So you’ve finished turning your car into a fire breathing monster and now you’re ready to hit the road. There’s just one little problem standing in your way… exhaust emissions. The challenge we have as tuners is that we have to achieve the same level of emissions performance from a 500hp turbocharged monster as your average family hatchback. This guide will talk you through some of the tips and tricks that can be used to improve emissions. If it sounds too complicated, don’t panic! We still offer a remote tuning service to either fully map your car or just optimise your emissions.
Catalytic converters
Unless your car is exempt from requiring a catalytic converter, it is a legal requirement in most countries to have one fitted regardless of whether it passes the emissions test. It is also extremely difficult to pass the emissions test without one, so it is highly advisable to have one fitted to any car that needs to pass emissions testing. The effectiveness of a catalytic converter is dependant on its cell count/density, length and material quality. It is also important to ensure your catalytic converter is hot prior to an emissions test.
Tuning for emissions reduction
Understanding your targets
It’s common sense that running rich is likely to hurt your car’s emissions performance. Not only does this result in increased carbon monoxide and hydrocarbon emissions but it will also coat your catalytic converter in a layer of carbon, causing it to reduce in effectiveness. In some countries such as the UK, running lean can also cause your car to fail an emissions test due to an upper limit on lambda. The first step in optimising emissions to pass an emissions test is to understand the test.
In the example above, we can see that at the natural idle, there is no limit on lambda, whereas for the “Fast Idle” test (car in neutral and RPM raised using the throttle), there is an acceptable range of 0.97-1.03. This is important to understand so that you do not fix the CO and HC emissions by reducing the fuel only to fail on lambda.
Fuel Tuning
AFR Targets
The first and most obvious step in tuning for emissions is to aim for the correct air/fuel ratio, but which cells should you adjust and which should be left alone? Emissions testing on individual vehicles is only performed at no load conditions up to 3000RPM. For cars using an MAP sensor as the primary load, the primary load is typically at 45kPa or less in low load conditions unless the engine has been fitted with exceptionally high lift/duration camshafts. Any cells at a higher RPM than 3000 or higher load than 45kPa can be left alone.
Choosing air fuel ratio targets in the emissions test area is to some extent a judgement call. For a vehicle with strong dynamic compression and a high cell catalytic converter, the “standard” stoichometric air fuel ratio of 14.7 can be used as the target. However, if it is likely that some air and fuel may remain unburnt (eg due to cam overlap or large / poor spray pattern injectors), then the target air fuel ratio can be pushed higher to minimise the amount of unburnt or poorly burn fuel. Typical maximum lambda limit is 1.03, which converts to an air fuel ratio of 15.14. To allow for some slight measurement discrepency and variation in actual air fuel ratio, it is recommended to use a target AFR of 15 for the fast idle area of the map (2000-3000 RPM).
Since there is rarely a limit of lambda at idle, this is primarily a trade off between emissions and idle smoothness. The target AFR below 1500RPM can be safely increased to reduce carbon monoxide emissions as long as it does not cause any misfiring. Note: exhaust gas temperature peaks at ~14.7 AFR and drops above or below that. Running lean at low load will not melt pistons.
Closed loop lambda correction
Of course, optimising the target AFR table is only useful if the ECU is able to reach those AFR targets. A well tuned Volumetric Efficiency (VE) table will ensure that the air fuel ratio is close to the target but subtle variations such as fuel temperature, humidity, barometric pressure or even fuel ethanol content can all cause day to day variations in air fuel ratio even if the injector output is identical. It is recommended to have either a narrowband lambda sensor or wideband AFR gauge/controller connected to the ECU in order to compensate for these variations. A narrowband lambda sensor can only tell the ECU if the AFR is greater or less than 14.7, so a target AFR of exactly 14.7 must be used in all areas where emissions are critical. A wideband AFR gauge/controller (or ME442 built in wideband) gives the ECU a reading of air fuel ratio, so any air fuel ratio target can be used.
