Theoretically a change in elevation should do nothing to your trims. Massflow takes density into account and so does the map.

 

heh, looks like i'm going to be taking a trip from chicago to monterey, ca this summer, so i'll take some logs in every state, and see what really changes. my money is on MAF changing, and as long as i adjust that, VE should stay inline without any changes.

 

Well, see if this makes sense:

MAP/TPS Airflow Filter: This filter is used to filter the MAF airmass in response to a MAP or TPS failure. It is also used to filter the MAF airmass when the engine RPM is greater than the High RPM Disable limit.
High RPM Disable: Above this RPM use filtered MAF airmass for airmass prediction calculations.
High RPM Hysteresis: Hysteresis subtracted from High RPM Disable to re-enable dynamic airmass calculation.

This is from the HPTuner help file. So, the only "filtering" done to the MAF signal is from the MAP/TPS airflow filter. Dynamic airmass calculation from the VE table is most definitely disabled. The only role the VE table plays in MAF operation above High RPM Disable is error checking.

 

Yes, but I retired my MAF quite some time ago.

Or am I misunderstanding?

 

Doesn't matter. Speed density takes air density into account in the airmass calculation.

There are other things that could throw off your ltrims besides the airmass calculation, like the 91 octane used at higher altitudes, change in humidity which the pcm can't sense, and less backpressure on the exhaust due to less atmospheric pressure.

 

It sort of makes sense, but it doesn't really explain how it is filtered. Notice that the airflow above the disable is still filtered, and dynamic airflow is "MAF based". If it was MAF airflow, it would just be MAF airflow.

 

Well I just looked at a log and maf airflow and dynamic airflow are still different above 4000 rpm. But that doesn't necessarily mean the unfiltered maf airflow isn't just used instead of dynamic airflow up there does it?

What I mean to say is, is dynamic airflow the filtered airmass calc, or is it what gets filtered into the airmass calc?

 

What it is saying it that the MAF airmass reading above the High RPM Disable is filtered using data provided by the MAP and TPS sensors. So yes, the MAF airmass reading is being filtered, just not by the VE table.

 

Dynamic airflow is the MAF airmass reading, adjusted by the mass flow calculation derived from the VE table, to the extent designated by what MAP vs. RPM boundary the engine is operating in. This is disabled above the High RPM Disable value.

The filtered MAF airmass is the MAF airmass reading, filtered by data provided by the MAP and TPS sensors. This takes place above 4000 RPM, or in response to a MAP or TPS sensor failure.

 

I stand corrected.

It's actually quite easy to see when you look at the mass flow calculation, as the PCM uses both MAP and IAT values, along with VE, to derive the mass flow.

 

if it's so easy to see, care to put up an equation? i think we could all use it

 

Your wish is my command!

Mass flow = (VE * MAP * RPM * Displacement) / IAT

which is derived from this equation:

VE = ((mass flow * IAT / (MAP * RPM * Displacement))

So, the PCM is back-calculating mass flow from the VE value. Hence the importance of having a good VE table. It quite clearly shows that the PCM takes MAP and IAT into consideration.

 

Ok I follow all of this (I think) but my impression is that the SD mode does not deal with enviromental changes as well as the MAF/SD stock setup does, is this correct?


Thanks

More Than Zero

 

So was I.

However, it's all there in the mass flow equation. My own experience would seem to bear this out somewhat, as my trims don't really vary much due to temperature changes. But, they got shot all to hell by the altitude change. I think that maybe the relationship between MAP and actual barometric pressure (and hence air density) is a little more complicated than is immediately apparent.

 

umm...so if you're calculating MAF values from ve/map/rpm/iat then this mean it's in SD mode, right?
and the second equation is used in MAF mode, and use the MAF sensor values for the mass flow portion of the equation?

 

you said it before i could..

 

The second equation is the main airmass calculation equation used by the PCM. It back-calculates from the VE value using the values from the MAP, IAT, etc. to get mass flow. This is how it sets fueling in SD mode. It is also doing this during normal MAF operation (under the High RPM Disable setting), applying the calculated mass flow value against the measured value supplied by the MAF, to a greater or lesser extent as dictated by what MAP vs. RPM zone it is in.

The first equation is merely the second rearranged to more easily show how the PCM does this back-calculation.

 

I live at 5600' elevation and frequently go down to ~ 1200' elevation to the track. After a day of tuning, I finally got all my LTFTs in the -2 range, car runs great. From what I've read here, going down to 1200' will cause my LTFTs go to positive quite a bit. This makes sense due to the increased barometric pressure, but does the VE of the engine really change? Would MAF scaling take care of the pressure differential?

The oxygen content of the air shouldn't change much between the two elevations, so I wouldn't think the VE of the engine would change much per part of air. Shouldn't the MAF (using the maf table and IAT) take into account the density of the air?

I understand that there is more oxygen at lower elevations, but there is also more air. This is where I'm confused. Can someone give some insight into how the PCM deals with this?

 

I don't think it makes sense. Your map can sense the increased barometric pressure, and the maf can sense the increased mass flow, meaning the pcm should be able to compensate for it through the airmass calc instead of the ltrims.

The only way i can see atmospheric pressure changing the ve of the engine at a given map is by decreasing back pressure on the exhaust, which would increase ve at higher alititudes.

VE has nothing to do with the oxygen content of the air. All it means is how much volume of the air at intake manifold conditions actually makes it into the cylinder compared to the displacement of the cylinder.

It does. Massflow = volume flow * density. If the maf isn't doing its job right, that's a different issue.

The pcm is concerned with air/fuel ratio, not necessarily oxygen/fuel ratio although they're related. So all the pcm is worried about is calculating how much air is going into the cylinders. To do that it can use massflow directly from the maf, or it can calculate massflow by multiplying volume flow (which comes from ve) times density (which comes from air temp and pressure via the ideal gas law). density = Pressure/(temp * gas constant)

 

Points well taken. Only thing that still throws me for a loop is that when driving at 1200', there is more air and (roughly) correspondingly more oxygen. Saying the pcm doesn't care about the oxy/f ratio, but rather the air/fuel ratio, doesn't seem 100% true. The pcm will adjust the Ltrims based on the oxy/fuel ratio it sees at the O2 sensors while attempting to maintain ~ 14.7:1, correct? It would seem the PCM would have to back-calculate the a/f ratio from the oxygen it sees at the O2 sensors...? So, if the pcm is adjusting for the volume of air entering the engine, I would think the O2 readings would follow the increase in massflow, thus fully compensating for the increased air volume of the lower elevation (as far as Ltrims are concerned). If O2 sensors don't actually read O2, but rather air, excuse everything prior.

Guess I'll just have to do some logging at 1200' and see how the LTrims are effected.