The plan for the ‘Vette in the near future is to run a 100-150 shot of nitrous for the strip. This wouldn’t play well with regular cats, so he had to go out and buy a set of special Random Tech catalytic converters. The rest of the exhaust is a stock L98 exhaust system (editor’s note: Random Tech has since stopped selling specialty automotive parts).
Getting Fueled Up
The original fuel system on the Corvette was a return-less regulator on the pump, something LS engines don’t support. So, a Racetronix fuel pump with Teflon fuel lines was installed in the tank to supply the fuel needed for his 390 horsepower demands.
Rafael ran custom-made fuel lines (supply and return) to a Wix 33737 C5 filter and used AN fittings at both ends. He then ran a custom-made line from the filter up to the fuel rail, also connected with AN fittings. A Nook and Tranny was used to adapt the 3/8-inch quick connect (PN 644123) on the rail to the AN fitting on the fuel line. At the filter, he used Aeroquip 90s and unions.
Slowing Down The Extra Power
The ‘Vette now has a set of big Baer six-piston calipers all around. These press the pads against cross-drilled and slotted Kinetic rotors. Not being one to work in half-measures, Rafael ran all-new Goodridge SS hard lines to all four corners.
Lessons Learned And Shared Recommendations
According to Rafael, at the beginning of this project, the hardest part was the lack of information, however, with information currently available online, he believes anyone with a modicum of mechanical knowledge can pull this type of project off. If you’re not comfortable doing wiring stuff, he highly recommends buying a pre-made harness. “Corvette accessories fit best and using an aftermarket adapter will get your A/C back in place, he said. “Later C4s like my ’91 have a CCM; it’s best to leave the C4 ECM in place because the CCM does a handshake with it at start up. Without it, many functions won’t work correctly.”