Questions about LS tuning for optimum performance have been coming up since the LS1 debuted in the 1997 Corvette. Over the years, various LS siblings and cousins have been factory-installed in passenger cars, SUVs, and pickups, leading to even more questions. While all LS-based engines share the same basic architecture, differences that affect tuning abound. This raises a number of questions, and five of the most common are answered below.

Question: “I’d like to get a little more power out of my Corvette and truck. Both have LS engines. How much of an increase can I expect if I get a custom tune?”

It’s like asking, “How high is up?” because there are so many variables. With a stock vehicle, whether it’s a Corvette, pickup, or SUV, you’re probably not going to see enough of a power improvement—if any—to justify the cost of a custom tune. Reviewing an Engine Control Module’s (ECM’s) calibration data with systems like EFILive’s FlashScan clarifies that.

The ECMs that control LS engines have a lot of capability to optimize engine operation for varying operating conditions. Assuming all other variables are the same, power output is strictly a function of air/fuel ratio and spark timing. There isn’t much to be gained with changes in spark timing because these ECMs have two spark tables (labeled “High Octane” and “Low Octane”), which serve to optimize timing across all operating conditions. The ECM constantly varies spark timing between the settings in these tables, depending on the amount of spark knock it detects.

If you fill your vehicle up with regular fuel (87 octane or lower), the ECM will bias spark timing towards the settings in the low-octane table. Similarly, if the tank is filled with premium-grade fuel, timing will be shifted to favor the settings in the high-octane table. Provided there is little or no spark knock at wide open throttle, spark timing will be very close to, if not right on, the values in the high octane table. Typically, the differences between the values in each table will be 6 to 15 degrees in areas corresponding to high-RPM/high-load operation. The effect of low-octane fuel can be clearly seen by viewing a data log. With low-octane fuel in the tank, actual timing will be well below the high-octane timing table values.

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LS engine ECMs have two spark tables identified as “high-octane” and “low-octane”. Actual spark timing varies between the values in each table depending on the amount of knock detected. As can be seen in this high-octane table, timing is much more aggressive than in the low-octane table. Note the values in the red circle for comparison.

Custom tuning for optimum performance can increase wide-open power by modifying fuel flow. In most cases, the commanded air/fuel ratio at a wide-open throttle is a bit richer than needed to produce maximum power. Factory tunes are calibrated this way to protect the engine from the damage that can result when running at wide-open throttle with an overly lean air/fuel mixture and/or low-octane fuel. The engineers who develop factory calibrations understand that some drivers don’t understand that only diesel engines should rattle under heavy load, so two spark tables are included to minimize the potential for knock-induced engine damage.

It is possible to gain some horsepower by commanding a leaner (than factory-commanded) wide open throttle air/fuel ratio, but engine damage is a potential result. Again, you have to consider the cost/benefit ratio.

With a modified engine, it’s a whole different ballgame. A custom calibration (tune) is necessary when injector size, camshaft, throttle body size, or other significant engine modifications have been made. Any mechanical modification that significantly alters air flow or fuel flow will result in existing calibration data calling for fuel flow and ignition timing that are off the mark.

Question: “When an LS engine is in obvious need of a rebuild or replacement, what are the best options?”

There are two old sayings in the high-performance world: “Speed costs money. How fast do you want to go?” and “There’s no replacement for displacement” (or liters, if you speak metric). With respect to costs, if the existing engine doesn’t have a connecting rod hanging out of the side of the block or any other significant damage, a rebuild is usually the least expensive option.

There are plenty of LS-suitable performance parts available, so whether you’re rebuilding the original engine or installing a replacement, you have the option to improve performance, drivability, and fuel economy. For example, several years ago, we installed a 5.3-liter (325 cubic-inch) truck engine in a 1987 Corvette. At the time, some people criticized us for replacing a 350 cubic-inch. (Tuned Port) engine with a smaller one. However, with a mild camshaft and a “fluff and buff” on the cylinder heads, the engine produced 350 horsepower at the wheels.

We also modified a 5.7-liter LS1 in a 2001 Corvette with impressive results. With a reasonably aggressive camshaft and fully CNC-ported heads installed, the engine produced 450 horsepower at the wheels. With these modifications, we had a textbook example of the need for reflashing the ECM. As a result, even though the exhaust note leaves no doubt there’s a high-performance camshaft installed, the engine delivers smooth power from off-idle to redline.

