1991 Corvette C4 LS Swap

LS Swap
Rafael’s LS1 install is a thing of beauty.

LS swaps are all the rage these days. There are groups on Facebook where the saying is “If you’ve got it, swap it!” Once upon a time, the dream of hot rodders, especially those with GM vehicles, was to stuff a ‘Vette engine in their Camaro, Firebird, Nova, Chevelle, El Camino, etc. These days, the LSx engine family is the engine everyone is dreaming of having. More power and better fuel economy are the usual reasons given. My friend Rafael is one of these, and he owns a C4 ‘Vette that already had a pretty nice engine in it. Now it’s got a really nice engine and gets pretty good mileage to boot. This article will take you through the process of swapping out your engine like we did here.

The Parties to the Swap that Took Place

LS Swap
The original L98 engine is removed.

Rafael’s ’91 C4 came from the factory with a 350 cid, 245 horsepower L98 V8engine with a manual transmission. This was tuned port injection (TPI) engine that developed 345 pounds-feet of torque. He yanked these in favor of a 5.3-liter LS that was bored out to a displacement of 5.7 liters.

LS Swap
The LS1 is installed and being wired. Rafael just has a few finishing up things to do before starting it up for the first time.

He crowned the forged 347 iron block with 241 heads. These have a cathedral port and typically have a 68 cc combustion chamber.  Operating the valves in his new 5.7 LS is an MS3 (Magic Stick 3) cam from Texas Speed & Performance. This cam has intake specs of .603 inch lift and 238 degrees of duration and .609 inch lift and 242 degrees of duration on the exhaust side. The MS3 cam, it should be noted, requires at least double valve springs instead of stock-style single springs. He’s running a stock LS6 intake up top.

Translating the Torque from the LS to the Ground

LS Swap
The 4L60E that was installed in Rafael’s 91 C4 Corvette with the new LS1 engine.

Rafael decided to stay pretty vanilla when it came to the transmission behind the LS he was installing. However, he did go with GM’s venerable 4L60e transmission over the Vette’s standard 700R4. He also had it built up and had a shifty kit installed. The torque converter is a 3000 stall speed Greg Slack model.

LS Swap
When converting from a 700R4 transmission to a 4L60E, this tailshaft adapter is required if you want to keep the torque arm the 700R4 uses.

Out back is a low mileage Dana 36 differential with 3.07 gears. In order to keep his original driveshaft, he picked up a 4L60e to 700R4 tailshaft adapter from Advanced Adapters. However, this adapter placed the reluctor ring for the VSS in the wrong spot. He had to take it to a transmission shop to have it lined up correctly with a VSS out of a ’96 Corvette, which lined up perfectly. The shop also had to do a little machining to the inside of the housing to keep things from rubbing.

LS Swap
Rafael discovered that the shift arms on the 700R4 and the 4L60E aren’t identical. This caused him to have problems getting the transmission into gear properly. Luckily shiftworks makes this new shift adapter to correct the problem.

Installing the Motor Required Motor Mount Modifications

LS Swap
These are the motor mount adapters required to mount the LS1 into the 91 C4 Corvette. Also pictured is the Racetronix fuel pump Rafael used.

It’s a rare LS swap that doesn’t require some sort of work to the motor mounts to get the engine and transmission to mount up properly. Rafe’s ’91 C4 was no exception. The brand and model of LS swap mount kit that you can use will depend on what car the LS is going into. Rafael went with an LS1 swap motor mount kit from Speed Hound Racing and Performance of Georgia. These mounts properly place the motor so only minor mods are required to get the 4L60e transmission mounted in such a way as to enable the use of the stock driveshaft.

LS Swap
Although the motor mount adapters helped ease the installation of the LS1, the C-beam was still off by half an inch as this image shows.

As it was, he had to modify the C-beam because it was off by half an inch. He also discovered, the hard way I might add, that the C-beam and driveshaft have to be installed together. He discovered this when he pulled the C-beam to clean and modify it and discovered it wouldn’t go back together. He took it back out and put the driveshaft in. Again, no joy. He finally put C-beam in, moving it back enough to give him room to install the driveshaft and was then able to bolt the C-beam into place.

Rafael was delayed in installing the LS engine because the Powerbond SLP 100225 balancer he bought needed to be pinned and for some reason it didn’t have one when he bought the new balancer. While waiting for this, he power washed the engine bay and then applied a coat of POR-15 heat resistant engine enamel paint. He also coated his C5 exhaust manifolds with this paint to protect them and give them a better color. He says this stuff worked great and goes a long way.

Getting Wired

LS Swap
Rafael’s wife let him take over the table in their exercise room while he worked on converting the wiring harness for the LS1. This harness came out of ’99 Z28.

