We Put The Modular FAST LSXR Intake To The Test With A Runner Swap



Fuel Air Spark Technology, better known to the masses as simply FAST, has been a leader in LS intake manifold design for almost as long as the LS itself has been around. Never one to rest on their laurels, FAST has been constantly innovating since their inception. Some of you may remember the classic FAST 78, which seems like a distant memory at this point but was revolutionary when it first hit the market. Flash forward to the new LSXR intake manifold and it is easy to see just how far the design of FAST’s flagship manifold has come. Combining three separate runner systems into one modular manifold system, the LSXR can be custom tailored to almost any situation, making sure your combination is always maximized.

However, just like FAST is always innovating, we are never ones to take a product at face value. When we heard that FAST’s new intake would come with individual, interchangeable runners, we were anxious to put one to the test. But not only did we do that, we took it one step further, CCing and measuring the runners themselves to make sure you knew exactly what you were working with if you opt for this manifold.

Screen Shot 2017-02-08 at 5.09.53 PM

Weighed and Measured

When we received our FAST LSXR intake, one of the first questions around the office was “well, just how long are the runners anyway?” It seemed like a simple enough question to answer so we got out our handy dandy measuring tape. We measured all of the runners on the inside curve of the runner, as we felt that this gave the best indication of how the runner would perform. Afterward, we used tape to seal off the bottom of the runner and a beaker, measured in cubic centimeters, to fill up the runners to see exactly how much volume they would hold.

The standard runners measure in at 8 inches and their total volume measured in at around 680 cc. Obviously, these are the standard set of runners that you will receive with any LSXR manifold, as the additional runners are sold separately. These standard runners have been optimized for both on- and off-track use and strike a balance between low-end grunt and peak power gains. Typically, as shown by our test, they are more than capable of supporting stock cubic inch engines without a problem while retaining good street manners and power down low. Even with increased head flow and larger camshafts.

Next, we checked out the mid-length runners which FAST calls their “high-RPM” runner. They measured in at 6.25 inches on the runners with a volume of around 460 cc, a considerable decrease from the standard-length runners. Obviously these runners are designed for higher RPM engines that will take advantage of increased rev limits. These runners also typically shine on larger cubic inch engines with increased head flow and larger cams. If you’re going to run these runners, be aware that they will shift your powerband higher typically leading to an increase in peak horsepower but with a slight decrease in low end grunt—just like we saw in our test.

Last but not least, the “race” runners measured in at just 3.5 inches and 300 cc. To put that number into perspective, some LS heads out there have port sizes of close to 300 cc by themselves. These runners are designed with maximum performance in mind and should be reserved for serious high revving engines. While, in our test, peak horsepower was attained at the same 7,000 rpm mark as our mid-length runners, these short length runners would have a lot more to give on an engine that was capable of taking advantage of the short length and increased plenum volume.

Part numbers

  • LSXR Intake LS3, L99, L76 P/N 146102B
  • LSXR Intake LS1, LS2, LS6 P/N 146302B
  • LSXR Intake LS7 P/N 146202B
  • Interchangeable runner kit (mid-length and short runners) P/N 146070
  • High HP runners only 146070
  • Race runners only 146071
  • High HP complete manifold 146103B
  • Race runner complete manifold 146104B

“We spent a lot of time in Computational Fluid Dynamic (CFD) development with the runners,” explained FAST’s Ed Monte. “The original street runner went through several iterations of CFD and prototyping with physical parts in engine testing for validation. The process became more difficult with each port design, as the OE has done a better job of squeezing all they could out of the stock manifold.”

We weren’t out to try and create a runner that would be best suited for the street – we already have it in the standard LSXR.”– Ed Monte

“The time spent in CFD and proving out the results with physical testing on the engine certainly paid off. That’s the reason the FAST LSXR is the best manifold on the street,” continued Monte. “Knowing how well the standard LSXR performs on the street, we knew it would be hard to beat. That’s why the approach to the High-HP mid-length runner and the Race runner was different. The same process was used, CFD and physical testing, but our objectives as far as engine usage and RPM was different. These runners are designed to perform where the street runner starts to fall off. The test engines were much rowdier than what was used during the street development. We wanted to develop a runner that would really perform for the racer; the racer at the drag strip who’s engine spends more time above 5,000 rpm than below it and the racer at the road course who is really winding out the engine down the straights. These new runners really perform on engines that like to turn a lot of RPM. We weren’t out to try and create a runner that would be best suited for the street – we already have it in the standard LSXR.”

