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Fast Under Boost: LS Intake Test Part 2

Words & Photos By: Richard Holdener

Back in Part 1, we compared a couple of popular intake manifolds on a normally aspirated 5.3L. The FAST LSXR and LSXRT have long been the go-to intake of choice for performance LS applications, but with so many new intakes available, we wanted to see if the FAST design was still a player. More than that, we wanted to demonstrate the effect runner length had on the power curve.


Not wanting to limit the test with a stock cam, we installed a healthy COMP cam. Aggressive for a 5.3L, the COMP Cams 54-454-11 featured a .614./.624 lift split, a 227/243-degree duration split, and 113-degree lsa.

Working with the cam profile and head flow, the intake manifold determines the effective operating RPM of the motor. It is less about flow than it is about wave tuning. The runner length determines the effective operating range of the motor, with longer runners enhancing power production lower in the rev range than shorter ones. The terms long and short are relative here, as the FAST manifold offered more power in Part 1 of our test all the way up to 7,000 RPM. Obviously, the displacement of the test motor also has something to do with the effective operating range, but the intake should be designed to produce power where it will be most beneficial.

We have included the graph from Part 1 to allow us to compare the results in normally aspirated trim to the results once we applied boost. Not only did we want to show the effect runner length has on power production of the normally aspirated 5.3L, but we also added boost to the equation. Many LS enthusiasts think all bets are off once we add boost, as the positive pressure somehow overcomes the inability of the short runner intake to properly fill the cylinders at lower engine speeds. While boost is always welcome, the reflected waves responsible for enhancing cylinder filling at specific engine speeds remain in effect, even under boost. The only thing boost does is potentially magnify the gains, though the temperature can slightly alter the speed of the waves and change the effective RPM (but not significantly).

After comparing the FAST LSXRT and fabricated intake on the normally aspirated 5.3L, we decided to illustrate that the effect of runner length remains even under boost.

To illustrate that reflected wave tuning continues under boost, we had to supply some positive pressure. In this case, the pressure was supplied by a single Precision turbocharger. The T4-based 7675 turbo was capable of supporting over 1,100 HP, but we would be taking our SBE 5.3L nowhere near that output. The effect of runner length could be easily demonstrated at a much lower boost level, so we opted to run the 5.3L at just 7 psi.

The turbo was installed onto the 5.3L using a set of DNA turbo manifolds and custom Y-pipe. The Y-pipe featured the T4-turbo flange, as well as a pair of waste gate flanges designed to accept the HyperGate 45 waste gates from Turbo Smart. Boost was fed to the 102mm throttle body through an air-to-water intercooler supplied by Procharger. Like the Precision turbo, the ATW cooler was oversized for the application and fed a steady source of ambient (85-degree) dyno water. The dual waste gates ensured a rock-steady boost curve, while the 4.0-inch exhaust maximized exhaust flow out of the turbo.

With plenty of fuel flow from our oversized injectors, we were able to compare the two intakes under boost. The boost pressure, air/fuel, and timing were all kept constant during the testing. Equipped with the fabricated intake, the turbo 5.3L produced 717 HP at 7,100 RPM and 609 lb-ft of torque at 5,300 RPM. The boost pressure never varied by more than .5 psi through the whole run.

After installation of the FAST intake, we immediately noticed that the relative power curves of the two intakes remained constant under boost. Just as it had on the normally aspirated 5.3L test, the FAST intake improved power production through most of the power curve, in fact all the way up to 7,000 RPM. Equipped with the Fast, the turbo motor peaked at 735 HP at 6,600 RPM and 643 lb-ft at 5,000 RPM. Had we run the fabricated intake higher, it would have eventually pulled ahead, but that wasn’t the point of this test.

Note from the two graphs that the charge filling associated with the difference in runner length remained consistent, even under boost. So when you are considering an intake manifold for an NA or boosted application, remember it’s all about engine speed.

Graph 1: NA 5.3L-FAST vs Fabricated Intake

Back in Part 1, we compared the fabricated intake to the FAST LSXRT on a normally aspirated 5.3L. 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, but we wanted to find out if this trend continued under boost.

Graph 2: Turbo 5.3L-FAST vs Fabricated Intake

After running the intake test on the normally aspirated 5.3L, we duplicated the test with forced induction. Would the intakes perform differently under boost? Actually, the difference between the intake manifolds remained pretty constant once we added the single 76mm turbo to the mix. The relative shapes of the power curves were simply elevated. The reflected wave tuning offered by the change in runner length remained constant even under boost. Equipped with the FAST LSXRT intake, the turbo 5.3L produced more power than the fabricated intake up to 7,000 RPM. For elevated engine speeds (past 7,000 RPM), the short-runner, fabricated intake might offer power gains, but below that point, the FAST design is tough to beat, even under boost!