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Correcting Lean AFR without dyno- Motec M1

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Hi Everyone

We have a car here with an Nissan SR20 Turbo. Has been tuned by a Motec tuner here, who has targeted Lambda 0.85 from 160kpa onwards (0.93 at 100kpa/transition to boost) Far too lean by my understanding. Is there any reason why they would target such a lean AFR? Looking at other tunes done by that tuner, this appears to be their 'go to' afr for turbo cars. I feel like I must be missing something?

Without access to a dyno at the moment, can I change the AFR Target table to something more suitable, roughly add the same percentage of change to the fuel map then road test/log and make adjustments from there to try and get it close to the target?

The car is equipped with LTC and is running closed loop fuel control. What is an acceptable amount of trim? I feel like you would not want to rely on the closed loop system, and would want to get the fuel map as close as possible, but with limited time and no access to a dyno this could be tricky.

Without more info on the setup and fuel I couldn't say if 0.85 is prudent or not, but it might be fine.

That said, if you want to make it richer and this is a VE based tune, it's easily done. Simply adjust the lambda target.

You don't alter the fuel table if it's a VE based tune, because engine volumetric efficiency is not changing.

If this is an old gold box and the tune is injector on time based rather than VE based, then yes you would need to retune the fuel table as well.

To follow on what Mike said, why are you calling 0.85* " Far too lean" - that's around what I would expect, depending on the specific installation and build of your vehicle. The implication is that you want to run less than, say, 0.8 or richer which may actually cost you power/torque, bore and ring wear from bore-wash, oil dilution, plug fouling, etc.

You may, indeed, find you can run more boost and/or timing with a richer mixture, but that should generally be considered a band-aid to compensate for other issues - and running richer while keeping the rest of the engine assembly/installation and tune the same, for no apparent reason, seems rather daft.

Why, exactly do you think 0.85 is "Far too lean"?

*personally, I don't agree with tuners using a default lambda - it's fine for initial settings, but it's unlikely to be the best value for that particular engine. Same thing with ignition timing - the engine will tell the good tuners what it wants.

Bit more info, SR20DET in an old datsun circuit race car, 98 fuel, 8.5:1 comp, forged motor, 19psi max boost at the moment

Good point on being able to just change the target table - Can't believe i missed that one.

"far too lean" is probably the incorrect wording. But the AFR course would lead me to believe that in such a setup, something like 0.8 or even 0.78 might be more suitable? But it was a more a question than a statement. Hence I said am I missing something?

Even a quick google search on SR20DET reccommended AFRs are all much richer than 0.85, most saying that anything around 0.85 is inviting trouble. Hence the confusion.

Most European turbocharged cars are running 0.75 and richer but that’s for component protection, i. e. the catalytic converter and turbocharger. I do usually target a 0.8 +/- 0.02 lambda when installing a downpipe which removes the cat. Most turbocharged cars will be happy around this lambda, which might cost you some power but will be totally safe. Never heard of any bore washing occurring at this target, you will have to go way past 0.7 to achieve that, in my opinion.

0.93 at 100 kpa absolute is fine in turbo application.

Thank you for the additional info. Since this is a circuit car, fuel consumption is often a concern and can lead to selection of leaner AFR than other scenarios in order to prolong drive time between stops. That MAY have been a consideration the tuner took into account, even prioritized over other things.

As a general rule, air/fuel ratio is one of many factors that impact the outcome of how the engine runs. I wouldn't suggest being aggressive with lambda target, ignition timing, boost, RPM, on low octane fuel, with high charge temps, but engines can be tuned successfully with different combinations of boost, timing, lambda target. Sometimes being a little more aggressive in one area, bit safer in another, makes sense. If you give two competent tuners the same engine to tune, they won't end up with exactly the same result, and both tunes will likely work fine.

I don't know how long the car has been running that way, but if it's been that way for a while without falling apart, I'd say that's a good sign.

If you want to run it a little richer, I'm not trying to talk you out of it, and trying things out is how we all gain experience and learn.

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