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Understanding AFR: Idle, Cruise and High RPM Vacuum

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Idle, Cruise and High RPM Vacuum


00:00 If you are tuning a road car, then the two areas that the engine will spend 95% of its time in are the idle and cruise regions.
00:08 It makes sense that we are going to focus on these areas first as they will have a large influence over how the engine performs as well as the fuel economy.
00:18 If our AFR isn’t on point here, the car will either drive erratically or exhibit poor economy and potentially foul spark plugs.
00:27 In a stock, unmodified engine, you will usually be aiming for lambda 1.00 in both of these areas.
00:34 As we have already discussed, this will give us good fuel economy and more importantly from an OEM perspective, it will minimise emissions.
00:43 If you want to try and achieve a small economy improvement you may prefer to try targeting a leaner lambda of 1.03-1.05 in the cruise areas but I don’t recommend using a leaner target under idle.
00:58 From experience I find that small capacity engines can often feel hesitant and rough when they are run leaner than 1.00 so it always pays to check your particular engine and how it responds.
01:10 Large capacity V6 and V8 engines will generally perform smoothly at lambda targets of 1.05.
01:19 With an aftermarket ECU, we may also want to target a slightly richer than stoichiometric AFR at idle of around 0.95.
01:28 The reason for this is that if the engine is tuned perfectly to lambda 1 at idle, we can often see the engine run leaner after a hot restart.
01:36 This can end up with the engine hunting and running rough until the heat soak dissipates.
01:41 A richer target means that even if the AFR moves slightly lean occasionally, the engine will still be running an acceptable mixture and should perform smoothly.
01:52 If you are using closed loop lambda control, this step isn’t necessary as the closed loop control can take care of any discrepancy during this time.
02:00 In a heavily modified engine, particularly one with large cams with a lot of overlap, you may find that you need to target a richer mixture at idle.
02:09 With these sort of engines, we generally have a lot of reversion occurring at idle and a lot of unburnt fuel will be passing through the engine.
02:17 This effects our AFR reading, making it seem like the engine is running richer than it actually is.
02:24 I usually find that an idle AFR of between 0.9 and 0.95 works best and this often needs to be coupled with a slightly higher idle speed than what you would use in an otherwise stock engine.
02:37 The high RPM vacuum area of the map is an area that the engine won’t be driven in under normal circumstances.
02:44 Nobody really drives a car at 6000 RPM and part throttle for extended periods of time.
02:50 We can’t ignore this area though as the engine will operate here during a throttle lift or perhaps a gear change.
02:58 Given that the engine won’t be held here, we are not going to be so worried about fuel economy.
03:03 Also consider that if the engine is running in this region there is a good chance it is being driven hard.
03:09 In this instance I like to target a slightly richer AFR of 0.95 to help cool the engine when it is in this area.
03:18 Since the engine will only ever operate here briefly, we don’t need to be too fussy with the actual AFR.

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