Lambda = proxy for 'safe tune'?

[The American]

I've attached a copy of a dyno run for my car - Is it true that a lower lambda number is richer, therefore cooler/safer tune? How much headroom is there in these results safety wise? I'm planning to run this car on track, and don't want to worry if it is bouncing off the limiter.

MX5 Dyno.pdf

[Locutus]

Yes, lower lambda is richer and generally safer.

As for headroom, there are at least half a dozen other factors to consider such as timing, compression, etc. and every engine is different. Best way to determine this for your car is on a dyno.

To answer your question, the easiest way to introduce safety is to run your richer map and retard timing.

[Magpie]

Get a proper tune is the best advice!

http://www.endtuning.com/afr.html

Inside the cylinder will be a cloud of vapour, with some patches richer in fuel, and others leaner. The fuel can be accurately controlled, so the limiting factor in a Normally Aspirated petrol engine is the amount of air. To achieve the best power, all of the air has to be used as efficiently as possible, so the mixture will be enrichened by adding more fuel. The AFR for maximum power is usually between 12-13:1, depending on how well mixed the vapour was beforehand.

The extra fuel increases the likelihood of the free oxygen being closer to a fuel molecule for the reaction. Some fuel will be wasted as there is an excess in the cylinder, but it helps ensure all of the air is used to it's full effect.

Likewise, running lean is better for economy. Where economy is concerned, you are looking to use all of the fuel injected by having an excess of air. Free oxygen taken into the cylinder will be wasted but the priority for an economic tune is to avoid wasting fuel.

Lean running will increase the temperatures of the combustion, and will also make more of the Nitrogen in the air oxidise into Nitrogen Oxides (NO and NO2, Nitrous Oxide is N20 and a different kettle of fish). The excess heat if unchecked can cause engine and Catalytic Converter damage, and the NOX is an undesirable air pollutant.

Rich mixtures are cooler but you can see the increased Hydrocarbon emissions as the excess fuel is unused. Nitrogen oxides are low from the cooler temps, but Carbon Monoxide is far higher with the lack of free oxygen to convert the CO to CO2.

Lean mixtures around 16:1 AFR produce the best economy, but the extra heat oxidises the Nitrogen in the air increasing air pollution, but with low CO levels. Leaning the mixture further past this point creates lean misfires with the mixture failing to completely combust, lowering temperatures (and therefore NOx levels). The Hydrocarbon (HC) levels start to rise as unburnt fuel exits the exhaust and power drops off.

[Sean]

Whilst generally speaking a richer tune is considered 'safer', as pointed out above, a tune is much more than simple targettting a single AFR value.

Also while that looks rich, you need to consider what sort of fuel - A high ethanol blend (EFlex/E70/E85) will read differently on an AFR metre that is configured for regular fuel/98.

Safest bet is to make sure the tuner knows your intentions for the car and engine and keeps the appropriate level of safety in the tune - when I had my turbo MX5s, the tuner always seemed to tune a richer than most others, yet I made similar power and economy numbers.

His theory was you cant win a race if you cant finish the race.

[The American]

Thanks all, I'll have to find a good local tuner. The ECU (software at least) is Sakura Motorsport, originally supplied by Mx5Mania, but no longer supported. Does anyone know anyone who uses this ECU/software in Qld?

The software is titled 'BP Tuner v2'.