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Understanding AFR: Setting the Stoichiometric AFR

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Setting the Stoichiometric AFR

04.26

00:00 - Now that we've discussed the concepts of AFR and lambda, we're going to talk about how a wideband meter can be used, when tuning on fuels with different stoichiometric AFRs.
00:12 First of all, we need to understand that the wideband meter doesn't have a clue what fuel you're running your car on.
00:18 All it knows is the quantity of unburnt fuel, or oxygen, in the exhaust system.
00:24 Essentially, the wideband meter is measuring whether we have an excess of oxygen, or fuel, in the exhaust system.
00:32 When the exhaust case components are perfectly matched, and we don't have an excess of oxygen or unburnt fuel, the engine is operating at stoichiometric AFR.
00:42 Since the wideband is measuring how far away from the stoichiometric AFR the exhaust case is, it's actually natively measuring in units of lambda.
00:52 This is a fundamental concept, and most tuners incorrectly assume that the wideband is measuring directly in units of AFR.
01:02 For the wideband meter to be able to display in units of air fuel ratio, we must tell it what the stoichiometric AFR for the particular fuel we're running on is.
01:14 The wideband meter can then convert from lambda, and display AFR.
01:19 For example, if the wideband is measuring lambda at 1.0, then it would simply multiply this by the stoichiometric ratio programmed into it, which let's say is 14.7 to one.
01:32 The result would be displayed as 14.7 to one AFR.
01:37 So if you now want to use the wideband meter on E85, and you want an accurate measure of the air fuel ratio, you'd need to adjust the stoichiometric AFR to 9.8 to one.
01:49 Now at a measured lambda of 1.0, the meter would display 9.8 to one.
01:55 Most tuners around the world tend to be lazy, so instead of constantly adjusting the stoichiometric AFR, they simply leave it always set to 14.7 to one.
02:06 There's nothing specifically wrong with doing this, provided you understand the implications.
02:12 If you don't have a solid understanding of the implications though, you can get yourself into a lot of trouble, so let's discuss it in more detail.
02:21 Leaving the stoichiometric AFR set permanently to 14.7 to one as we switch between different fuels, really has the meter working the same as if we're using units of lambda.
02:32 This means that if the engine is running on pump fuel at 0.85 lambda, it will always display 12.5 to one, regardless of the fuel.
02:42 This means that if we switch to E85, and tune the engine to 0.85 lambda, The meter will still display 12.5 to one, because the meter is still multiplying the measured lambda value of 0.85 by the programmed stoichiometric AFR of 14.7 to one.
03:00 In reality though, the actual AFR is 0.85, multiplied by 9.8 to one, which equals 8.3 to one.
03:08 As you can see, it's a big difference.
03:12 Probably the two most common fuels in use right now are pump gas and E85.
03:18 When tuning on the lambda scale, both usually perform well at very similar target numbers of around 0.90 for a naturally aspirated engine, and around 0.80 for a turbo charged engine.
03:31 This means that if we leave the wideband meter scaled for a stoichiometric AFR of 14.7 to one, we can just aim for the same numbers on both fuels.
03:42 13.2 to one for naturally aspirated, or 11.8 to one for a turbo engine.
03:48 It isn't technically the correct way to use a wideband, but this is what many tuners do, and to save confusion when talking with other tuners, it's important to understand it.
03:59 This is just one more reason why I always recommend learning to work in units of lambda is there can be no possible confusion.
04:09 If you want to use units of AFR for your tuning though, decide early on if you'll adjust the stoichiometric AFR to suit the fuel, or leave it fixed.