The first order of business was to run the 4.8L in stock trim to establish a baseline. Though we have factory power ratings, the rated output is not what we see on the engine dyno for a couple of reasons. The major differences between the rated output and the power curve achieved on this engine dyno include lack of accessories, exhaust and induction system. Combine these with an optimized tune (the factory is conservative) and reduced water temperature, and you have the makings of a significant difference in power. For our testing, the difference between rated and as-tested output is irrelevant, as we will use our power numbers for the baseline and the test procedure will remain consistent to illustrate the differences offered by the proposed performance modifications.
Both the 4.8L and 5.3L were run under identical conditions. Up first, the 4.8L was equipped for dyno use with an aftermarket electric water pump, a set of 1-3/4-inch Hooker headers feeding Borla XR1 race mufflers, and a manual Accufab throttle body to replace the drive-by-wire version that came on the test motors when delivered from the wrecking yard.
Run in this trim and tuned to perfection with the Holley EFI system, the 4.8L produced peak numbers of 333 hp at 5,400 rpm and 343 lb-ft of torque at 4,700 rpm. Torque production exceeded 320 lb-ft from 3,600 rpm to 5,400 rpm.
With the motor still warm from running in stock trim, we yanked off the stock intake, heads, and out came the wimpy stock cam. These were replaced by a set of CNC-ported (Stage 2) heads from Total Engine Airflow. Full CNC porting, a 2.00/1.54 stainless steel valve package combined with a multi-angle valve job to produce exceptional flow numbers. The heads were combined with a healthy Crane hydraulic roller. The Crane cam offered .590 lift, a 224/232-degree duration split at 0.050 and 115 LSA.
The final upgrade was the LSXRT intake from FAST combined with a 102mm throttle body. Testing on the 4.8L and 5.3L showed that the LSXRT was a little too much manifold for the mild truck motor, as the stock intake offered more power up to 5,500 rpm, with the FAST intake producing an extra 15 hp at the top of the rev range.
For this test, we liked the extra top-end power, as it helped further demonstrate the effect of the displacement. Equipped with these mods, the 4.8L produced peak numbers of 476 hp at a lofty 7,000 rpm and 392 lb-ft of torque at 5,900 rpm. Torque production exceeded 350 lb-ft from 4,500 rpm to 7,000 rpm. Despite what would be a mild performance cam for an LS2 or LS3, these mods transformed the little 4.8L into a high-rpm screamer.
After running the 4.8L, we replaced it on the dyno with the 5.3. Like the 4.8L, the 5.3L was a high-mileage unit from a local wrecking yard. The 5.3L was equipped with the same headers, water pump and throttle body as the 4.8L and first run in stock trim with the Holley EFI system. The larger 5.3L offered only slightly more peak power (344 hp vs. 333 hp), but offered significantly more torque, especially lower in the rev range.
With a peak of 379 lb-ft at (a lower) 4,300 rpm, the 5.3L was up 36 lb-ft of torque over the smaller 4.8L, but the largest difference came at 3,700 rpm where the 5.3L offered 47 more lb-ft than the smaller 4.8L. It is not surprising that the two made similar peak power numbers given the same heads, cam and intake, but note that peak power came 300 rpm sooner with the larger 5.3L.
Torque production was equally skewed, as the smaller motor not only produced less overall torque, but did so at 4,700 rpm compared to 4,300 rpm for the 5.3L. Displacement not only improved power production through the entire rpm band, but lowered the effective operating range.
As with the 4.8L, the 5.3L was subjected to the same heads, cam and intake upgrade. We were thankful to have the Aeromotive fuel system feeding our 42psi injectors. Adequate fuel delivery is critical for maximum performance. After swapping out the heads, cam and intake, we were rewarded with a substantial jump in power, from 344 hp and 379 lb-ft of torque to 484 hp and 424 lb-ft of torque. As with the 4.8L, these mods substantially increased the effective operating speed, upping peak power from 5,100 rpm to 6,800 rpm, and torque from 4,300 rpm to 5,700 rpm.
Though the gains were most prevalent past 4,200 rpm, the modified motor equaled the low-speed power of the stock motor down to 3,000 rpm. It is interesting to note that the relative gains and basic shape of the respective power and torque curves remained constant. The TEA heads, Crane cam and FAST intake improved the power output of the 4.8L by 143 hp and the 5.3L by 140 hp. Peak torque improved by 49 lb-ft on the 4.8L and by 45 lb-ft on the larger 5.3L.
The mods increased the effective engine speed (where each made peak power) by 1,600 rpm on the 4.8L and 1,700 rpm on the 5.3L. Using the exact same components, peak power and torque occurred higher in the rev range on the 4.8L than the 5.3L. As expected, bigger engines indeed make more average power production, but don't count out the little 4.8L when it comes to making power. It just needs a little more engine speed.