5.3L LS Small Block Build - Twice As Nice: Part 5

Modern Mouse, Part 5: Getting Carbs

Richard Holdener Mar 14, 2012 0 Comment(s)
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Modern Mouse has a busy life. Just a few short weeks ago, our 5.3L LS-based 383 stroker engine was sporting a massive Whipple supercharger and pumping out over 800 hp. Superchargers are capable of some serious power, but the fact that the big numbers came from less than 10 psi of boost made it all the more rewarding.

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Before that, we saw additional displacement and a wide variety of different bolt-ons. Having run through cams, intakes, and a couple of power adders (oh, there will be more), we decided it was time to cater to the carbureted contingent.

Given the popularity of LS engines as swap candidates in early muscle cars, it seemed only natural to take a look at carburetors. While a number of carbureted intakes are currently available for LS applications, Holley recently introduced a trick dual-quad intake for LS engines equipped with cathedral-port heads. Since man does not live by fuel injection alone, we decided it was high time to equip Modern Mouse with some old-school induction.

As much as we love the original small-block, it has just been so easy to make power with Modern Mouse. So far, we have easily exceeded 500 hp in normally aspirated trim, and topped 800 hp with boost. More than just big (one-time-only) dyno numbers, these came as realistic and wholly usable outputs that could be enjoyed on a daily basis. We demanded no less from our carbureted intakes, despite the fact that dual-quad induction systems are often chosen on looks alone. It's hard to argue with the performance and reliability of a single carb on any engine, but they certainly rank behind a trick dual-quad setup when it comes to the all-important "wow" factor. The problem with multi-carb induction is that they often lag behind the single-carb setup in terms of driveability and performance. For some, looking fast takes precedent over actually being fast, but what is wrong with wanting both?

To illustrate that it is possible to offer both form and function, we took Holley up on its offer to test its new dual-quad intake system. Designed to fit LS applications with cathedral-port heads, the new mid-rise, dual-quad intake was listed for LS engines up to 427 cubic inches with an effective operating range to 7,000 rpm. The mid-rise design positioned the intake between a standard low-rise, dual-plane and a dedicated (high-rise) tunnel ram. Holley now offers a dual-quad tunnel ram for rectangular-port LS3 applications, which we have tested with excellent results.

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Having run that tunnel ram on a stroker LS3, we were anxious to see what the mid-rise design had to offer on our 383 stroker. Our one (very minor) complaint about the intake involved carburetion. Holley designed the intake to receive a pair of 4160 carbs or (of all things) Carter AFBs (or Edelbrock equivalents). Some feel it would be sacrilege to run anything but Holley carbs on a Holley dual-quad intake, but that said, we must admit that the induction system worked extremely well with the pair of 390-cfm 4160s.

Rather than compare the dual-quad intake to the FAST (or other) EFI system, we decided to select a typical carbureted single-plane intake. We figured that the choice would be less about carburetor versus EFI than different carbureted intakes. We also wanted to see how well the dual-quad induction stacked up against a known entity. For this test we selected what is considered by many to be the mainstay of single-plane manifolds, the Edelbrock Victor Jr. intake. We know from experience that a single-plane intake will usually offer impressive top-end power but often sacrifice low-speed torque. For most street applications, the dual-plane is a better choice thanks to improved torque through most of the rev range, but it is hard to top the single-plane intake for peak power.

Naturally the single-plane intake was run with another Holley carburetor: in this case a 750 HP equipped with Percy's external Adjust-a-Jet system. The external adjustment screw eliminated the need for bowl removal to facilitate jet changes to dial in the air/fuel mixture.

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Loyal readers of Super Chevy should be familiar with our test rodent, but a brief rundown will be provided for those who just tuned in. What started out as a bone-stock 5.3L eventually evolved into a TFS-headed 383 stroker thanks to a 3.902-inch bore and 4.0-inch stroke combination from Procomp Electronics, Probe Racing, and L&R Automotive. The stroker kit kicked up the displacement from 324 cubic inches (5.3L) to a full 383 inches (6.27L). The GenX 215 heads from Trick Flow Specialties combined with an Xtreme Energy cam from Comp Cams and a Fast LSXR intake to push the power output well over 500 hp.

