Diesel Tuning and Air-Fuel Ratios

Air-fuel ratios (AFRs) are extremely important and often misunderstood

Words by Andrew Leimroth

If you’re getting your diesel engine tuned or considering increasing the power of your diesel, it’s important to consider the exhaust gas air-fuel ratios (AFRs.)

AFR is the mass ratio of air to fuel present in an internal combustion engine. It is an important measure for reducing pollution and performance tuning. AFR is commonly misunderstood, but it is extremely important if you are towing a heavy caravan or have a heavily loaded motorhome.

I have many years experience in tuning. It is best said that a diesel begins to smoke when the AFR gets below 15-14:1 for a non-turbo and 16-15:1 for a turbo-diesel.

Remember, a diesel doesn’t need a perfect AFR to run and operate, but a petrol engine cannot run much either side of its perfect AFR of 14.7.

In a diesel, too much fuel (a low AFR) means smoke and engine heat. Not enough fuel (a high AFR) means clean running and lower engine temperatures. The following is a guide for tuning diesels.

A non-turbo diesel, such as a Toyota Coaster with 1HZ engine, would generally be tuned in between 15-16:1. This means it is running just on the clean side of exhaust emissions. Any lower in AFR and it may begin to smoke. An aftermarket turbocharged diesel, such as a Toyota LC100 with 1HZ engine, would be best running at around 19-20:1 AFR or higher under full load. The reason for the higher and leaner (oxygen rich, fuel lean) AFR in this application of aftermarket turbo is so that you know the engine will last for a very long time at that exhaust gas level. While it turbos safely, it wasn’t made for turbocharging. A factory turbocharged diesel, such as a Toyota LC100 1HD-FTE or an Isuzu NSeries, would be running an AFR from new of about 20-22:1 or higher under load. While you may get a tune up performed or a chip fitted on your diesel, always make sure you check what the AFR reading is before and after. So often I hear about big power gains had from

a diesel tune or performance modification. Then I find out that there were no AFRs supplied to support the gains and changes made. Remember, for the safety of your diesel always ask for the behind the scenes figures. These include the AFR, to assure you really know ‘how’ the power was achieved.

Safe Driving

Andrew