Runner Runner

To put the manifold, and the individual runner combinations to the test, we headed over to Cunningham Motorsports where they had a cam-only LS3 car waiting to put the runners to the test. Normally, the car runs a healthy dose of nitrous, but for these tests we decided it would be best to keep things naturally aspirated and see where the chips fell with the stock displacement LS3.

Screen Shot 2017-02-08 at 5.08.13 PM

As we predicted, when we decreased the runner length in the manifold we gained up-top horsepower and lost low-end torque. This is pretty common when decreasing runner length. That’s because every engine has an RPM where it functions the most efficiently, which can be modified by the length of the runners. As the RPM in the engine increases, so does the velocity in the intake runners. The longer runners tend to increase that velocity more quickly than the shorter runners. This causes the cylinders to fill more easily in the low RPM due to the air velocity being higher in the longer runners. However, when the air velocity hits a certain point, the air speed can no longer increase and the longer runners become a restriction in the higher RPM. The shorter runners lack the velocity down low, but are less of a restriction in the upper RPM range, allowing the engine to make more top end power at the cost of low-down torque.

The Power Difference

The different lengths of the runners also act like a tuning fork of sorts. As the cylinder intakes air, at some point the intake valve slams shut, this creates a reverberating air pressure wave that travels back up into the intake plenum. Depending on the length of the runners, they can take advantage of these pulses and actually use the rebounding wave to force more air into the cylinder. Increasing or decreasing the length of the runners effectively tunes them to a different frequency at a different RPM.

powertv fast ls3 runner test full length

Our first test showed us that the standard length runners really are the best of both worlds, producing a healthy 439.4 horsepower at 6,700 rpm and 392.5 lb-ft of torque at 5,400 rpm. If your car’s main duties are street driving on a stock cube engine, this might be the best selection for you. Since most of the car’s time on the street will be spent in the mid-range, the increases there will be the most pronounced with these runners.

powertv fast ls3 runner test mid length

Next, we dyno’d our mid-length or “high-RPM” runner. As we expected horsepower increased, as did the RPM at which it was attained, but torque decreased. These runners are based on Daytona Prototype units and have been optimized for high RPM engines with high head flow. Our test resulted in 451.6 horsepower and 377.7 lb-ft of torque, an increase of 12.2 horsepower over the standard runners but a decrease of 14.8 lb-ft of torque. As we mentioned before, the shorter runners typically sacrifice low-end torque for high-end horsepower. These runners would be best suited to high-RPM, large displacement motors that will spend a good amount of time in the upper RPM range such as in racing applications.

powertv fast ls3 runner test short race

Last, we bolted on the set of “race” runners and headed back to the dyno. As expected, our peak horsepower increased yet again to 455 rear wheel horsepower, a gain of 15.6 horsepower over the standard runners and 3.4 horsepower over the mid-length runners. However, torque yet again dropped to 365 lb-ft, a decrease of 27.5 lb-ft compared to the standard runners and 12.7 lb-ft down from the mid-length runners.

powertv fast ls3 runner test compare

“The beauty about the LSXR is not having to buy three different intakes, but just runners as a customer’s engine evolves,” said Monte. “Even beyond that, the engine builder can develop a seemingly infinite combination of intakes by mixing and matching the runners. That’s the beauty of the modular system; the torque curve can be tailored to any number of desired curves by mixing the Race, High-HP, and Street runners within the intake.”

Like many things in the automotive world, this one ended up being a bit of a give and take as we knew it would. The interchangeable length runners allow you to realistically fine tune your intake to produce power where ever you’d like to. If you’re looking for a torque monster, standard length runners will do just fine, but if you want peak horsepower and will be keeping the car in the upper RPM one of the shorter length runners might be your jam.

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About the author

Chase Christensen

Chase Christensen hails from Salt Lake City, and grew up around high-performance GM vehicles. He took possession of his very first F-body— an ’86 Trans Am— at the age of 13 and has been wrenching ever since.
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