Just how much power can you bolt on to a high-mileage 5.3L?

By Richard Holdener

Some time back, we answered a simple question. Was the 4.8L LR-4 too small to take advantage of the power potential offered by the LSXRT intake? As it turned out, even the little LS (LR-4) took full advantage of the LSXRT to the tune of 24 extra horsepower. In fact, a top-end package that included Stage 1, TEA-ported 706 heads, a COMP 223/231 cam and FAST intake increased the power output of the 4.8L from 334 hp to 477 hp. Every bit as important was the fact that the upgrade transformed the diminutive but torquey truck motor into a 7,000+ rpm, street/strip terror of the highest magnitude. With the success of the 4.8L fresh in our minds, we couldn’t help but wonder what might happen if we applied the same test to the larger 5.3L? Would the increase in stroke change the requirements of the motor or would the larger displacement revel in the newfound flow and resonance tuning offered by the LSXRT? Basically, if less was more on the 4.8L test, would more be more on the 5.3L? Only one way to find out-it’s off to the dyno.

As indicated right on the casting, the 4.8L and 5.3L share the same iron block. The difference between the LR-4 and LM7 is the stroke and connecting rod length.

For the uninitiated, The 5.3L differs from the smaller 4.8L by way of stroke. The two share the same block and bore size of 3.78 inches, but the 5.3L was blessed (or cursed) with the same stroke used in the larger 5.7L, 6.0L and 6.2L LS engines. The increase in stroke length from 3.267 inches to 3.622 inches is sufficient to increase displacement from 4.8 liters to 5.3 liters. Since the two share the same piston compression height, the difference in stroke length is made up by a longer rod in the short-stroke 4.8L. Basically the 5.3L is a stroker version of the 4.8L, not unlike the difference between the original 350 and smaller 327 small blocks. That the two share the same heads, cam and intake means peak power production is very similar, but (stock for stock) the larger 5.3L makes peak power earlier and is up on torque production. Despite the change in displacement, the static compression ratio is similar between the two engines thanks to the use of flat-top pistons in the smaller 4.8L.

Having successfully run the LSXRT on the smaller 4.8L, it was time to see if the performance versatility continued when put to the test on the larger 5.3L. Like our test on the LR-4, we first needed a test motor. Once again, our dyno mule came from a local wrecking yard in the form of a high-mileage, take-out, LM7 5.3L. The 5.3L has quickly become the go-to small block of the modern era, with popular swaps involving everything from 50s, 60s and 70s muscle cars to imports and even 5.0L Mustangs. LS swaps have become so popular that companies like Holley are offering swap kits to help install the little wonder motors into your favorite muscle car chassis. Considering that our stock 5.3L produced 345 hp and over 380 lb-ft of torque, even a stock LM7 would be a good choice for powering a street rod or street machine. We know of a lot of small block crate motor that don’t put out those kinds of numbers, let alone idle smooth, offer a tremendous torque curve and knock out such impressive fuel mileage.

As impressive as these little 5.3Ls are in stock trim, we wanted to see how well they responded to a few simple mods including the LSXRT intake. Like the previous 4.8L, the 5.3L took a quick trip to the machine shop for similar upgrades prior to testing. The upgrades included a fresh deck and hone, new Total Seal rings and a set of rod and main bearings. Also included was a new oil pump. The heads were treated to surfacing and a quickie valve job to make sure everything seated properly during the baseline test. Truth be told, we weren’t as concerned about the 178,000 miles logged on the motor as we were about the apparent lack of attention. A peak under the valve covers before the rebuild revealed some sludge build up. Though we reused the major components (crank, rods and pistons), we had a fresh 5.3L ready to hit the dyno.

Tuning for the test was provided by this Holley Dominator EFI system.

Like the previous 4.8L test, the 5.3L was configured for dyno use with a Meziere electric water pump, long-tube headers and a Holley Dominator EFI management system. Also present was a fresh pan of Driven oil, an Accufab manual throttle body and 36-pound FAST injectors. After tuning, the otherwise stock 5.3L produced 345 hp at 5,100 rpm and 381 lb-ft of torque at 4,300 rpm. Using the same heads, cam and intake as the smaller 4.8L, the 5.3L produced peak power 300 rpm lower and peak torque 500 rpm lower. Thanks to a long-runner intake, the 5.3L offered a healthy torque curve, bettering 350 lb-ft from 2,700 rpm to 5,300 rpm. After verification of the baseline numbers, off came the stock heads, cam and truck intake and on went a set of CNC-ported 706 heads from TEA and COMP LSr cam. The TEA Stage 1 heads featured full CNC porting, a 2.0/1.55 valve combo and dual spring/titanium retainer package that allowed up to .650 lift. The heads flowed over 300 cfm or more than enough to support over 600 hp. These were combined with the COMP 273LRHR12 cam that offered a .610/.617 lift split, a 223/231 duration split and 112-degree LSA.

The FAST LSXRT intake demonstrated its worth by pushing peak power to 486 hp at 6,700 rpm and torque to 424 lb-ft at 5,700 rpm.

Still on the dyno, the new TEA heads and COMP Cam were quickly installed along with COMP hardened pushrods. For this first test, the ported heads and new COMP Cam were topped with the factory truck intake and Accufab throttle body. This combination upped the peak power output from 345 hp and 381 lb-ft of torque to 472 hp at 6,500 rpm and 423 lb-ft of torque at 5,200 rpm. The TEA heads and COMP Cam package increased the power output by 127 hp over the stock stuff. Though the majority of the gains came past 4,200 rpm, the new combo matched or exceeded the stock components all the way down to 3,000 rpm (the lowest we tested). Unlike the stock motor, the modified torque curve exceeded 400 lb-ft from 4,300 rpm to 6,100 rpm. Next up, we installed the LSXRT intake. As before with the 4.8L, the peak numbers were up substantially, from 472 hp to 486 hp, with most of the gains coming past 5,500 rpm. Oddly enough, the low-speed power suffered slightly more on the larger 5.3L than the smaller 4.8L, but it is clear that the LSXRT will make more power on the 5.3L with more displacement than the smaller 4.8L. I guess sometimes More really is More.

Sources

Aeromotive
aeromotiveinc.com

COMP Cams
compcams.com

FAST
fuelairspark.com

Holley/Hooker
holley.com

Total Engine Airflow
totalengineairflow.com