LS engine swaps are all the range, and while there is certainly a case to be made for the power potential of a solid LS-based small-block, the need for complicated electronic fuel injection scares many enthusiasts away. Many companies now offer carbureted intakes for LS applications and thanks to the ignition controller offered by MSD Performance, LS performance is now much more attainable for traditional enthusiasts.
Working with the factory cam, crank sensors and coil packs, the MSD controllers provide the necessary ignition portion of the carbureted performance equation. With fuel coming from the carburetor and intake and spark coming from the controller, hooking up a carbureted LS motor couldn't be easier. With carbs and LS engines being peanut butter-and-chocolate good, what about forced induction? Can the carbureted counterparts enjoy the same boosted benefits as EFI motors? In short, how do you add boost to a carbureted LS?
Being real-world enthusiasts, not only did we want to add boost to an LS motor, but we wanted to do it on the cheap. Anything is possible with an unlimited budget but what about boost for the average guy? With this in mind, we started not with a high-dollar crate motor, but with a junkyard 5.3L LM7. Just as we did with our Modern Mouse series, we purchased a high-mileage, iron-block 5.3L from a local supplier, upgraded it with the intake, carburetor and ignition controller for normally aspirated use, then added the necessary components for turbocharging.
When we say upgraded, we mean only those things that were absolutely necessary, including a turbo kit, intercooler and CSU carburetor, but no changes were made to the long-block itself. Normally we would at least swap out the factory head gaskets and studs for Fel Pro and ARP units, but for this adventure we stuck with the stock stuff. Running a modest boost level, the stock components were deemed more than adequate. The lone upgrade made to the motor was a cam swap, which we tested in normally aspirated and turbo trim to illustrate the gains offered to each.
We purchased the iron block 5.3L for $300. That the motor came without the trans, harness or other components usually involved in a swap did not bother us since it was destined for the dyno use and carbureted dyno use at that. After an oil change to 5W-30 synthetic, the LM7 was equipped with an electric water pump, 1 3/4-inch long-tube headers and the Edelbrock Victor Jr. intake. The intake was installed with a set of Fel Pro intake gaskets, to which we added a Holley 750 HP carburetor, and the MSD ignition controller. An Aeromotive fuel system was on hand to ensure adequate fuel delivery in both normally aspirated and boost conditions.
Having never run this 5.3 before, we were excited when it fired up on the first crank of the starter and settled into a nice idle with plenty of oil pressure. The 750 HP carburetor was equipped with an external jetting system from Percy's, making changes in air/fuel a simple turn of the screw. Dialing in the timing curve was even easier with the MSD, and very soon we were rewarded with peak numbers of 335 hp and 366 lb-ft of torque. In truth, the single-plane Victor Jr. was not the ideal choice for this otherwise stock 5.3L, but we chose it over the more effective dual-plane design for use with the turbo. We were concerned about fuel distribution (and signal) after we added boost.
Though cam swaps on the LS are the easiest in the industry, there is a reason why others wouldn't go to all the trouble to properly test cams on a turbo motor. The cam swap is easy enough, but testing the cam in both normally aspirated and turbo trim required either swapping cams or turbo systems twice. We opted to perform the cam swap once and install and remove the turbo system, allowing us to run both the stock and mild Crane cams in NA and turbo configurations.
Since we already had the factory cam in place, on went the single turbo system from CXRacing. Designed to fit a fourth-gen F-body, we figure the kit might also fit common muscle-car swap applications. The kit was both simple and effective, consisting of dedicated tubular exhaust manifolds, a common Y-pipe to channel the exhaust to the single 76-mm turbo and a 3-inch down pipe. Also available (but not used) was an air-to-air intercooler. On this low-boost application, the carburetor was acting as the intercooler. Having measured temperature drops across the carburetor exceeding 100 degrees on high-boost application, we knew it would suffice on our mild turbo combo. The kit also included a dedicated wastegate set to provide 10 psi and since it was designed for an EFI application, we sourced some additional tubing, silicone couplers and clamps from CXRacing to complete our carb kit.
We ran the normally aspirated 5.3L with a 750 HP Holley but swapped this out in favor of a dedicated blow-through carb from Carb Solutions Unlimited (CSU). Changes by CSU to the standard Holley 850 were considerable, including the boosters, power valve and metering blocks to name a few. Bottom line: every time we used it, it flat out worked. Whether on a mild small-block with a turbo or a massive big-block with a blower, the CSU carb has never failed to deliver the goods. Testing on this 5.3L LM7 was no different. CSU also supplied the necessary carb bonnet to connect our turbo to the carburetor. Despite having come off a blown big block, minor jetting was all that became necessary to dial in the air/fuel mixture on the LM7. With our waste gate spring (no controller) set to provide 10.2 psi, we ran the turbo 5.3 first with the stock cam.
Using the map sensor capability on the MSD, we programmed the controller to retard timing under boost, dialing it back to 22 degrees (from 32). The turbo system provided a slightly rising boost curve and rewarded us with peak numbers of 527 horsepower and 520 lb-ft of torque. Registering a peak of just 10.2 psi, the turbo kit added an easy 192 hp and 154 lb-ft of torque.
After cooling the turbo and associated exhaust system, off cam the Y-pipe to provide access for the cam swap. We chose a mild, streetable cam from Crane. Designed for mild performance street use, the Crane grind offered .585 lift, a 216/224 duration split and turbo-friendly 114 LSA. Swapping cams in an LS (especially on the engine dyno) is something we look forward to rather than dread. There was no need to drop the pan, damage any gaskets or even remove the intake manifold.
After swapping in the cam, we reassembled the turbo system and ran the new configuration in boosted form. Equipped with the new Crane cam, the peak power jumped to 611 hp and 565 lb-ft at a peak pressure of 9.9 psi. The cam swap netted a gain of 84 hp and 45 lb-ft of torque, clearly demonstrating that turbo LS engines respond very well to cam swaps. Now the question was how would these improvements under boost compare to the normally aspirated gains.
Off came the CXRacing turbo system and on went the long-tube headers and 750 Hp carburetor. No changes were necessary to the carburetor, but we did adjust the timing curve to provide 32 degrees (where best power occurred). Upgraded with the Crane cam, the peak power numbers checked in at 411 hp and 393 lb-ft of torque. The power was actually still climbing at our shut off point of 6,000 rpm. In normally aspirated trim, the cam swap was worth 76 hp and 27 lb-ft of torque. This compares to 84 hp and 45 lb-ft of torque with the turbo. We actually expected a slightly bigger difference in power with the turbo, but that is why we go to the trouble to test these theories on the dyno.
What the test did show was that it is possible to take a cheap, carbureted LS motor and add boost. If EFI systems are too complicated or expensive, why not take the alternative fuel route and stick a carbureted LS under the hood of your early Camaro or Chevelle? Need more power? With the right carb and MSD controller, you can always add boost.