We get an understandable amount of flack from readers about doing so much editorial on the GM LS-series engines, but there's a reason behind our madness: They're the most popular performance engines today and make completely stupid power when you throw boost at them via a supercharger or turbocharger. Internet forums are all atwitter about just how far up the power scale you can go before the LS finally yells ôuncle,ö but we have yet to see any real proof of its limits. So we decided to find out for ourselves. How much boost can we throw at one, and how much power will it make (and for how long?) before it goes pop in one way or another?
Our guinea pig was a 5.3L truck motor, while the punishers were a pair of 76mm turbos from CXRacing plumbed into a homemade twin-turbo system. When we got it, the engine was a grimy core from a junkyard. We took it apart and found a pretty significant rust spot in the No. 8 bore that a ball hone wouldn't remove, but it was halfway down the bore so we thought, What the hell, we're trying to kill this thing anyway. It won't matter. The motor was reassembled using the original crank, rods, flat-top pistons, rings (a little rusty but they cleaned up OK), and bearings. The sole modification to the short-block (at the advice of Jerry Stetler from Ace Machine) was to increase the gap of the top rings from 0.015 inch to 0.028-0.030 inch (we reused the original rings). The second (factory gapped at 0.028 inch) and oil rings were left untouched.
Noticeably absent from this combination are an RHS block, billet crank, and ARP main studs.
Since the exercise was designed as a strength test for the stock short-block, we opted to make a few changes topside. The stock 5.3L heads were ported by Total Engine Airflow and received larger intake valves and an appropriate spring upgrade. Fel-Pro MLS head gaskets and ARP studs hold the heads on, and the cam is an off-the-shelf 228/230 grind from Comp. The cam was run with a new set of Comp hydraulic roller lifters, as the originals were clogged and unusable, but we retained the stock pushrods and rockers. The final upgrade was a swap of the stock truck intake for a FAST LSXR intake, 75 lb/hr injectors, and matching 102mm throttle-body. In normally aspirated trim, the 5.3 produced 451 hp and 380 lb-ft of torque. It seemed odd to us that the motor produced peak power all the way out at 7,000 rpm, but we had a lot of boost to add and little time to ponder the issue.
The pair of 76mm turbos was combined with a twin-core, air-to-water intercooler, a pair of 45mm Hyper-Gates, and a single RacePort blow-off valve, all from Turbo- smart. A pair of JBA shorty truck headers with short tubing sections feed the turbos. Run with the wastegate springs set to provide just 7 psi of boost, the engine easily produced more than 600 hp. With proper tuning, we safely worked our way past 10 psi up to nearly 15 psi, where we ran into our first issue. The combination of 7-psi springs and the manual boost controller would only allow us to reach 15 psi. While this may seem like a lot of boost on the stock short-block, the 900hp mark had come and gone, and the motor was just laughing at us.
We stuck in a pair of 14-psi springs that, when combined with the manual controller, allowed us to reach nearly two bar-just over 26 psi. Surely that would be enough to break something. It bears mentioning that we performed all the testing on 118-octane Rockett Brand race gas, as we wanted our big bang to come from lack of component strength and not detonation. Our respect for the designers of the LS engine family grew with each successive pound of boost. Stock internals be damned, the 5.3 had come to party. The 1,000hp mark was eclipsed, followed soon by 1,100 hp and then, finally, the peak power of the night, an amazing 1,203 hp at 26.8 psi! Remember that this was on a well-worn, stock 5.3 short-block with nothing more exotic than increased ring gap. Think about that next time you hear of someone complaining about exploding an LS with only 8 psi of boost.
The stock heads were shipped off to Total Engine Airflow for Stage 1.5 porting. Upgrading the heads included larger intake valves (from 1.89 inch to 2.0 inch) and CNC work to the combustion chambers and ports. Intake flow jumped from nearly 230 cfm to just shy of 300 cfm, while the exhaust flow checked in at 220 cfm.
After eclipsing 1,200 hp, we kept going. Raising the boost beyond 27 psi resulted in what we suspect was an ignition failure. The motor was still in perfect shape internally and worked well up to 26 psi, but unfortunately we were out of available dyno time. The motor survived more than 60 runs, the majority of them more than 1,000 hp. We hammered this motor like we stole it, with repeated back-to-back pulls, but the little LS never missed a beat and was ready for more.
Then something dawned on us after pulling it off the dyno: 5.3s don't come with flat-top pistons (we know the few exceptions). Combine this with the fact that this combination produced peak power at 7,000 rpm, and we started to suspect that it might not be a 5.3 after all. Off came the oil pan and sure enough, connected to those flat-tops was set of 121 connecting rods and a crank that featured a casting number of 12553482. That's right, this 1,203hp 5.3 was actually a 4.8L. That's just 294 ci for a stunning 4.09 hp/ci.
The longevity of the combination is still in question, but having survived about 60 pulls under big boost means it could easily run a full season at the strip, making at least 1,000 hp. It might lose ring seal and drop power, but you can simply replace the rings. If the whole motor grenades in spectacular fashion, replace it with another $500 takeout. We'd caution that eventually you might drive over the crank at the drags and get oil under the tires, but as a detuned 700 or 800 streeter, this little sucker will be amazing.
