Words and Photos: Richard Holdener
The internet is great for a number of things, not the least of which is spreading rumors. The information superhighway is certainly king when it comes to getting info out to a vast audience, but therein lies the problem. As efficient as it is at getting info to millions of people almost instantly, it is somewhat less efficient at ensuring the accuracy of said info! Whether that information is biographical, political, or extraterrestrial matters not; people love a good story.
Naturally, this simple fact of human nature carries over to the automotive world. Case in point, the introduction of the Dorman composite LS2 intake. Car guys will recognize Dorman as a supplier of a great many items that help us fix things, but they also offer replacement intake manifolds for a number of applications, including the ever-popular LS engine family. In fact, they offer no less than three different intakes for the LS, one designed to replace the TBSS, one for the LS6, and, the one we tested here, designed for the LS2.
While the information about the availability of the Dorman intake is nice and all, where is the conspiracy we want? Well, the word on the street is that the Dorman LS2 intake (pt#615-901) is actually less of a direct copy of the factory LS2 as it is a hybrid of sorts. The internet rumblings seem to suggest the Dorman intake appears to have taken design cues from not only the factory LS2 intake, but also the original FAST manifold. Obviously, none of this information has ever been substantiated (or the question even posed to the folks at Dorman), but a good conspiracy theory needs no facts. In fact, much like Big Foot and the Loch Ness Monster, it is often much better without them.
The great thing about the conspiracy theory is that it leads to a number of ancillary questions, the kind that can only be answered on the dyno. If the Dorman manifold is indeed an improvement over the original LS2 (an easy job given the LS2 is only marginally better than the LS1), could it actually be better than original FAST? I mean come on, it does have a larger throttle opening!
To find out, we set up some good old-fashioned, intake-on-intake action to compare the two manifolds. Actually, we ran three manifolds, as, like any good one, the conspiracy theory was not super specific about which FAST intake the design cues were lifted from (obviously, if at all).
The test was run on a 5.3L that came from the same source of all great test motors, our local LKQ wrecking yard! Rather than run the truck motor in stock trim, we made a few upgrades prior to the test. The high-mileage 5.3L LM7 was first treated to a cam upgrade in the form of a 54-454-11 grind from COMP Cams. The healthy hydraulic roller cam offered .614./.624 lift split, a 227/243-degree duration split, and 113-degree lsa. The cam was teamed with the necessary 26918 valve-spring upgrade to ensure both coil bind clearance and plenty of RPM potential. The test mule also received long-tube headers, 89-lb FAST injectors, and a FAST XFI management system. All three of the intakes were run with the same air/fuel and timing curves.
To get things started, we installed the Dorman LS2 intake with a FAST 92mm throttle body. The intake also required the use of different 83-lb injectors fed by a Wilson fuel rail. One thing we didn’t care for on the Dorman intake was the lack of threaded inserts to secure the fuel rails. The kit came with self-tapping screws, but in its defense, the Dorman was priced much more affordably than the FAST.
Equipped with the Dorman and tuned to perfection, the modified 5.3L produced 465.5 HP at 6,600 RPM and 412.5 lb-ft of torque at 5,300 RPM. We then replaced the Dorman with an original FAST/Wilson intake that featured the minuscule 78mm throttle opening. The OG FAST was run with the same injectors, but a much smaller 78mm Accufab throttle body. Despite the handicap in throttle body size, the FAST intake produced 471.7 HP at the same 6,600 RPM and 413.7 lb-ft of torque at 4,900 RPM.
Obviously, the FAST held its own against the Dorman, but just to be sure, we threw the big-boy, 102mm LSXRT intake in the mix. Run with the LSXRT, the 5.3L produced 479.2 HP at 6,800 RPM and 421.6 lb-ft of torque at 4,900 RPM. The big FAST offered more peak horsepower and torque, but is the power difference enough to guard the door when it comes to cost?
Graph 1: 5.3L Intake Test-Original FAST vs Dorman LS2
The talk around the shop (meaning the internet) is that the Dorman LS2 intake might have taken design elements from both the factory LS2 and FAST intake. Tested against the original FAST/Wilson intake, the Dorman did well, but couldn’t quite match the original in terms of peak power. The modified 5.3L produced 465 HP and 412 lb-ft of torque with the 90mm Dorman, but these numbers increased slightly to 471 HP and 413 lb-ft with the 78mm FAST.
Graph 2: 5.3L Intake Test-FAST LSXRT vs Dorman LS2
Despite the fact the original FAST held up the family honor, we decided to check out how the FAST LSXRT compared to the new Dorman. Unlike the original FAST, the LSXRT featured a 102mm throttle opening and throttle body, but the intake design itself was different, as well. Where the Dorman LS2 produced 465 HP and 412 lb-ft of torque, the LSXRT stepped things up to 479 HP and 421 lb-ft of torque. The modern FAST not only offered more horsepower but more peak torque as well.