The stock wiring harness was too much of a hassle to modify for use with the LS and 4L60e, so he picked up a harness (and PCM) from a 1999 Camaro Z28 to make things easier. I say easier, but that’s relative. That’s because even with a wiring harness and PCM that were designed for cars with LS engines, the harness was designed to mount the PCM on the passenger side. IF you’ve ever seen the engine compartment on a Vette, you know how laughable the idea of doing that is due to space constrictions.

LS Swap
Rafael found a number of “duplicate” wires when he opened up the bulkhead pass-thru. Turns out these wires were simply looped through the bulkhead for some reason.

So, Rafael repurposed his exercise/workout room to convert the harness for use in his ’91 C4. He had to take it all apart, labeling every sensor as to which pin it went to on the PCM. This occasioned a trip to the store to buy a bunch of wire in the proper gauges and colors because every wire that goes to a sensor, a switch, and or pick-up on the passenger side had to be lengthened.

LS Swap
Once he had the harness figured out, Rafael laid it into the car to check fitment. The wires in the yellow circle needed to be lengthened a bit.

He had to match wire size (gauge) because not doing so would change the voltage/current values which could cause problems with how the PCM interprets signals. He also deleted anything that he didn’t need to use. Rafael recommends EFI Connection for needed wires because they sell factory colors and gauges in five foot lengths.

LS Swap
The LS1 engine uses an IAT while the L98 doesn’t. Rafael modified the MAF sensor location on the cold air intake with a drill at low power and a rat tail file to make the new IAT fit.

Rafael decided that instead of overtaxing a single circuit for power, he would power certain circuits through a new fuse panel he bought. He also decided that the ignition and coil circuits would be hooked up to the PCM’s injector 1 and 2 outputs instead of where they would normally be connected. One word of warning that he had for others that might want to try this is to not delete circuit number B8. “It feeds the VSS signal to the PCM. You definitely need this.”

LS Swap
The Vette’s PCM tray needed to be cut down to properly fit the ’99 Z28 PCM.

There’s a wiring bulkhead near where the steering column passes through the firewall/bulkhead that Rafael had to modify because the LS1 engine sits quite a bit farther back in the engine bay. After deciding which wires/circuits he needed to keep and which he could delete, he had to modify the bulkhead so the wires exited out the top instead of the center/back.

LS Swap
The PCM has just enough room for the hood to clear it when closed.

It should be pointed out that Rafael laid the Z28 harness out and mocked it in before doing any of this. He also picked up the PCM end of the harness out of a late model truck so he could keep the stock colors and gauges. Rafael highly recommends ringing out all your wires for continuity before installing the harness in the car. He also said to be very careful with your wire colors as quite a few of them are very close and easily confused.

Corvette Accessories Were Used Due to Fitment Problems

LS Swap
Rafael had to wait a few days for the balancer pin kit to arrive before he could finish up the installation of the accessories and serpentine belt.

Rafael originally tried to use the GM F-body accessories that came with the engine he bought. However, again due to the smaller engine bay, this wasn’t possible, He had to use a Sanden 508 type air conditioning compressor with a special high mount bracket, to overcome the clearance issues he had.

LS Swap
Rafael says that the hardest part of installing the accessories was getting the hi-side line to thread into the pump. It took him several hours of carefully bending the hard line with a wrench to get the bends just right.

He also used a Corvette alternator and power steering pump, pulley, and bracket. However, space restrictions (again) meant that he spent all day getting the power steering installed and getting the pressure and return lines hooked up. Rafael told me that it took him two hours alone getting the pressure side line to bolt up to the pump because he had to carefully tweak it to get it to fit correctly and not kink the hard tubing. He got the power steering reservoir from TPI parts.

Getting Exhausted

LS Swap
Other than the headers he installed as a last minute idea and the special catalytic converters, the exhaust on Rafael’s ’91 C4 is stock.

Instead of stock exhaust manifolds, Rafael went with the biggest set of Melrose headers that would fit in the excruciatingly tight Corvette engine bay. It should be noted that because he went with headers, he had to use the oil pan out of a late model F-body. The Vette oil pan worked, but didn’t allow clearance for the headers.

LS Swap
Due to his desire to use NOS at a later date, Rafael had to install a set of special catalytic converters.

Because he plans on eventually running a 100-150 shot of NOS on race day, he had to get special Random Technologies CDS 3500 catalytic converters. Unfortunately it seems that shortly after he bought them, Random Tech stopped making automotive exhaust parts. The rest of the exhaust system is pretty stock.

About Mike Aguilar 202 Articles
Mike's love of cars began in the early 1970's when his father started taking him to his Chevron service station. He's done pretty much everything in the automotive aftermarket from gas station island attendant, parts counter, mechanic, and new and used sales. Mike also has experience in the amateur ranks of many of racing's sanctioning bodies.
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