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Practical Standalone Tuning: Idle Mixture

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Idle Mixture


00:00 Once we have the idle timing sorted out, we can look at the mixture.
00:04 The air/fuel ratio you choose will depend to a degree on the engine you are tuning and how modified it is.
00:10 All factory engines are tuned to run at stoichiometric air/fuel ratio or idle to meet admissions targets.
00:17 This also happens to produce a good idle quality and good fuel economy so it isn't a bad place to be.
00:23 You will find with a heavily-cammed engine though that the engine will struggle to idle cleanly at a stoichiometric ratio.
00:30 These engines will run smoother with a slightly richer AFR target so we need to consider this when choosing the mixture.
00:37 As a guideline for a stock engine, then we would normally be aiming for an idle mixture of lambda .95 to 1.0.
00:45 On a modified engine, I would generally target an idle mixture of around .90 to .95.
00:52 If you consistently run much richer than .85, you risk failing spark plugs over time.
00:58 When you're tuning the idle mixture, it's important to make sure the engine is up to normal operating temperature and no compensations are being applied that could affect your mixture.
01:08 We would now adjust our VE map or our main fuel table until our measured AFR matches our target.
01:15 A tip here is to target slightly on the richer sides of the ranges I've suggested.
01:20 The reason for this is after a hot restart, it's normal for an engine to run slightly leaner for a minute or so.
01:27 While there are tables in the ECU that can help smooth this out, if your engine runs leaner after a hot restart, it will feel erratic and rough.

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