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Tuner Tech: Air / Fuel Ratio

Posted by ID Motorsports on

Today we are discussing AFR (Air Fuel Ratio). As a “tuner”, you may be rolling your eyes now, saying “I already know about this”. This may be true, but are your customers educated on AFR when you are performing a remote tune? That’s what this article is geared towards. When the customer are acquiring data logs, they can abort if the engine is in unsafe conditions.

First off, know your stoich!!! This will change for every fuel. For example standard gasoline is 14.64. This is the ideal fuel to air ratio for gasoline. So if you are running gasoline, then this will be your target for idle and part throttle. However, when it comes to full throttle, you will want to see a 15% richer AFR for NA (Naturally Aspirated) . So, if you are running gasoline you will be targeting 12.44 AFR. As you can see, it will be a lower numeric number. The lower the number, the more fuel is consumed. The higher the number, the less fuel is consumed. If you are naturally aspirated and doing full throttle and your AFR is reading 12.9, it is lean and you should abort the pull and send that data to your tuner.

For supercharged engines your will want your AFR 20% richer. If you are on gasoline then you will target 11.72 AFR. It is critical on a supercharged engine to not run lean. As a rule, if during full throttle testing your AFR goes leaner than 12.0, you should abort the test and send that data to your tuner. By not following these guidelines, it may result in engine damage.

These same rules apply for every fuel. For example,States are Ethanol based. You will see at the pump a sticker that says the th On that same supercharged vehicle, your target is no longer 11.72 AFR, it is now 11.25 AFR. Therefore anything leaner than 11.50, the test should be aborted and the data sent to the tuner.

If your wideband is set up for Lambda., it’s just as easy. 1.0 Lambda is stoich. Regardless of fuel, 1.0 will always be stoich. Anything less than 1.0 is rich, anything above 1.0 is lean. For example .96 Lambda is 4% rich and 1.04 is 4% lean. So when looking at full throttle for NA, you will target .85 Lambda. That is 15% richer than stoich. The same applies for supercharged engines. .80 Lambda is 20% richer. If you want to convert Lambda to AFR. Take Lambda and multiply it by stoich. For example, for E10 based fuel, stoich is 14.07. For a supercharged engine, we are targeting .80 Lambda at full throttle. So , we’d take the .80 labda and multiply it by stoick of 14.07, which equals 11.24 AFR. (.80*14.07=11.24 AFR)

Following these guidelines will help keep your investment safe and your tuner happy.

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