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EFI Tuning Fundamentals: Lean Tuning Myths

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Lean Tuning Myths

04.26

00:00 - At this point you should be getting the idea that choosing the right air fuel ratio for a particular engine isn't black or white.
00:06 There's no single magic number that you can apply to all engines and all situations.
00:11 You need to consider the engine design, the fuel and what the engine is being used for before you make your decision.
00:18 Getting the air fuel ratio wrong can quickly result in expensive damage to engine components.
00:24 Melted pistons, melted exhaust valves or even pistons missing most of one side can all be the result of getting it wrong, however often I find the diagnosis of what has caused an engine failure is incorrect and the wrong conclusions are jumped to.
00:39 Let me give you an example of what I mean.
00:41 One of the most common myths I hear is that lean mixtures blow up engines.
00:46 This actually couldn't be further from the truth and I want to spend a little time investigating this further.
00:52 As the air fuel ratio becomes leaner combustion temperature increases and combustion speed increases.
00:58 As we saw in the last module, if we continue to reduce the amount of fuel injected eventually we'll also begin to see a drop off in power.
01:05 Contrary to common belief the most dangerous aspect of a lean mixture is that the engine will become a lot more susceptible to suffering from detonation.
01:14 The higher combustion temperatures coupled with the faster combustion speed can exceed the ability of the fuel to suppress detonation.
01:23 In my experience, it's detonation that most often causes the damage that results from a lean mixture.
01:30 It can be tempting to look at an engine which has had the side melted out of a piston and decide that the damage has been due to excessive temperature thanks to a lean mixture.
01:40 More often than not though, the truth is that the damage is due to detonation stripping away the boundary layer of gases that protects the piston from the full heat of combustion.
01:50 This can very quickly melt the piston causing a huge amount of damage.
01:54 In some motorsport environments like endurance racing, it can be an advantage to tune an engine with what would normally be thought of as a very lean mixture.
02:03 Often teams will do this, even at the expense of a little horsepower to gain an advantage in pitstop strategy over the course of a long race.
02:11 The difference is that these teams understand the effect of this lean mixture and what it will do to the engine and they make allowances for this in the design and the tuning.
02:21 Let's have a look at how they could approach this.
02:24 Firstly let's consider the engine design.
02:27 A lean mixture will result in a higher combustion temperature and higher exhaust gas temperature.
02:33 This will result in the pistons and valves operating at a higher temperature and this heat needs to be managed somehow.
02:39 The higher piston temperatures will result in more expansion of the piston and this may require and increase in the clearance between the piston skirt and the cylinder wall.
02:49 Thermal coatings may also be added to the crown of the piston to help reject more of the combustion heat.
02:55 The valve temperature may be handled by using superior metals for the valve construction, such as stainless steel or Inconel.
03:02 Sodium filled valve stems may also be used to help transfer the heat away from the head of the valve.
03:07 The actual valve seats may also be cut wider which allows more heat transfer from the valve to the cylinder head when the valves are closed.
03:16 Now let's look at the tuning approach.
03:18 Since we know the engine will be working with a much higher combustion temperature, it's important to consider how this will affect the tuning.
03:24 Firstly the higher combustion speed will require less ignition advance to achieve maximum torque.
03:31 Secondly, the engine may also be more prone to detonation and we'd want to account for this by reducing the ignition advance to provide a larger safety buffer.
03:41 So with this in mind it's quite possible to take a naturally aspirated race engine and tune it to run with an air fuel ratio as lean as perhaps 0.95 to 0.98 Lambda.
03:52 What's important though, is that the engine and tuning strategy have been designed around managing the heat that this will produce.
03:59 You can see that there's a little more to tuning than just selecting an air fuel ratio and hitting the track.
04:05 There's no magic number to aim for and we have a lot of decisions to make as there may be several different approaches to getting a safe and reliable tune.