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Understanding AFR: Step 4 - Evaluating Changes in AFR

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Step 4 - Evaluating Changes in AFR


00:00 Every engine is a little different, and trying to tune every engine with exactly the same AFR target, isn’t always going to work.
00:08 Once we have the AFR dialled in in from the last step, its time to see how the engine responds to different AFR and from this we may decide to settle on a different target.
00:19 There are a couple of aspects that we want to focus on during this evaluation.
00:24 Firstly we want to see how sensitive to AFR the engine power is.
00:28 By this I mean we want to see how much the measured power output changes as we move the AFR richer or leaner.
00:36 I like to try moving both richer and leaner by about 0.2-0.3 lambda to see what the effect is.
00:44 Provided you have tuned using a sensible AFR from the start, you shouldn’t see the power move by more than 1-2%.
00:51 Engine power is usually not very sensitive to AFR and hence you shouldn’t expect to see a large change.
00:58 What this means is that we have some flexibility in setting the air fuel ratio to ensure engine safety and this comes back to how the engine will be used.
01:08 Occasionally though, we will see an engine make better power at a certain AFR.
01:14 The second aspect we need to consider, is how the AFR will effect the knock threshold of the engine.
01:20 As we already found out, as we richen the mixture or add more fuel, the combustion temperature will reduce.
01:27 In a heavily knock limited engine, this can move the engine away from the knock threshold and allow a little extra timing to be added safely.
01:36 If the engine was previously knock limited, this will often result in an increase in power.
01:42 To test this, we can aim 0.2-0.3 lambda richer and adjust the ignition timing to optimise power.
01:51 If you see a positive result from moving the target AFR richer and adding timing, but you are still knock limited, it is worth repeating the process and trying an even richer mixture.
02:03 In general a naturally aspirated engine will start to show a significant drop off in power when tuned richer than approximately 0.85.
02:12 A turbo or supercharged engine will show the same result when tuned richer than approximately 0.75.
02:19 Testing the performance on the dyno will quickly show you how your particular engine responds and optimising the AFR to maximise your engine performance is relatively straightforward.
02:31 So by the end of this section you should have gone through HPA’s 4 Step process to choose the AFR, enter it into the ECU, tune the engine and finally evaluate the effect of different AFRs.