On the other hand, swapping a 4.8-, 5.3-, or 5.7-liter engine for a 6.0- or 6.2-liter version is a good choice for trucks and SUVs or other applications in which mid-range torque rather than top-end horsepower is the priority. Obviously, replacing an original engine with one of larger displacement requires ECM recalibration to address the difference in displacement and any change in injector flow rate.

 

Remember that early LS engines have a 24-tooth reluctor wheel, while later models use a 58-tooth version. That can be a problem because of ECM compatibility or the lack thereof. If you’re installing a later-model engine in an early-model vehicle, you have two choices: change the reluctor wheel or install an appropriate ECM. You definitely don’t want to do the latter, as the wiring harness and some sensors won’t be compatible with the new ECM.

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With a fully assembled engine, reluctor type can be determined by the color of the crankshaft position sensor. A black sensor identifies a 24-tooth reluctor. A grey sensor identifies a 58-tooth reluctor.

Question: “We don’t have emissions inspections where I live so I’d like to remove the catalytic converters on my Camaro. I’ve heard that the ECM will have to be reprogrammed if I do. Is that true?”

Yes it is- BUT, the requirements to run all originally installed emissions control equipment come from your friends at the federal government. Consequently, even if there’s no emissions inspection in your area, removal of the converters or any other emissions control equipment would be a violation of federal law.

When an LS engine is swapped into a vehicle produced before 1975 (the first year of EPA-mandated use of converters), most owners don’t install catalytic converters. The legality of that is murky. Federal law prevents the removal of any emissions control equipment. However, catalytic converters were most likely not supplied with “crate engines” or engines purchased from wrecking yards, so anyone installing an LS engine in an older vehicle would not be guilty of removing equipment not supplied with the engine. However, EPA or local law enforcement may not see it that way, so it’s best to determine whether requirements for catalytic converters apply to specific engine swap installations.

Assuming a specific LS engine installation does not legally require installation of original emissions control equipment, the ECM will have to be reflashed, or it will set trouble codes relating to the equipment it thinks is installed but isn’t. It will also be necessary to reflash an ECM used in an engine swap to eliminate VATS (Vehicle Anti Theft System).

Question: I want to install a supercharger on an LS engine without adding a bunch of electronics. Can a stock ECM handle the requirements of a supercharged engine?

In most cases, a stock ECM and a supercharger will live happily ever after. The 1999-2006 Silverados and GMC or Sierras have LS1B ECMs, which don’t include provisions for recognizing more than 1 bar (the equivalent of 14.5 psi or 105 kPa) of manifold pressure. To handle MAP (Manifold Absolute Pressure) measurements above 1 bar, it’s necessary to install a 2- or 3-bar MAP sensor and reflash the ECM with the appropriate EFILive custom operating system- which can adequately interpret the readings when manifold pressure is above atmospheric (14.7 psi) maximums.

Later model ECMs (E38, E67, and E92) have the capability to correctly interpret the pressure measurement values produced by 2- or 3-bar MAP readings. Still, in most cases, the MAP calibration data was set for a 1-bar MAP sensor unless the ECM was originally calibrated for a supercharged engine. In that case, the sensor calibration data in the appropriate tables would have to be changed to interpret the voltage output of the 2- – or 3-bar MAP sensor being used.

 

Question: Is it true that the only way to reflash an ECM for an LT1 or LT4 is to send it off to have it modified?

Whether an ECM for an LT1/LT4 engine needs to be physically modified before it can be reflashed depends on the model year. The E92 ECMs in the 2019 model year and older vehicles with LT1/LT4 engines can be reprogrammed using the same process as older ECMs. In 2020, GM switched to E90, E93, and E99 ECMs, incorporating extremely high-level security algorithms. Presently, getting by them is between difficult and impossible, so the most common work-around is sending an ECM off to be modified. In addition to leaving a vehicle undrivable with the ECM removed, the modifications are fairly pricey, and the cost of custom tuning is additional. However, several more reasonable alternatives to current options have been developed, and many tuners are now offering their services for 2020 and later ECMs.