When the 1997 Corvette was new it was fielding a very potent powerplant: the Gen III all-aluminum LS1. At 345 hp it made the Corvette pretty fast. But as they say “that was then and this is now” and nowadays 345 hp is barely more than some V-6s put out. Our ’99 FRC project had pretty much the same LS1 in it that came in that first year of the C5. And even though it had just over 80k on the odometer, it only felt moderately fast. Most likely this wasn’t because it was slower than it was in 1999, but more likely we had become speed-numb from driving faster Corvettes. What our lightweight, 3,046-pound Corvette needed was a boost in power output. The big question was how to go about it.
At first we considered pulling the LS1 and giving it a makeover, including bearings and rings. Some ported heads and a cam would’ve yielded a nice gain, but we wanted more. So we considered stroking the LS1, but all that cash would have only gotten us to a displacement of around 383 cubes, just a bit bigger than a stock LS3. At that point we did what any self-respecting American hot rodder would do; we decided to go big. You see, during the long run of the LS series of engines, the GM engineers were particularly bright. All the variants, in terms of long-blocks, are dimensionally identical. The bosses for pulley systems, transmission mounts, and engine mounts are all in the same spots. This means a pulley system from a C6 LS3 will bolt onto a C5 LS1, or LS6, and vice versa. See where we’re going here? Yep, dropping an LS3 (or even a wet-sump LS7, LS9 or LSA) into your C5 is cake in terms of mechanics.
As for electronics, GM did make some notable changes. Sensors moved around, some plugs changed shape, and most notably during the run of the LS2, GM switched from a 24x reluctor to a 58x version. The reluctor wheel is on the crank, so it’s the most difficult item to change. If you decide to swap to a 58x based LS engine, and don’t feel like yanking the crank, then a few companies offer electronic widgets to convert the signal from what the 58x reluctor is putting out into something the 24x-based ECU can comprehend. We built our engine from scratch, so it was easy to make sure it had the proper 24x reluctor, but we’ve used the electronic gizmo before and it works well. The nice part is that even though our engine is a forged LS3 stroker, the same tricks we used will work on a stock crate LS3 from Chevrolet Performance, or even something sourced from a salvage yard.
The key to working around all the electronic issues was the various adapters and extension harnesses available from Racetronix. The products they offer don’t cost a lot of cash and make evolving to a newer LS engine a plug-and-play deal. And, since anyone who has ever done a clutch job on a Vette knows how much it sucks, we went ahead and replaced the clutch with a Stage 2+ unit from SPEC. The swap itself took two days and was pretty much drama free. Follow along as we cruise our C5 FRC over to Don Lee Auto for an engine switch up.
01. Not much to see here, just a 1999 LS1 with 83,000 miles on the odometer. With an advertised 345 horsepower at the crank it was pretty fast when it first came out, but has since been eclipsed by more modern variants of the LS1. One option would be to pull it out and do a complete workover, but we had another plan.
02. And that plan revolved around the 415 LS3 stroker we recently built over at CBM. With a stock 4.065-inch LS3 bore (Chevrolet Performance block PN 12621769) and a forged 4.000-inch-stroke Eagle crank, it churned out 576 hp and 566 lb-ft of torque thanks to a moderate Comp cam and some free-flowing Trick Flow LS3 heads. To make our lives easy, we had the foresight to build it with the 24x reluctor and 1x cam sprocket that the 1999 GM PCM was designed to work with.
03. Without these adapters from Racetronix the swap from LS1 to LS3 would still be possible but it would be a lot more difficult. We weren’t positive what we would need so we had Racetronix send over most every adapter they make for this application. They also sent us two widgets (PN SSE-001) to eliminate the annoying First to Fourth lockout (Skipshift) deal without having to cut or splice any wires.
04. Of course, GM made a few changes to the LS platform between the LS1 and the LS3, but thanks to companies like Racetronix they’re easily worked around. To get the new mill in we needed to eject the old one. C5 engines come out through the bottom, so we started by unplugging the harness, evacuating the A/C system, draining fluids and removing anything that attached the LS1 to the rest of the Corvette.
05. Once everything was disconnected up top we started unbolting the suspension cradle from the car, including the shocks and upper control arms. We also unbolted the clutch bellhousing from the torque tube and removed the exhaust from the headers back.
06. Some people lower the engine on the cradle as one assembly, but we did it a bit different. This isn’t the first time Don Lee Auto has done this so they have an engine stand to support the engine without the cradle. This allowed us to drop cradle without worrying about the weight of the engine.
07. With the cradle out of the way we used the hoist to slowly lower the engine, checking along the way that we didn’t forget to detach any wires or hoses. Keep in mind we already had long-tubes installed, so we didn’t need to mess with the O2 sensors. But, if you’re doing long-tubes at the same time as your swap, Racetronix has a full line of O2 extension harnesses and plug adapters.
08. And just like that the old LS1 was out of our C5. Now it was time to address some of the sensor changes between the two LS engines.