The settings below are a good starting point to use for closed loop lambda control in the emissions area of the map:
If using an external wideband AFR gauge/controller, it is important to ensure that the wideband HRT has been calibrated and that the “Lambda Ext. AFR” reading in MEITE matches the AFR reading on the wideband gauge. This is not necessary with the ME442 internal wideband.
VE table correction
After enabling closed loop lambda control, you should see a reading on the “Mapping” tab labelled “Inj. Lambda Trim” that will read a positive or negative % value depending on whether the trim is adding or removing fuel. A large and consistent trim (eg >10%) suggests a significant error in the VE table. You can correct this by adjusting the cells that close to where the engine is running. it is advisable to blend the changes into adjacent cells to avoid spikes or dips in the VEtable. A good VE table should appear smoothly curved in the load axis but can show dips and peaks with changing RPM due to engine resonances.
Ignition timing adjustment
The effect that ignition timing has on emissions is a less obvious one but can have a major impact. A low ignition timing will result in much of the air/fuel mixture being burnt later in the expansion stroke where cylinder pressure is lower. This lower pressure can result in a less complete burn due to a slower rate of reaction and can leave unburnt fuel (HC) or partially burnt fuel (CO) even if sufficient oxygen is present. You can find the maximum ignition timing that should be used experimentally by holding a constant idle duty and adjusting the ignition timing until the RPM stops increasing.
To start with, set the idle mode to manual and adjust the “PWM Manual Duty” to give a steady idle RPM that is close to your target RPM.
Now select a group of cells in the ignition timing table that are close to where the engine is idling
Slowly add ignition timing using the “w” key on your keyboard to increase by 1 degree at a time. You should see the RPM begin to rise. Reduce the manual idle duty in order to bring the RPM back to target and then continue to add ignition timing. Repeat this process until the rpm stops increasing with timing. This timing is known as “MBT”. Because ignition timing is also used for rapid correction of idle RPM, it is necessary to reduce the timing from MBT by ~10 degrees. Press the “=” key on your keyboard with the adjusted cells corrected and type -10 to remove 10 degrees of timing.
The same process can be followed for the fast idle area of the map. In this area, timing is not used for idle RPM adjustment and so a smaller reduction from MBT (eg 5 degrees) can be used.
Once the natural and fast idle areas of the map have been adjusted, the table can be smoothed to eliminate any jumps in ignition timing that may result in a less smooth driving experience. Remember to revert your idle settings back to their original values (eg closed or open loop mode).
Ensure that the idle is able to reach target RPM without applying negative spark scatter. If “Ign. Adv. Spark Scatter” is consisently reading negative by more than 2 degrees after 30 seconds of idling, either reduce the PWM Min Duty in the idle settings or reduce the bypass flow of the throttle body.
Other Causes of emissions failures
A common cause of emissions test failure is a mismatch between what the ECU believes the air fuel ratio to be and what the emissions test equipment believes it to be. This can be due to an inaccurate lambda sensor but is more commonly caused by exhaust leaks mixing fresh air into the exhaust stream. In some cases, the size of the tailpipe can also cause problems. A large tailpipe (particularly one that creates a step change in cross sectional area) can cause fresh air to be sucked back into the tailpipe due to exhaust pulses. If the test probe is too short or cannot reach far enough into the exhaust past the flow step change, it can lead to inaccurate readings.
For cars with individual throttle bodies (ITBs), throttle balance is key to passing an emissions test. If one cyclinder is rich while another is lean, the rich cylinder will increase HC and CO emissions and the lean one will prevent increasing air fuel ratio without failing on lambda limits. A synchrometer is a worthwhile investment for any ITB car owner.
Summary
While the above steps can’t guarantee that your car will pass emissions testing, they will give it the best possible chance of doing so. If done correctly, it should be possible to pass the test with minimal impact on the rest of the map. The high load areas of the map should remain untouched.
If you need some help or can’t get the car to pass, we can offer a remote tuning service to give your car the best possible chance of passing emissions. You can find more information on our remote tuning services and contact us here.