Words & Photos By: Richard Holdener
For some time now, the LS engine family has been the proverbial hot button of the automotive industry. The LS took the reins from the original small-block Chevy and has never looked back. Even after the introduction of the direct-injected, Gen V LT engine, the LS remained the engine of choice among Chevy enthusiasts, hot rodders, and engine swappers. Not surprisingly, the aftermarket has wholeheartedly embraced the LS, offering all manner of performance components, including a plethora of intake manifolds.
While the available intakes differed widely in design and construction, we will focus mainly on changes in runner length. To illustrate the effect of changes in runner length, we set up a test between two popular intake designs, the FAST LSXRT and a fabricated aluminum intake.
Popular among LS enthusiasts for different reasons, the two intakes differed enough to show dramatic changes in the power curves. To spice things up, we decided to first compare the two intakes in normally aspirated trim in Part 1, then follow up in Part 2 with a little boost! Would things change under boost? You’ll have to check out Part 2, but let’s first find out how they compared on our normally aspirated LM7.
Before we could test our two intake designs, we needed a suitable test motor. The test motor has a decided effect on the results, so we selected one according to popularity. Far and away the most prevalent LS engine out there was the 5.3L. It was offered in (literally) millions of Chevy trucks, compared to just thousands of LS1s and the like. Typical of most LM7s, our 5.3L had logged over 150,000 miles, but was still ready for action.
For this test, we performed a few upgrades, including a cam, valve springs, and ARP head studs. Truth be told, the ARP hardware was not necessary for the low boost run on this test, but we also performed a head test on this motor, so it received Fel Pro head gaskets and the ARP studs during the process. To ensure plenty of power, we installed a healthy COMP Cams grind. The 54-454-11 cam offered a .614./.624 lift split, a 227/243-degree duration split, and 113-degree lsa. It was teamed with a set of matching 26918 beehive valve springs.
To get things started, we installed the 5.3L on the engine dyno with a set of 1 7/8-inch, long-tube headers, a Meziere electric water pump, and 120-lb injectors (89 lbs for the FAST intake). The injector sizing was overkill for the normally aspirated combination, but would come into play once we added boost. Even running just 7 psi of boost, this little 5.3L produced some impressive power numbers. The engine was configured with a FAST XFI management system and fed by a 102mm Big Mouth throttle body.
First up on the dyno was the fabricated intake. The design featured short(er) runners, a common plenum, and radiused air horns. This intake design is available though a number of sources and offered a combination of good looks and fitment under most production hoods. Run in normally aspirated trim with the fabricated intake, the 5.3L produced 486 HP at 7,200 RPM and 403 lb-ft of torque at 5,300 RPM. The combination of short runners and a healthy cam profile allowed the 5.3L to produced peak power past 7,000 RPM.
Now, it was time for the FAST. The FAST intake family has been with us for some time and continues to be popular among LS enthusiasts — and for good reason. Both the LSXR and LSXRT offer an unmatched combination of power and torque up to 6,500 RPM (even higher on our 5.3L). Credit the long, free-flowing runners that offer substantial power gains over even the best factory intakes.
For this test, we chose the LSXRT intake, though the two intake versions offer surprisingly similar power curves when tested back to back. The LSXRT was configured with the same 102mm throttle body run on the fabricated intake, but the FAST intake required an injector change from the 120-pounders run on the fabricated intake. As with every test, the air/fuel and timing values remained identical for both intakes. Equipped with the FAST LSXRT, the normally aspirated 5.3L produced an identical 486 HP, but at a slightly lower 6,800 RPM. The FAST intake greatly enhanced torque production through the rev range, with a peak of 425 lb-ft coming at 5,000 RPM.
With the power and torque curves now generated in normally aspirated trim, check out Part 2 where we added a single turbo then tested the intakes under boost!
Graph 1: NA 5.3L-Fast vs Fabricated Intake
Before adding boost, we ran the 5.3L with both intakes in normally aspirated trim. This way, we could demonstrate that the gains from the intake design continue under boost. Run on the dyno in normally aspirated trim with the fabricated intake, the COMP-cammed 5.3L produced 486 HP at 7,200 RPM and 403 lb-ft of torque at 5,300 RPM. After replacing the fabricated intake with the FAST LSXRT, the peak power remained at 486 HP, but the peak torque jumped to 425 lb-ft at a slightly lower 5,000 RPM. The longer runners in the FAST intake improved power production up to 7,100 RPM. The short-runner, fabricated intake was better suited to greater displacement and higher engine speeds.