Power Enrichment Table
One of the HP Tuners VCM Suite's main tables is "Power Enrichment" in the engine tab. The Power Enrichment Fuel Multiplier vs. RPM table is used to program the air/fuel ratio under wide-open throttle. Calculations are based off a multiplier of a stoichiometric mixture that is equivalent to an air/fuel mixture of 14.7:1. The 14.7:1 A/F ratio is the goal for best emissions and economy at part throttle. The factory programmed wide-open throttle A/F ratio of 12:1 is not optimized for performance, so naturally aspirated ones of between 12.5:1 and 12.9:1 are the goals of modern tuners. Since the stock GTO values are not optimized, additional power can be made by leaning the air/fuel mixture into the target zone. Changes to the Power Enrichment table can also be displayed as deltas, if desired.
Catalytic Converter Overtemp Enrichment Table
Catalytic Converter Overtemp Enrichment (COT) is factory programmed to add a multiplier of fuel (1.35) to the Power Enrichment table. The ECM calculates cat converter core temperature using load/fueling calculations to determine when the catalytic converters have reached approximately 1,787 degrees Fahrenheit. In theory, by adding additional fuel to the engine, its cooling properties will cool the catalytic converter. At a predetermined rpm from the stock tables, the air/fuel ratio will be richened by 135 percent when a COT condition is met, cooling down the converters and reducing the power.
In general, COT protection would only be enabled in the stock programming if the engine was running lean, leading to elevated catalytic converter temperatures. Unfortunately, other conditions, such as a heat-soaked engine on a warm summer day or light detonation, will trigger COT as well. In RPM's experience, COT protection raises its ugly head all too often at the end of a quarter-mile and even when accelerating to pass another car on the highway. The COT protection will be modified to prevent the ECM from creating an over-rich engine condition.
Inside the VCM Suite are various...
Inside the VCM Suite are various tabs, at top, including fuel system, engine and transmission. Under the engine tab, the Power Enrichment Fuel Multiplier vs. RPM table is employed to program the WOT A/F ratio. The stock GTO WOT values (14.69971/1.080.) range from 13.6:1 at 2,750 rpm, followed by increasingly rich mixtures until the A/F ratio is very rich at 12.0:1 from 5,000 rpm to redline.
In order to reach a target...
In order to reach a target of 12.9:1, a factor of 1.140 is entered into the Power Enrichment Fuel Multiplier vs. RPM table. Brian divides stoichiometric (14.69971) by 1.140 and he is commanding the PCM to achieve an A/F ratio of 12.894:1 at WOT. In this case, the changes to the PE table represent leaning the air/fuel mixture by 7.45 percent at the top of the rpm curve. (It should also be noted that simply inputting the correct calculations do not in themselves ensure the targeted A/F ratio. Although the factor is important, oftentimes fine-tuning is necessary to achieve the targeted A/F ratio.)
Catalytic Converter Overtemp...
Catalytic Converter Overtemp enrichment (COT) protects the car from overheating during lean conditions by adding fuel (1.35 multiplier as shown). Since the rich condition cuts power, COT seems to engage at inopportune times. Because short temperature spikes over the intended operating parameters of the catalytic converter will cause no long-term durability issues, RPM and the majority of Gen III/IV tuners disable the protection.
Spark Control Table
Engine timing is programmed on the Spark Control table, which contains a low- and high-octane table. Several factors influence timing including, but not limited to, intake air temperature and coolant temperature. In general, the LS2 seeks the highest available timing between the low- and high-octane tables. Changes to the high-octane table to put in a more aggressive timing ramp and/or to increase the total timing are common methods to increase power. The low-octane table exists to allow the customer to safely run a non-premium gas or withstand a tank of bad gas and is also an alternative to the high-octane table.
A certain amount of knock sensor activityover and above temporary "burst knock" will trigger the ECM to move off of the high-octane table and command the low-octane table. Over the course of many miles and drive cycles, if the octane is adequate, the computer will gradually ramp timing back in until it is in the high-octane table.
Changes to the low-octane table are not as prevalent among late-model tuners, but additional power can be achieved with low-octane table changes, according to RPM. In practice, it is very easy for the ECM to gather enough data to revert the timing onto the low-octane table. This approach requires a delicate balance between the high- and low-octane tables because the knock on the low-octane table does not have a safeguard like the knock on the high-octane table. Therefore, high-quality premium fuel must be used.