Is the LSXRT too much intake for a modified 4.8L?

By Richard Holdener

Why is it that the little guys get no respect? One need look no further than the 4.8L LR-4 or (late model) LY2 truck motors. Sure, we all long for the larger 6.0L LQ motors, but given the dramatic price difference, most enthusiasts wind up settling for the medium-sized 5.3L. Often left out in the cold are the plentiful, and potentially powerful, 4.8L variants. Make no mistake, when it comes to performance, bigger motors make more power, but is that reason enough to dismiss the potential of the little 4.8L? Remember, bigger motors that make more power also require more fuel, so the 4.8L usually offers improved fuel mileage, but we are here to tell you that it is also a viable candidate for a performance street/strip application. The little 4.8L motors found their way into thousands of truck applications. It is that availability in wrecking yards combined with the current pricing that makes the 4.8L such a sweet deal. Though down on displacement compared to its larger siblings, we are here to demonstrate that with the right combination of components, the 4.8L is proof positive that sometimes less really is more.

Looking at the tale of the tape, we see that at 293 inches and 324 inches, the 4.8L and 5.3L are close to being metric equivalents of the original 283 and 327 small blocks. Technically speaking, a better comparison might be the 327 and larger 350 small block, as the 4.8L and 5.3L both share the same bore (3.78 inches), the difference comes from a change in stroke (3.267-in for the 4.8L and 3.622-in for the 5.3L). Further confusing things is the fact that the 4.8L and 5.3L share the same block (externally marked 4.8L/5.3L), heads and intake manifold, making it difficult to externally distinguish between the two. We know from first-hand experience that more than one LS enthusiast has purchased a 5.3L only to be surprised by the receiving the smaller 4.8L. The best way to distinguish the two is by taking a peak inside, as the 4.8L will be configured with a distinct crank, rod and piston assembly. The 4.8L can be easily identified by their flat-top pistons, though the H.O. version of the 5.3L also featured theses as well. To be certain, check the crankshaft and connecting-rod casting numbers, as the 4.8L will feature 482 and 121 casting numbers on the crank and rods, respectively. By comparison, the 5.3L will be sporting a 216 crank and 143 rods.

Hardly a beauty queen, the high-mileage 4.8L came as a complete take-out motor from the local wrecking yard for just $350.

Despite the displacement deficiency, the little 4.8L can be made to produce some serious power, especially if you favor high-rpm screamers (and who doesn’t?). For this test we decided to illustrate not just how impressive a modified 4.8L can be, but also the important role the induction system plays in the overall performance. Not just any intake mind you, but a 102-mm FAST LSXRT intake. Our concern was that the high-flow intake might be too much for the little 4.8L, but this test clearly illustrated that FAST did their homework when designing the manifold and made it versatile enough to run on a wide variety of different displacements. To get things started, we needed a suitable test motor and ours came from the source most often used by enthusiasts, a high-mileage take out from a local wrecking yard. For the cheap sum of just $350, we received a complete 4.8L LR-4 sporting 157,000 miles. The purchase included everything we needed, from throttle body to oil pan, including injectors, coil packs and cast-iron exhaust manifolds.

In preparation for dyno use, we ditched the accessories, stock cast-iron exhaust manifolds and drive-by-wire throttle body and replaced them with a Meziere electric water pump, long-tube headers and Accufab manual throttle body. Since the LR-4 would be seeing extensive dyno time, we also elected to disassemble the motor and treat it to a quickie ball hone and deck surface before the installation of a fresh set of Total Seal rings and new rod and main bearings. We’ve successfully run high-mileage motors like this right from the yard, but wanted to give this one a new lease on life before the punishment. The stock crank, rods and pistons were retained, but the heads were given a fresh surface and valve job. What we wanted was a fresh 4.8L to demonstrate the bone-stock power production without concern that the accumulated mileage had taken a toll on performance. Run with a Holley HP EFI management system and 36-pound FAST injectors, the stock 4.8L produced peak numbers of 335 hp at 5,400 rpm and 345 lb-ft of torque at 4,800 rpm. The difference between this power output and the factory rated 270-290 hp can be attributed to the headers, lack of accessories and optimized tune on the engine dyno.

The TEA 706 heads were also treated to a dual spring upgrade that included titanium retainers.

Satisfied that we had a solid baseline, it was time for some modifications then a back-to-back intake test with the FAST LSXRT. In an effort to coax more power out of the little 4.8L, we elected to upgraded the top end by replacing the wimpy stock cam and 706 cylinder heads with something decidedly more sporty. The stock 706 heads were shipped off to Total Engine Airflow for their Stage 1 porting designed for the small-bore 4.8L/5.3L applications. The head work included full CNC porting, a 2.0/1.55 valve package and back-cut intake valves. The TEA 706 heads were also treated to a dual valve-spring upgrade capable of taming cams up to .650 lift. The 64-cc chambers were slightly larger than stock, but who can argue with flow numbers that exceeded 300 cfm on the intake and nearly 250 cfm on the exhaust? The TEA Stage 1 heads were teamed with a Comp hydraulic roller, LSr, cathedral-port cam that offered a .610/.617 lift split, a 223/231 duration split at .050 and 112-degree LSA. Though mild for a typical 6.0L LS application, this powerful cam pushed peak power up to 7,000 rpm on the little 4.8L.

Installation of the cam and heads also included a new set of 7.4-inch hardened pushrods and hydraulic roller lifters from COMP Cams. The factory rockers were retained and we made sure to check lifter preload during the installation. The CNC-ported, Stage 1 heads were installed using a fresh set of Fel Pro MLS head gaskets and ARP head studs. For this first test, we configured the little 4.8L with the factory truck intake and Accufab throttle body. Tuned to perfection using the Holley HP EFI, the modified 4.8L produced peak numbers of 453 hp at 6,900 rpm and 387 lb-ft of torque at 5,500 rpm. Power production equaled the stock motor up to 4,200 rpm then pulled away by as much as 150 hp at 6,500 rpm.

The intake upgrade also included a FAST 102-mm Big Mouth throttle body.

Next up we replaced the factory truck intake with the FAST LSXRT. The intake swap also included the matching 102-mm, Big-Mouth throttle body. The intake upgrade increased the peak power numbers to 477 hp at an even 7,000 rpm and 393 lb-ft of torque at 5,900 rpm. Though certainly not designed for the smaller 4.8L, the LSXRT offered serious power gains above 5,500 rpm and only dipped below the truck intake in a few spots. When all was said and done, the heads, cam and intake upgrade netted us an additional 142 hp (even more at higher engine speeds), transforming the little 4.8L truck motor into a screaming street/strip motor. I guess sometime less really is more! Check back with us next time and see what happens when we run the same test on the larger 5.3L!

Sources

Aeromotive
(913) 647-7300
aeromotiveinc.com

COMP Cams
(901) 795-2400
compcams.com

FAST
(877) 334-8355
fuelairspark.com

Holley/Hooker
(270) 782-2900
holley.com

Total Engine Airflow
(330) 634-2155
totalengineairflow.com