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LS1 Carburetor V.S. Computer Performance Test - Retro Modern Motivation

The Eternal Fight: Carbs vs. Computers.

Richard Holdener Apr 1, 2011
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Who isn't a fan of the original 23-degree small-block Chevy? After all, it revolutionized the automotive aftermarket and has truly earned its legendary status. For those new to the Chevy scene, ask any small-block owner and chances are they have at least one cool story involving their beloved mouse motor. Never mind all the past racing success in nearly every form of professional and amateur motorsports, the original small-block continues to be a mainstay of the performance aftermarket industry, despite being phased out in favor of the modern LS family.

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The LS engine series offers everything we knew and loved about the original and made it not only more powerful, but lighter and more environmentally friendly to boot. As much as we loved the Gen 1, the LS is certainly a worthy successor.

The LS looks bound and determined to make its own mark in history. Despite an impressive combination of power, mileage and low emissions, there are many Chevy enthusiasts who refuse to embrace the LS family for one simple reason: They were brought up on carburetors and electronic fuel injection is simply too complicated.

Having run untold numbers of both small-blocks and LS motors on both engine and chassis dynos, we can vouch for the fact that setting up any EFI motor is considerably more complex that its carbureted counterpart. From a sheer simplicity standpoint, it is tough to beat a carbureted engine. That ease of installation has carried over to the new LS engines, thanks in part to companies like Edelbrock, who have seen fit to produce carb conversions to replace the factory fuel injection. More than just bolting on a carbureted intake and calling it good, the Edelbrock conversion includes a plug-and-play ignition system that connects directly to the factory coil packs, cam, and crank sensors.

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It is certainly true that simplicity is on the side of the carbureted combo, but does the modern EFI system have anything to add? In truth, a factory or even aftermarket EFI system has a lot going for it. That the OEMs have abandoned the carburetor in favor of electronic fuel injection should tell you something about its potential. The great thing about a sophisticated system is the ability to properly dial in the air/fuel ratio and timing curves for any combination of load and engine speed. The carburetor is very effective at metering fuel under most conditions, but it can never match fuel injection for precise metering under all conditions.

The one benefit carburetors often have over their fuel-injected counterparts is power production at wide-open throttle-assuming the same type of intake manifold is used. This advantage comes not from fuel metering as much as the position of the fuel supply. A carburetor supplies fuel to the top of the intake manifold, where it provides a significant drop in the inlet air temperature. Typical fuel injection systems (other than TBI) inject the fuel directly into the intake or head port (usually aimed at the back of the valve). This greatly minimizes the cooling effect offered by the carburetor.

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All the speculation and conjecture are all well and good, but what happens when you run the same motor with both fuel injection and a carburetor in a back-to-back test? Actually this test was not designed to crown a winner, as both systems provide benefits that may be more or less desirable to the individual. You will never convert a die-hard carburetor guy over to fuel injection, nor will a true-blue EFI guy likely replace his system with a carburetor. Rather than provide an answer to the "Which is best" question, we decided to provide the data from both systems and let the readers judge for themselves. Along the way, we decided to set a goal for ourselves and see if we could reach the magical 500hp mark using nothing more than a stock 5.3L short-block. Sure, it would be easier with a 5.7L LS1, LS6 or even a 6.0L LS2, but the 5.3L LS motors are considerably more affordable, and making big power with fewer cubes is all the more rewarding.

Sucp_1104_13_o LS1_carburetor_vs_computer_performance_test Running_with_the_carb 14/22

Our 5.3L test mule was yanked from a local wrecking yard and then rebuilt back to factory specs by replacing the stock rings and bearings. Truth be told, the motor could have run as is, but we decided to freshen it up since it would be seeing more than its fair share of dyno time. The bores were lightly honed but care was taken to allow use of the standard bore-size pistons (we reused the originals). Run on the dyno in stock trim, the injected truck motor produced just over 350 hp and 380 lb-ft of torque.

First on the dyno was our carbureted combination. As indicated previously, Edelbrock supplied one of its Performer RPM LS1 conversions that included a plug 'n' play ignition system. The Performer RPM intake was ideally suited to an engine designed for street use, as the dual-plane design offered exceptional torque production, especially below 4,000 rpm. The Edelbrock combination was topped off by a Holley 750 HP carb. Rather than run the stock 5.3L with a simple carb swap, we decided to upgrade it with a set of ported heads and high-performance cam.

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Since this was intended for street use, we chose our cam profile with both performance and streetability in mind. A little searching brought us to the XFI RPM Hi-Lift section of the Comp Cams catalog, where we found the XR281HR. This cam featured a .571/.573 lift split, a 228/230 duration split, and a 112-degree lobe separation angle. This cam is skewed toward the performance end of our performance street description (especially for the 5.3L), but we have run this profile successfully in the past and for a stick car it really rocks.

