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Lamda target and air density


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I hope this doesn't come across as confusing. I am just trying to use an extreme example to see if I get it. I'd appreciate some comments....

Case1: I'm tuning my Falcon on a scientifically prefect day of 15°C, 0% humidity, 0m altitude, 1013hPa. On this day I target and hit 0.82λ consistently throughout many runs and lock down the tune. 

Case2: Then for some stupid reason I take my Falcon over to Alaska in a time machine to January 1989 where the town of Northway had a day that was -52°C, 1079hPa, 521m altitude & humidity around 30%. 

Using my RAD meter with the above inputs (save for temperature which only goes down to -25°C) for case 1 I get an air density reading of 100% and for case 2 I get a reading of 116%. 

Question: I haven't changed my tune between the 2 cases. In case 2 will I see my actual lambda be something like 0.95λ? I.e. 0.82λ + (116%-100%)

Reason I ask is should we be taking RAD into account when trying to hit a certain Lambda? Researching realistic weather values for Victoria across the seasons, relative air density swings of up to +/-8% are not out of the question. If we run our tune on the edge like 0.84 to 0.86λ that RAD swing could cause issues if we're not mindful. Am I making sense or thinking about it to much again?

Edited by hjtrbo
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They measure estimated barometric pressure at engine startup to account for altitude changes. You can datalog it. 

It might even re calculate it at run time under certain circumstances. I can have a look later. 

Temperature is already taken into account via IAT. 

Humidity I don't believe can be estimated. 

If you fix all the variables except for humidity how much does the density change? Probably not that much I assume? 

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At STP a humidity swing of 0 to 100% only results in a 0.5% change in RAD or closer to nothing to worry about. 

Am I reading you as saying that the PCM already accounts for temperature and pressure variations and will vary the open loop fuelling accordingly? I.e. if I tuned in the middle of summer at 0.82λ then went for a run in winter instead of it reading heaps leaner it will still read around 0.82λ?

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Yes it does. Air temperature is critical to determining the air mass (and hence fuel mass) of a given volume especially when you have massive changes in air temp due to a turbo. 

Have a read through the speed density howtos, the equations are in there. 

Open loop and closed loop are no different in how the air model works, closed loop just varies the target. 

If you have 0.82 at 8psi in winter, you should have exactly 0.82 at 18psi in the middle of summer after heat soaking everything. 

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Will do. Have just been noticing lately since mucking around with closed loop boost the wideband has small changes day to day. Some days actual might be around 0.78 and others it'll be at 0.82. 4% day to day variance is quite a lot so have been looking for answers. Thought it might have been RAD and open loop WOT fuelling so was starting to read up on it and make a note of what it is day to day and compare it to the wideband readings. . 

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Your wideband is probably not that accurate. Also you really need to do steady state on a dyno to ensure its not a transient issue. It takes several seconds for valve stretch to stabilise and as such you do get variations in transient conditions.

FYI these are the three temperature speed density table multipliers.



The X axis is actually IAT, however Ford do something weird and use a ECT axis, but feed in the DMR for IAT only for this one table. The editor is not aware of this and hence has ECT for both axis.



This is the barometric correction table


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I think I found the problem. I added ECT to my log chart. I reckon the previous owner has either installed a cooler thermostat or it has no thermostat at all. Only noticing this now as we are hitting cold air temps with this southerly blast coming through. Didn't even think to click what other tables might be effected when last week I was adjusting the closed loop boost target ECT multiplier axis as I was never making it into the OEM temp region for 100% commanded boost. Some logs my ECT is 68-72degC and others its a 80 deg.


Can we please have math parameters added to the logger so I can create a math parameter for EQ error. This will save me from myself. 


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Thanks for the tables.

Just having a look at them to see how much real world influence they could have.

auF0058. Looking at the slope and the presumption that typically 990hPa to 1030hPa is our pressure range the correction factor could be anywhere from 0.98 to 1.02. Reading the description I assume that this will act as a global multiplier I.e. shifting the offset of the air charge curves?

auF0051 would normally not be in play under normal conditions

auF0052 big influence. My air charge temps are typically 15 to 35degC depending on what's happening. So a rough +10% correction to air possible.

auF0053 small influence. +/-2% air charge multiplier on my typical ECT and air charge temperatures. 

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