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DeTuning an engine for efficiency

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Hey !

I need to detune a turbocharged engine to improve efficiency for emissions using Motec. What all factors should I consider for detuning and how lean can I run ? Thanks.

Detuning and efficiency are usually oxymorons, but...

First things first, you don't give any information on the actual needs for the 'emissions', or the engine in question?

Most vehicles emissions tests are done at idle, or at a specified rpm. and usually test for carbon monoxide with, sometimes hydro-carbons (unburned fuel) or oxides of nitrogen (not really a concern because it forms at high temperatures that aren't normally reached off-load.

Do you need a catalytic converter to be fitted - some countries/states require them, some do not? There are lot of idiots that will just gut the case, but purchasing and fitting a SUITABLE high performance cat' will have neglible affect on power/torque (they are required for many race series!) but definitely help with emissions.

NOTE, AFR comments for petrol/gasoline, alcohol containing fuels will be richer! For idle you may be best with around 14.7, for best idle quality and minimal CO, but some engines will tolerate leaner without problems - depending on the engine characteristics, you may be able to run as far as the 16-17:1 range, and if the test is done at idle, may as well try and get it as close as practical right there. Oh, most people with forced induction engines will run very small spark plug gaps - ~25 thou' (0.6mm) and this can sometimes cause a problem with misfires - try 35- 40 thou, if the engine will accept it - any misfire is going to result in emissions problems!

Much the same for high idle tests.

My personal preference would be to run the fueling close to 'best lean power' - this is where the engine makes the most power/torque per unit of fuel used and will generally be in the 16-17:1 range - for light throttle/load where fuel efficiency is important. It is NOT the same as the peak power/torque values, obviously, but as the load on the engine increases and certainly when producing boost, you're going to be running it a little richer and richer, as the engine demands it.

The ignition timing will be set as normal - best torque without pinking/detonation.

If you have the time, patience, and need, you can play around with the fueling and timing as they will have a small interaction with each other.

Thank you so much for the reply ! The engine used is a rotax 900 ace turbo and need to achieve an E-score of 175+ and less than 210. The Catalytic converter will be used for the test and also the emission test is at idle as well as increasing speeds upto 100% throttle.

Sorry, man, I'm in NZ and that doesn't help me that much - I can't seem to find any info' on that test.

Is that the 3 cylinder, turbocharged ROTAX engine as fitted to the Can-Am maverick X3 x RS - amongst other applications? If so, you may find the December 2019 issue of Racecar Engineering of interest, it has a feature on the preparations and modifications being made to one for endurance off-road racing - specifically the Paris-Dakar race.

I would start by bringing it back to around the stock values, or as best you can manage using the previous suggestions. It would seem that ROTAX sell the engine to several off-road vehicle builders, including personal snow, and water, craft - you could try checking such forums for their experiences with those engines. I think the last ROTAX I had anything to do with was a kart 2 stroke single cylinder around 35 years ago - a very different sort of beastie!

I dont have much experience with emissions as the laws are fairly relaxed here is NZ, but my understanding was for a catalytic converter to be effective at all the mixture must be centered very close to stoichometric (within about 4% max) and must regularly swing a little either side of stoichometric (~2% each way).

As mentioned above, the key to minimizing exhaust emissions is to keep the air/fuel mixture as close to stoichiometric as possible throughout the tested range, because that is where a three-way catalytic converter is most efficient . . . . although, did you say that the emissions test in your area tests up to 100% throttle? Where do you live?

Here in California there are two different tiers of "smog" tests: basic, which is used in less-densely populated areas and only tests and unloaded engine in neutral at idle and at 2500 RPM. In "enhanced" areas the engine is loaded by placing the drive wheels on an emissions dyno at 15 and 25 MPH, which promotes the emissions of oxides of nitrogen (NOx), which requires an efficient cat and a slightly rich A/F mixture. Adam was on the money when he said that most OE calibrations deviate slightly lean and rich, with the average mixture being stoich. This provides the slightly rich mixture needed to reduce the NOx emissions, as well as the slightly lean mixture needed to oxidize the hydrocarbon (HC) and carbon monoxide (CO) emissions.

All of this said, I managed to pass the "basic" California smog test in my 1986 Porsche 944 Turbo with VEMS ECU and large, batch-fired 830cc injectors using the original catalytic converter and minor tuning in the idle range. If I lived in an "enhanced" area, I might have needed a new catalytic converter to meet the NOx requirements.

Good luck!

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