What is Autotune?

Autotune is the process where the fuel tables in the ECU are altered automatically based on the lambda reading, engine speed and load.  This can save time when tuning the fuel maps for an engine, especially the part throttle areas under normal driving conditions.

Requirements for Autotune

• Hondata s200 system
• RomEditor V3.2
• Lambda meter (wide band such as FJO or PLX)
• Real-time emulator (Romulator)

How Autotune works

The operation of Autotune is fairly simple: Each time you press or hold down the Autotune button (Alt+A), RomEditor looks at the current lambda reading and compares it to the target lambda (as entered in Settings dialog 'Target Lambda').  RomEditor calculates the percentage change to the fuel tables necessary to make the lambda reading the same as the target lambda, then applies the change to the fuel table cells which are currently being read by the ECU. 

To prevent the lambda overshooting the target, the fuel table is only changed part of the way towards the target lambda (this can be set in Settings dialog under 'Percent change per cycle'). Thus over several iterations the lambda will converge on the target lambda.  To prevent the fuel tables being altered when the lambda meter is not reading the real air fuel ratio (for example, when decelerating), you can prevent Autotune for making changes at light throttle or when the lambda readings are outside a certain range (Settings->Autotune dialog).  Finally you can limit the percentage change made to the fuel tables (under Settings 'Maximum change from base map').

Setting up for Autotune

1. Make sure that lambda reading as datalogged by RomEditor is the same as that shown on the display of the lambda meter.  If necessary, adjust the conversion table until the readings are the same.
2. Check the target lambda table is suitable for the engine.  For any engine running closed loop you must set the target lambda to 14.7 for all areas where the engine will run in closed loop.  Normally this is everywhere below about 800 mbar manifold pressure.
3. Check the other Autotune settings are correct.
4. Make sure all ROMs which you use have been updated.  If the ROM has not you will receive a warning when you load the ROM into RomEditor.  If this happens, use the 'Convert ROM Version' menu to bring up the convert/update dialog, then make sure V3 and the correct ECU type are selected and click OK, then save the ROM.  This will update the ROM to the latest version.

Using Autotune

1. Download the ROM to the Emulator. 
2. Set the fuel base maps by using the menu 'Emulator -> Set as Base'.  Note that the base maps are automatically set when the ROM is loaded, but after making manual changes you will need to update the base maps.  You can view the base maps with the 'View -> Autotune Base' menu.
3. Warm the engine up fully, and bring the engine speed to the tuning point.  Press (and release) Alt-A to change the fuel tables. If you want Autotune to cycle repeatedly, hold down Alt+A  You will need to do this over the
4. After using Autotune over an area of the fuel table, it is best to manually edit the table to correct any obvious spikes or dips, then set the table as the base Autotune table and repeat the Autotune process.  Usually two of three passes will result in a fuel table which is close to target.

The final result depends on the experience of the tuner and characteristics of the engine, but we found it was possible to street tune a naturally aspirated engine  from 1000 - 6000 rpm in about 1 hour to without 0.2 air/fuel ratio of target.

Tuning Tips

• Set the 'Percent change per cycle' to about 75%. 
• Set the 'Maximum change from base map' to about 15%
• Try to prevent heatsoak by not letting the engine idle for too long, and keeping the vehicle moving. Heatsoak will generally lean the engine out at idle and make it richer under other conditions.  If on a dyno, use a fan.
• If you find Autotune is overshooting the target lambda and creating a saw-tooth fuel map, first check that you are using the latest ROM version.  Then reduce the rate of change of engine speed so that the engine spends more time on every rpm point - you can do this by using a higher gear (if on the dyno) or dragging the brakes with your left foot (if on the street).
• The best method of using Autotune on the street is to make RomEditor zigzag thought the fuel tables.  To do this start at a low rpm and vary the throttle so that the 2nd and 3rd columns of the fuel table are being read.  Hold down Alt+A, and slowly increase throttle so that the activity indicator goes from left to right smoothly until it reaches column 10.  Release Alt+A, release the throttle, and start again at a slightly higher rpm.  Dragging the brake with you left foot to keep the rpm from rising helps, but beware of overheating the brakes.  After you tune a region of the fuel tables (say from 2000 to 5000 rpm), go back and manually correct and spikes and dips with Interpolate (Alt+E).  Set the base Autotune maps, and repeat if necessary.