by Geoff Way

At our recent technical day at AVO in Melbourne there was much discussion about fuel mixtures and resultant power output.  Terry Wilson showed several people a “Lambda” readout on their car and explained that their particular car was reading spot on or perhaps it was too low and the fuel mixture was very rich, robbing them of power!  I will endeavour to briefly and simply explain what a “Lambda” reading is so you can better understand your readout.  If you are somewhat of an expert on this stuff stop reading now, if I am going to keep this simple and brief it will also attract comments from better informed readers and my knowledge is not that good!

Lambda 1.00 represents the exact point in an engine when the correct amounts of air and fuel are present and all quantities of both are burnt during combustion.  If your Lambda reading was as low as 0.70 then your mixture is rich to the point of causing a loss of power and a high reading of 1.25 for example would be dangerously lean.  The Dyno hook up measures the unused oxygen and/or unburnt fuel in the exhaust to give that reading. Simple EH!

Having mastered that point, be aware that Lambda 1.00 is not always the perfect number for peak power and under full load conditions Lambda 1.00 may be lean enough to cause pinging (detonation).  For this reason the Lambda is adjusted to between 0.82 and 0.85 throughout most of the power band by the engine tuner.

Many manufacturers set the Lambda on Turbo versions well below 0.80 particularly in the top end of the power band to ensure that the engine does not run lean and detonate which can quickly destroy the engine giving rise to warranty claims. 

The Dyno read out shown with this article gives an example of  what  can be done.  This is a Dyno Run for a stock 1990 MR2 Turbo before and after tuning.  Note that on the first run the “Lambda” reading falls to 0.73 at 6000 RPM, the peak power is 126.4 Kw @ 5505 RPM with a Lambda of 0.75 and it decays from that point onwards as the fuel mixture gets richer.

The second run on the Dyno was done after fitting a Link “Interceptor” to the factory ECU.  This aptly named device intercepts the signal from the ECU to the engine and allows the tuner to adjust the fuel mixtures to optimum levels maintaining the Lambda readings within 0.80 to 0.90 range.  The result clearly shows what can be achieved using such a device.  Note the dip in power at 3600 RPM where the Lambda reading indicates more fuel is added to the mixture and the lean out at 5000 RPM to achieve 140 Kw at 5636 RPM.  No other work was done on this car other than fitting and tuning the Interceptor and the boost readings are almost identical on both runs.

Although not spectacular, a 14 Kw improvement is not to be dismissed as this power gain can be achieved with relatively small cost compared to an aftermarket ECU.  It is possible to obtain 150 Kw or more at the wheels on the same car with a fully programmable ECU but it will cost over twice the price of the interceptor so on a bang for bucks scale this device wins.

For costs and fitting details call Lee Wilson  at AVO on (03) 9584 4094 and quote your MR2 Club Membership number for a club discount.  You will need to produce a current membership card at time of purchase.