Tuning Out Rev Hang for Faster Gear Shifts

Many vehicles come tuned from the factory to have a gentile decrease in engine RPM when the clutch is depressed and the throttle is released. This allows the driver time to comfortably, and slowly, change to the next gear with a smooth transition. Once you are able to execute a quick and smooth gear change, you may start to notice that you are having to wait for the engine to slow down to rev match each up-shift. It is possible to increase the rate at which RPM drops after releasing the clutch by retarding ignition timing. This has a similar effect to a lightweight flywheel. In this article, I will be specifically modifying my Stage 2 tune for a 2004 Subaru Impreza WRX, but the concept applies across all gasoline internal combustion engine vehicles.

Requirements for this modification

1. Tactrix Openport or another means of connecting to the ECU
2. Reliable x86 based laptop or tablet PC running Windows for flashing the ECU and logging parameters.
3. ECUFlash software for flashing the ECU.
4. RomRaider for logging and monitoring ECU parameters.

Pros

1. Decrease up-shift time.

Side effects

1. Possible exhaust crackle, after-fire, and flames.
2. Harsher on/off throttle transitions.

Getting Started

Before you can start modifying your ECU you will need to read the ROM from the car's ECU and save it if you do not already have a copy, preferably on multiple drives to prevent unexpected data loss. You will need the original ROM in case you need to revert the ECU back to how it was before in the future. Make a copy of your ROM and open it in ECUFlash or your preferred ECU tuning software.

Ignition Timing

Retarding ignition timing to the point that combustion of remaining fuel happens during the power stroke causes the engine to produce no, or little power. Doing this during gear changes creates a similar effect to having a lightened flywheel, RPM begins to fall faster after releasing the clutch. For my Subaru ECU, the map I will be adjusting is called "Base Timing." Timing is reduced from between 0 degrees to -15 degrees starting at 2400 RPM. Timing is smoothed out to make the large timing transition more gentile over the RPM range. Leaving positive ignition timing advance below 2400 RPM allows the engine to act normally at low RPM making driving slow easier and prevents exhaust pops and flames while they may be undesirable. Please note that extreme timing jumps can introduce knock, jumping from 0 degrees or less all the way to 45 degrees is a big change. Use less aggressive retarded timing or smooth out the map into higher load ranges if the ECU is applying knock correction when blipping the throttle.

In addition to adjusting the base timing map, check any knock correction timing maps and ensure that timing is set to 0 degrees in the same load ranges modified in the base timing map. This will ensure the total ignition timing will not have any positive timing adjustments. For my Subaru, this is in the "Knock Correction Advance Max" map. Also retard timing in the "Base Timing Idle(In-Gear)(Above Speed Threshold)" map to ensure timing is also retarded when there is no throttle input and no load. I did not adjust the "Base Timing Idle (Neutral)" map because the maximum RPM value is 2000 and I do not want flames and pops coming from my exhaust while coasting out of gear.

Fueling

Using negative ignition timing can create excess heat. Log your vehicle to ensure exhaust gas temperatures are not increasing to dangerous levels while running the car hard through all the gears. Additional fuel can be added in this range to help cool the combustion chamber. Leaning out the air/fuel mixture will increase exhaust pops/crackles and potentially increase heat temperature.