Rb20det A/f ratios

[lg54]

Hey guys,

i recently installed my AEM uego wideband and tuned my car. my tuning guy said that the prime a/f ratios under boost would be from 11.8 to 12.8. ( under boost i am seeing from 11.8 to 12.8)

also, when accelerating normally not going into boost i am seeing anywhere from 14's to 16's and 17's . I know that the higher the number the leaner it is. am i running lean on the low rpms?

note: i am running 10psi of boost and tuned with an S-afc
rb20det

thanks

[240sxrb25]

low rpm no boost easy drivin areas are ok to be around 15 or close to 16 but it seems like your engine is running normally.

[Kingtal0n]

most ECU from japan are tuned blindly rich past about 4" of vacuum. You should be seeing about 14.5:1 anywhere from 20" of vacuum to about said 4" of vacuum. Once you hit that critical spot (around 4" of vacuum+) it should instantly flip to around 12.5:1, then drop from there to below 12:1 (anywhere from 10:1 to 11.5:1)

If you are in boost and you ever see the A/F go above 13:1 let off right away, that may cause engine damage.

[lg54]

If you are in boost and you ever see the A/F go above 13:1 let off right away, that may cause engine damage.

wow reallyy?? Didnt know this , in fact to be honest, i think i've seen 13 on boost for about a second or half.

[Kingtal0n]

wow reallyy?? Didnt know this , in fact to be honest, i think i've seen 13 on boost for about a second or half.

It depends on how much boost, what is the octane, what is the rpm, and what is the timing. may cause engine damage means that if the timing is a bit too far advanced for the boost given the octane at that particular rpm for that particular air/fuel ratio- the engine may knock. Basically that octane rating on the pump is there as a comparison for 2,3,3 trimethylpentane, a gasoline hydrocarbon soup of a particular arrangement that gives "100 octane" worth of knock resistance... based on a very particular engine with a specific compression ratio. As you raise boost pressure, you raise the compression ratio dynamically, and hopefully the temperature offset does not lower the mass of the air as much as the increase in pressure increases the mass of the air... and as these things change, temperature, pressure, ignition timing, etc... the event of combustion changes, and if it changes to the point where it happens too early, or too quickly, the engine may suffer damage.

Lucky for us, engines designed with the idea of turbocharging in mind are more tolerant of poor ignition timing and poor air fuel ratio choices. the way it is tuned from the factory, when I say "blindly rich" I mean they designed the engine so that with OEM plugs it will light off a mixture consistently around 10:1 during wide open throttle, with an ignition system capable of doing this under very high compression ratios (when compared to average naturally aspirated gasoline engines) all the while giving us a nice white-spark-plug thanks to the closed loop and off-boost tuning, that shows they know the difference between power and economy. So when I see them maintain closed loop 14.7:1 until around 4" of vacuum we know that we can safely do the same- but immediately they throw the rich stick at it- 12.5:1 instantly in that range. Your engine should generally do the same, just to be safe, just to remove the guessing, because no matter how large or small the turbocharger is, around 0-5psi you have pretty much the same conditions in the combustion chambers, except larger turbochargers will generally be less efficient (and you will make less torque) so we can only IMPROVE our conditions from the OEM, in other words if we keep the 0-5psi range (and likewise closed loop situations) identical to OEM we can NOT go wrong, NO mistakes there. But doing the same for 5+ psi is a bad idea, since a larger turbocharger may require much different fueling and ignition timings.

Now also keep in mind that transient delay and acceleration enrichment (throttle pedal movement enrichment based on predicted pressure differentials) may also factor into what you SEE on the wideband.

Also exhaust leaks and misfires will affect wideband sensor readings.

So at the end of the day, only a very experienced tuned can decide whether or not engine damage may occur around the 13:1 air fuel ratio range. 14:1 is unsafe at almost any boost as a general rule on just about any typical engine, however, and 15:1 is out of the question.