Compared to the 5.7L, the 5.3L LS motor offers a smaller bore size (3.780 vs. 3.898) and as such requires different cylinder heads for optimum performance. The production 5.3L heads feature smaller combustion chambers to help maintain compression ratio, and the installation of most factory LS1-LS3 heads will result in a drop in compression ratio-ditto for many aftermarket heads.

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Knowing we required a dedicated set of cylinder heads, we went right to Trick Flow Specialties. Our needs were met by a set of Trick Flow GenX 205 cylinder heads. In addition to the generous (but not oversized) 205 cc intake ports (flowing nearly 300 cfm) and a small-bore-friendly 2.0/1.575 valve combination, the Gen X 205 heads also featured ideally sized 58cc combustion chambers to ensure no loss of compression. We needed a small-chamber cylinder head that offered exceptional flow with minimal cross section and the Gen X 205 heads fit the bill perfectly. That they came fully assembled, bolted right on and worked with all the factory valvetrain hardware was something we have come to expect from Trick Flow.

With our heads, cam, and intake needs met, all we had to do was install everything onto the awaiting 5.3L short-block and run it in anger on the dyno. Installation went smoothly and required only new Fel Pro head gaskets (5.7L bore size for use with the Trick Flow heads) and ARP head studs. The factory O-ring intake gaskets were swapped over from the truck manifold to the Edelbrock intake and in no time our modified 5.3L was up and running. It should be noted that we used the factory rockers in lieu of a set of aftermarket roller rockers and retained the 1 3/4-inch long tube headers and Meziere electric water run on the stock 5.3L.

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Equipped thusly, the output of the 5.3L jumped to an impressive 462 hp at 6,600 rpm, while torque production now stood at 413 lb-ft at 5,200 rpm. The little 5.3L belted out over 400 lb-ft from 3,900-5,900 rpm, making for one heck of a solid street performer. The healthy Comp cam made itself known by allowing the motor to produce peak power fairly high in the rev range, but even down at 3,200 rpm, the 5.3L still offered 330 lb-ft of torque. It should also be noted that this carbureted 5.3L fired up on the first attempt after hooking up the Edelbrock conversion, an indication that ease of installation was high on the list when developing this system.

With our carbureted combo tested, it was time to swap over to fuel injection. To that end we replaced the Edelbrock intake and Holley carburetor with a 90mm FAST LSX intake and matching 90mm MSD throttle body. The FAST LSX intake was run with a set of 36-lb injectors and a Wilson billet fuel rail, though the stock rail would be more than sufficient at this power level. Naturally, the FAST LSX induction system was run with a FAST XFI management system. Westech's Ernie Mena had the system up and running in record time and after a few quick power pulls, we were rewarded with impressive peak power numbers. Obviously the 5.3L combination responded well to the FAST LSX intake combination, as the little LS motor thumped out 482 hp at 6,600 rpm and 436 lb-ft of torque at 5,300 rpm. The FAST intake improved the power output everywhere compared to the carbureted combination, but the EFI combination would require an expensive management system or its stock or other aftermarket equivalent. From a cost standpoint, the carbureted combination would ultimately be less expensive, but it is hard to argue with the extra horsepower and torque offered by the FAST LSX combo.

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Normally the story would end here with our carb vs. computer comparison, but 482 hp seemed ever so close to the magical 500-hp mark. Looking down into our bag of tricks, we found a few that might help us at least get closer to the elusive 500hp number. Remember, the 5.3L short-block was bone stock, which meant we were still running a static compression ratio of roughly 9.5:1. A bump up to 10.5:1 would likely get us to 500 hp or at least very close, but a compression hike was not in the cards. With all of our hard parts and tune previously optimized, we turned to a trio of tried and true racing tricks, namely cooling the water temperature, lightweight oil, and some VP Q16 (oxygenated) fuel. Motors always make more power when the oil is hot and the water temperature is cold. Lightweight oil helps minimize frictional losses and the power required to drive the oil pump, while the Q16 race fuel increased the number of oxygen molecules to the motor. More oxygen equals more power. With minimal dyno time available, we threw everything we had at the motor at one time and were rewarded with (drum roll) 501 hp and 452 ft-lb of torque. Imagine a stock short-block 5.3L producing 500 hp with just a set of off-the-shelf Trick Flow 205 heads, a streetable hydraulic roller Comp cam, and FAST LSX intake (and a few strip tricks).

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Trick Flow Specialties
Tallmadge, OH 44278
Comp Cams
Memphis, TN 38118
Demon Engines
Sante Fe Springs, CA
L&R Automotive
Sante Fe Springs, CA 90670
Holley Performance Products
Bowling Green, KY 42101



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