Running nitrous back in Part 2 produced over 600 hp, and we even managed to coax over 800 hp with the help of a Whipple 2.9L twin-screw supercharger. Modern Mouse had since lost the supercharger, but was still sporting the GenX 215 heads and Comp cam. Obviously, the FAST LSXR intake would give way to the dual-quad counterpart from Holley, but not before establishing a baseline with the single-plane Victor Jr. intake.

The 383 stroker was first equipped with the Edelbrock Victor Jr. intake. The intake was combined with a Holley 750 HP carburetor and run using an MSD LS ignition system (PN 6010). Designed as a plug-'n'-play system, the MSD ignition offered map-based, programmable timing curves and direct connection to the factory cam, coil, and crank sensors. All that was necessary was to hook up power and ground and your carbureted LS was up and running—what could be easier? Run on an engine dyno, Modern Mouse was equipped with a Meziere electric water pump and a set of 1-3/4-inch headers feeding Magnaflow mufflers. After testing a few different timing levels (30 degrees worked best) and minor tuning to the air/fuel curve via the Percy's external Adjust-a-Jet system, we were rewarded with peak numbers of 527 hp at 6,000 rpm and 484 lb-ft of torque at 5,100 rpm. Indicating the high-rpm nature of the single-plane intake was the fact that torque production exceeded 460 lb-ft from 4,400 rpm to 6,000 rpm.

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Having successfully converted Modern Mouse to carburetion, it was time to see if the dual-quad was indeed "twice as nice" as the single-plane. Off came the Victor Jr. and on went the Holley dual-quad intake. Manifold swaps were a breeze on the LS motor, as the swap involved disturbing neither a distributor nor any water passage.

The dual-quad intake was teamed with a pair of vacuum-secondary, 4160 carbs rated at 390 cfm each. The pair of 390s (780 cfm) nearly equaled the flow rating of the 750 Holley employed on the Victor Jr. It is possible to install larger carbs on the dual-quad intake, but the twin 390s performed admirably on the 383 stroker.

After a single jet change, we were rewarded with peak power numbers of 526 hp at (a slightly higher) 6,200 rpm and (more importantly) 490 lb-ft of torque at (a lower) 4,700 rpm. Not only did the dual-quads match the top-end power of the single-plane setup, but it offered more peak and average torque production.

Below 5,000 rpm, the dual-quad easily outpowered the single-plane, with gains exceeding 50 lb-ft near 3,200 rpm. Not very often do you get to combine form and function in one exciting package, but if this test is any indication, we should be seeing a lot more LS motors roaming the streets with these new dual-quad Holley systems.

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13. Dual-Quads Vs. Single Four-Barrel
The typical comparison between a single-plane and dual-plane intake involves a trade-off. Generally speaking, the single-plane offers more power at the top of the rev range, but the dual-plane provides considerably more torque at lower (and medium) engine speeds. This usually involves a choice of where you are most concerned about power production. In this case, the dual-quad Holley eliminated the need to choose, as it offered every bit as much peak power as the single-plane Victor Jr. but considerably more low-speed and mid-range torque. Dual-quad intakes are often more about looks than performance, but this Holley dual-quad LS intake offered both. Measured peak to peak, the dual-quad was up by just 6 lb-ft (490 lb-ft versus 484 lb-ft), but down near 3,000 rpm the dual-quad offered an additional 50 lb-ft of torque. The difference in the curves despite relatively similar peak numbers is why we go to the trouble of including a complete power graph (or numbers).

Sources

Trick Flow Specialties
Tallmadge, OH 44278
330-630-1555
www.trickflow.com
L&R Automotive
Sante Fe Springs, CA 90670
562-802-0443
www.lnrengine.com
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