Dyno Test: FAST LSXRT Intake

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by Rick Jensen  More from Author

30 Extra Horses For Your Late-Model GM

If you are the lucky owner of a GM truck or performance car built in the last 10 years or so, you may already know just how easy it is to modify it for huge horsepower. The so-called “LS” series of engines–starting in 1997 with the LS1-powered Corvette–are world-renowned for their pushrod simplicity, technology, and power output. Though there are differences between the earlier Gen III and more recent Gen IV versions–heck, some in this engine family aren’t even called LS–rest assured they all share the potential for huge power output beyond the factory’s already impressive numbers.

One of the keys to that potential involves the astoundingly good cylinder heads on these engines. The earlier Gen III-style heads are identifiable by their “cathedral”-style intake ports. The later Gen IV versions have rectangular ports, which are good for even more airflow. However, not everything on these engines flow like the heads–the factory intake manifolds are effectively bottling up horsepower on these GM cars and trucks.

Hot rodders wanted more out of these engines, and it wasn’t long before a fantastic aftermarket replacement became available: the FAST manifold. Available in 78, 90, and later, 92mm versions, these modular, molded polymer intake manifolds made a ton of power. In fact, this author saw FAST’s 92mm intake make 20 rear-wheel horsepower on his LS1 Camaro, with additional torque and low-rpm drivability as an added bonus.

To clear the hoods of the Camaros, Firebirds, and Corvettes that they will be installed in, FAST’s new 102mm LSXR manifolds utilize factory-style curved runners and keep similar low-profile shapes as the stockers. FAST’s engineers knew that power was being left on the table, and their next project–the LSXRT–was the most ambitious yet. The LSXRT was similar in many ways to the previous versions, but one look at it and you know that it holds some serious power potential. Instead of a low-profile design, the upper shell is massive–this intake is over 10 inches high, and it is able to fit straight runners because of that height. Anyone who has seen a race car’s dead-straight induction system knows that the straighter the air can flow, the more power it can make.

So all of you guys with 4.8/5.3/6.0-powered GM trucks, pay attention: your factory intake may be ugly and weak, but FAST has a great solution. Check out this before and after engine dyno test on a 5.3-liter V-8, and watch how the new LSXRT performs.

On a recent trip to Comp Cams’ Memphis headquarters, we were invited back into their top-secret Research and Development area. This is where Comp tests its new creations, and we would get a front-row seat for an LSXRT intake dyno session.

Here is the 102mm LSXRT intake manifold next to the stock 5.3-liter truck manifold we’ll be testing it against. The 102mm LSXRT (part No. 146602) is FAST’s latest version of the wildly popular manifold series for GM LS-style engines, aimed at GM trucks, as well as GM cars with more hood clearance. It is a multi-layer polymer design for cathedral-port heads, with runners that can be removed for easy disassembly or porting.

The LSXRT wears this massive 102mm throttle body, which complements the high-flow intake and dwarfs the stock 75mm TB seen to the right. At around 10.5 inches high, the LSXRT intake is much taller than FAST’s LSXR version typically used on car applications; however, cars that have been given more hood clearance will benefit from its additional flow.

The LSXRT is so effective because the taller intake allows it to use straight runners (bottom), as opposed to the curved LS3/LS7 runners used in stock and FAST intakes. Air doesn’t like to make turns, and this straight shot is great for making power.

A GM 5.3-liter (325 cubic-inch) V-8 will be used for the test. It runs stock “243” heads and a Comp Cams LSR camshaft with 219º/227º at .050 duration and .607/.614 lift. Dynatech 1.75-inch-primary headers, 2.5-inch dual cats, and a 3-inch Y-pipe will exhaust the fumes. Note the individual EGT probes at the top of each header pipe, and the wideband O2 sensors at the bottom, that Comp uses for precise data acquisition.

Comp’s Rich Smith fires up the sophisticated SuperFlow SF-902 engine dyno. Once the engine’s coolant has been warmed up to 175 degrees and oil temp is around 180, testing begins. The engine is run up to redline, and after three hard pulls, our baseline averages 400.1 horses at 6200 rpm, with 370.7 lb-ft of torque at 5000 rpm.

With the stock dyno pulls done, it’s time to remove the factory intake.

Rich starts by removing the 10 intake manifold bolts–there are five per side.

The eight fuel injector pigtails are unclipped.

On the driver’s side, the mass airflow (MAF), throttle position (TPS), manifold absolute pressure (MAP), intake air control (IAC), and intake air temperature (IAT) sensor clips are disconnected.

On the passenger side, the throttle body cable is removed. At this point the fuel pressure is relieved, and the fuel line is removed.

The positive crankcase ventilation (PCV) hose is removed as well.

Once everything is disconnected, Rich lifts the stock intake off of the 5.3.

Though our 5.3 dyno engine is using a cable throttle body, 2002-07 GM trucks have an electronic throttle body, and trying to swap to a cable TB makes the computer angry. Comp has a throttle body adapter (part No. 146029-KIT) that allows the electronic TB to be retained with the LSXRT intake’s different bolt pattern.

The coupler hose for the MAF sensor is slipped on and attached with a worm clamp.

Next, Rich installs and tightens the MAF sensor, after which he drops the 102mm intake onto the dyno mill.

This intake changes the 5.3’s look drastically–it almost looks like a new Hemi mill!

All of the connections that were undone to remove the stock intake are hooked back up to the LSXRT.

The LSXRT intake, ready for more dyno pulls. The FAST intake has all of the vacuum and sensor connections needed to pop right on. Two minor adjustments are needed for certain applications: a PCV adapter kit is included for 2002-07 GM trucks (the same applications that need the TB adapter plate), and anyone who wants to use OEM fuel rails will have to make a minor fuel rail tab modification.

When the engine is warmed up, Rich is ready to make some more pulls. He whacks the V-8’s throttle, and the healthy, throaty roar tells us that there is some serious power being made. After brief cool-downs between runs, he makes two back-up pulls.

The before and after dyno chart shows huge power gains from the LSXRT. From a baseline average of 400.1 hp @6200 rpm/370.7 lb-ft @5000 rpm, we were now seeing 429.7 hp @6500 rpm and 386.0 lb-ft @4900 rpm. That’s nearly 30 horsepower and 16 lb-ft of torque over the factory piece!


3400 Democrat Road
Memphis, TN 38118


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