OK LS enthusiasts, here is a simple question about intake manifold design. What is more important: airflow or runner length? It’s obviously a trick question, as they are both important. Maybe a better question would be, what changes the power curve more: an additional 80 cfm or eight inches of runner length? While no one would refuse runners that flow an additional 80 cfm (or more), the reality is that improvements in airflow of this magnitude can result in zero power gain, but change the runner length by eight inches and you will always see a huge change in power production. In fact, this test on the adjustable runners in the new FAST LSXR intake demonstrated that fact perfectly. A short runner of equal diameter will easily out flow a longer runner, but the airflow alone won’t improve power, at least not everywhere in the power curve. How can more flow equate to less power you ask? Let’s check out the results.
The math involved is very complex, but the upshot of changes in runner length is that the runners are used to tune the power curve. A quick explanation of one of the three different forms of charge filling associated with intake design will help illustrate what happens when we change runner length. Though Helmholtz and Inertial Ram both play a part, the reflected wave produces a form of supercharging in the cylinder to help improve cylinder filling and power production. This wave is initiated when the intake valve opens. The opening of the valve combined with the downward movement of the piston creates a negative pressure wave. This wave accelerates at the speed of sound from the open valve back up the intake runner toward the common plenum. When the wave reaches the plenum, rarefaction occurs and allows the (negative pressure) wave to expand. This expansion creates a negative pressure area which causes positive pressure to flow toward the open valve. It is this (reflected) positive pressure wave (if timed correctly) that improves cylinder filling.
While this might seem overly technical, the upshot is that these pressure waves help dramatically improve power production. Since the waves must travel up and back the intake tract length, the length of the tract determines how much time it takes to arrive. It takes longer for the waves to travel in long runners, so they are most effective at lower engine speeds (where sufficient time is allowed to cover the distance). By contrast, short runners are more effective at higher engine speeds. The downside to the pressure waves is that the long runners become less effective at higher speeds and the reverse is true of short runners. Thus, the runner length must be tuned to optimize power production for a given application (displacement, cam timing, and intended rpm range). The ideal intake would have self-adjusting runner lengths, but such a system is difficult to package and very costly. The next best thing is a manually adjustable system like the LSXR intake offered by FAST.
To illustrate the changes in the power curve offered by different runner lengths, we ran the 102mm FAST LSXR intake on a mild LS3. The test engine is important as the engine itself determines the optimum manifold (runner length). Our admittedly mild cam-only (from Brian Tooley Racing (BTR)) LS3 was ideally suited to the longest runner length that provided the best overall curve up to 6,500 rpm. The great thing about the FAST intake is that the different runner lengths would allow us to dial in the power curve on wilder and larger combinations. We suspect the medium-length runners might show well on a wilder 417 LS3 stroker, while the short runners could be utilized on an ultra-high-rpm (8,000+ rpm) or large displacement (495 RHS stroker) application. Check out the supplied power curves and note that the engine made the most peak power with the short runners, followed by the medium and then the long runners. Equally important is the fact that man does not live by peak power alone. In terms of torque, the opposite was true. The long-runner combo produced the highest peak torque reading, followed by the medium runners then the short runners. The short runners improved peak power by 20 hp, but were down by almost 50 lb-ft in the middle of the curve. Such is the tradeoff inherent in changes in runner length, but apply this attitude adjustment to the right application and watch the power needle climb.
FAST Adjustable LSXR (LS3) Intake Test—-Long vs. Medium vs. Short Runners
It should be obvious from the graph of the test on the three different intake runner lengths that the longest runner (blue) offered the best average power up to 6,500 rpm. In truth, the results were not surprising given that our test engine was a stock LS3 with a BTR cam. We all know how well even the stock (long-runner) LS3 intake does in this rpm range, to say nothing of the original 102mm FAST LSXR. FAST didn’t design the medium and short runners for stock or mild applications (they had that covered), but rather for the high-rpm and/or increased displacement applications. It is nice to know that with a simple runner change, the effective operating range of the intake can be adjusted to suit your performance needs.
01. The lone (major) upgrade to the LS3 crate engine from Gandrud Chevrolet was to replace the stock LS3 (single-bolt) cam.
02. The stock LS3 cam was replaced by a Stage 3 LS3 camshaft from Brian Tooley Racing (BTR). Effective even as a cam-only upgrade, the BTR cam offered a 0.615/0.595-inch lift split, a 229/244-degree duration split, and 113-degree (+4) LSA. The cam swap necessitated a valvespring upgrade as well due to the increased lift.
03. From the outside, the adjustable intake looked very much like the latest black edition of the original LSXR.
04. Remove the lid and the secrets were revealed. The new FAST offered bolt-in, interchangeable runners. This shot illustrates the long runners that best duplicated the original (fixed runner) LSXR intake.
05. The amazing thing about the new FAST intake was the ability to adjust the runner length and tune the intake to the needs of your specific engine combination. Long runners make more power at lower engine speeds, while short runners enhance power production higher in the rev range. Displacement and cam timing also play a big part in which runner length works best.
06. The runners (long, medium, and short) all featured O-ring sealing to provide a leak-free environment and make installation a snap.
07. Each individual runner was secured to the lower intake using a single retaining screw.
08. The medium-length runners might be a good choice for elevated engine speeds and/or a healthy stroker application.
09. Note that one of the medium-length runners featured minor clearancing to fit under the intake lid.
10. The short runners are best left to ultra-high-rpm or big-displacement race applications. They offered the most peak power on our LS3 test engine, but the rest of the torque curve suffered compared to the long runners. It is important to run the right intake runner length for the right application. What other intake on the market allows you to dial in your combination or change when you make additional displacement, camshaft, or cylinder-head upgrades?
11. All of the testing run on the new FAST intake included their 102mm Big Mouth throttle body.
12. Tested on our cam-only LS3, the long runners on the new FAST LSXR were the clear choice. Which runners will the engine want once we bore and stroke it to 417 inches and add an even wilder cam timing and ported heads? That is the beauty of the new adjustable FAST manifold—the ability to grow with the engine.