09. One of the biggest changes was that the LS1’s cam sensor is located under the intake manifold just behind the firewall end of the valley cover. On the LS3, both this sensor and the MAP sensor (on the back of the intake just above the brake booster vacuum port) are on the front of the engine, which is where the Racetronix extension harnesses come into play.
10. The LS1’s oil pressure sending unit was removed from the LS1 and installed on our new stroker LS3.
11. The Racetronix cam sensor extension/adapter harness (PN LS2-CSAH) was run to the front of the LS3 where the cam sensor is mounted in the timing cover. We used the direct mount version, but they make one that works with the short extension harness found on many LS2 and LS3 engines.
12. We broke our water temp sender pulling out the engine, but CBM (where we built the 415 stroker) was able to send us over a replacement. Since it’s in the same place on all LS engines no extension/adapter harness was needed.
13. We then pulled the intake from the LS1 to access its knock sensors, which were located in two recessed areas of the valley cover. On new LS3s they run a two-wire knock sensor (which isn’t easily compatible with the 1999’s ECU) on the side of the block.
14. We’re not sure if it was planning on GM’s part or just luck, but the LS1 knock sensors screwed right into two bosses on the side of the LS3 block, and they aren’t even the spots where LS3 knock sensors would mount. A pair of Racetronix knock sensor extension harnesses (PN LS2-KSRH) let us tie them into the factory harness.
15. We built our new engine with easier-to-find LS3-style fuel injectors. The problem is that the mini-min type plug on the LS1 harness won’t plug into them. There are three ways around this. The first is to run taller LS1 injectors and mod the fuel rail. The second is to get the right connectors and swap out the plugs on the LS1 harness to the LS3 style. We chose the easier, third option: getting a set of FAST plug adapters (PN 179604-8) that let us plug the LS3 injectors into the LS1 harness.
16. Doing a clutch swap on a Corvette is a time sucking pain. So, since we had the engine out, we decided to replace the stock clutch with something much nicer. The SPEC Stage 2+ clutch (PN SC093H-11) we chose was paired with this aluminum pressure plate (PN SC75A).
17. The multi-friction disc featured carbon semi-metallic friction material on one side and Kevlar on the other. This 2+ bridges the gap between SPEC’s Stage 2 and Stage 3 kits and offers the driveability and engagement quality of the Stage 2, but with 15- to 20-percent more torque capacity.
18. The made-in-the-USA hub is double sprung with spring cover reliefs for flexibility and heat-treated components for strength and long life. In short, with a torque rating of 782 lb-ft this should hold up to our 415 no problem, and the pedal requires about the same effort as stock.
19. Other wear-and-tear items we decided to replace while we had the engine out included the water pump, starter and clutch slave cylinder. We then transferred over most of the pulley drive system from the LS1, along with the engine mounts. All LS engines will accept drive systems from any other LS engine. Makes us want to hug a GM engineer.
20. Once the new 415 LS3 was looking like the LS1 we slid it under the Vette and prepared to slowly lower the C5 over it. The process was: lower the car an inch, adjust the engine stand, and repeat.
21. What we found was that the silencer cover on the LS3 intake hit the plastic cowl just above the firewall. Since the cover isn’t really needed, we removed it from the equation and finished getting the engine in place and lined up with the torque tube.
22. The C5’s factory fuel line plugged right into the LS3’s slightly different fuel rail. We then went about installing all of the LS1’s coil packs and plugging in the rest of the harness.
23. The LS1 MAP sensor was moved to the front of the LS3 intake and held in place with a smear of silicone and small bolt with a washer. Connecting the MAP to the harness was left to the Racetronix extension harness (PN LS2-MSEH). The 90mm LS3 electronic throttle body (make sure you have the earlier LS2 style unit that typically has a silver blade) was tapped into the LS1 harness with yet another Racetronix adapter harness (PN LS2-TBAH).
24. The old battery was, well, old, so we upgraded to an Optima YellowTop battery. The YellowTops are great if you’re running a larger stereo or, like us, don’t drive the car every day.
25. The stock air intake system (yeah, we need to replace that) did work with the LS3 and its 90mm throttle body, but we had to add a couple inches of 3-inch silicone hose down near the MAF sensor. Other than that we just went around and made sure everything was plugged in and connected. As you can see, it doesn’t look much different than stock, but it’s actually 66 cubic-inches larger. For now we used the stock MAF, but Racetronix has an adapter (PN MAF-003) to run the newer blade style.
26. The Corvette fired right up, but we were pretty sure the fuel tables were all wonky, so we cruised it over to Westech Performance for some chassis dyno tuning by Eric Rhee. After six pulls, Eric knocked down a best pull of 453 horsepower and 470 lb-ft of torque. From 3,400 to 5,500 the car puts over 400 lb-ft to the tires, which is just a good time waiting to happen and more than enough to help our C5 keep up with the newer C6s and C7s at the track.
27. To complete our stealthy swap we modified the stock LS1 fuel rail covers to fit the LS3. The passenger side was easy, but the fuel line on the driver side is a bit farther forward on the LS3, which made the clip not line up. A little cutting and a bit of epoxy made it work, and we feel that most people wouldn’t even know what was lurking under the hood of our C5.