the map sensor measures absolute pressure. therefofe it wery well takes altitude changes into account.

 

I'm trying to grasp the full picture here. I understand the MAP is going to interrupt the absolute pressure. My confusion is what if I tuned my VE at, say 5000ft alt. then moved to FL. would I be running rich or vice versus would I be lean? The pressure difference will put you in a different VE cell right? Well that cell was already adjusted for a certain fueling demand. So the MAP won't adjust the cell, just land in a different one for the different pressure. Right!?

I may be confusing myself!

 

I will open this post by saying I am not a degreed GM Calibration graduate...rather a tuner who has read, experimented and failed enough to develop my approach to tuning.

Armed with these 9+ years, I have received and provided guidance on the Forums for free as well as started a business which provides remote training and tuning. So my disclaimer for this post is it represents my understanding of the internal workings of the PCM and what we are trying to accomplish as we embark into the dark side of tuning.

I appreciate and respectively will accept any information which is in conflict with these facts as I am not too egotistical to believe I know it all....

Let’s start with a few known facts that I have subscribed to:

1. There are 3 Tuning Strategies we can utilize2. The GM PCM operates in 2 modes; Closed Loop (narrow band O2's provide correction information) or Open Loop where no correction is provided3. MAF Tuning4. VE/Speed Density TuningSo as usual, I am never short an to the point but I believe the above should support the two major questions and statements….Hope this helps…..The bottom line for me is to make sure that all the PCM functions are tuned and integrated as one to result in a smooth running vehicle that can idle, maneuver a parking lot, cruise and burn up the quartermile.

Tuning isn’t always as clear cut as it seems which is why we believe knowing the internals of the PCM and what it does and why is instrumental in providing you with the tools need to develop fully integrated tunes that can compensate for limitations without compromising safety of performance...need to think outside the box sometimes and no matter whether you tune 1 or 5000 cars, there is always something to learn.....

Ed M

 

Good breakdown Ed@TheVetteDoctors! Helps put a prescription on my tuning glasses. I have to say I feel you have really good knowledge of GM PCM tuning.

 

The key thing to remember is that the VE table's job is tell the PCM how much air is going into the combustion chamber at each cycle (based on the current MAP and RPM), so that it can inject the right amount of fuel to achieve the desired AFR. (An intake-air-temperature compensation will be factored in as well, but that's trivial math that doesn't need a table.)

Edit: Actually, I got ahead of myself there... The key thing to remember is that you can very accurately determine how much air an engine is ingesting if you measure the RPM & MAP, look up the engine's volumetric efficiency at that RPM & MAP, and compensate for the intake air temperature (IAT). The MAP & IAT tell you everything you need to know about the density of air available to the cylinders and the pressure that will push that air into the cylinders. Then it's just a question of how fast the crank is spinning and how efficiently that air can flow in during the intake stroke (which is what the VE table describes). And once the PCM knows the air flow, it can compute the corresponding fuel flow to achieve whatever AFR the tuner wants.

If you lose 5000 feet of elevation your MAP sensor will report a slightly higher pressure, so that when the PCM consults the VE table it will find a slightly larger value. The larger value indicates that more air is going in, and that in turn will lead the ECU to request slightly more fuel, by opening each fuel injector for a slightly longer period of time. So, in theory, it all works out perfectly.

In practice, your tuner at 5000 feet might not have much opportunity to test your engine at sea-level pressure, so there will be some cells in the VE table that reflect educated guesses rather than actual experimentation, so that part of the table might benefit from a little bit of fine tuning. But the differences are likely to be negligible. Partly because the manifold pressure change you get from dropping 5000 feet is tiny compared to the manifold pressure changes that you get from opening the throttle - your tuner can cover most of the VE table at any altitude, just by varying the throttle. And also because the educated guesses are not likely to be off by much. And also partly because the table shape just isn't going to change radically near the edges of the accessible area. A good tune will run well at any altitude.

 

That does make sence NSFW. I really would like to see someones scans of OL/SD tune at high alt, then untouched moving to sea level. I think they will be off some. A CL tune will have LTFT change, right?

Edit: My original tune was done in Dallas (460ft), then I took the car back to El Paso (3800ft). Big elevation change, and my car seemed really rich in El Paso. I had it dynoed again in Albuquerque (5000ft+) another elevation change and it was rich (11.5afr). Now knowing that the original tune was OL but MAF only after 1500rpm (I changed that) it appears the MAF might not be great at elevation change adjustment. I assume the MAP is a redundancy with sanity checks, but is it possible that the redundancy is more effective than the primary? I now wonder if my tune was SD if all those elevation changes would of resulted differently. I wish I had HP Tuners back then and I could of run the scanner and answered my own questions!

 

Altitude only matters because it affects the air's density.

The VE table has two axes: RPM and.... density.

Since the VE table contains different VE values for different densities, a speed-density tune is able to work just fine at different altitudes.

The PCM will factor the intake air temperature into the density calculations, so temperature changes are accommodated as well.

Exactly what ECU / PCM have you been dealing with? Apparently it has a really terrible implementations of speed-density, in fact it's so lacking that it really isn't appropriate to even call it speed-density. What you describe sounds like it might be "alpha-N" where the VE table's axes are RPM (the "N") and throttle plate angle (the "alpha"). But that's pretty old-school. I've never actually dealt with an alpha-N ECU but if I did I'm pretty sure the first step would be to replace it or reflash it with something that uses a proper VE table.

 

On one hand, higher altitude means lower air density, which means lower MAP, which means using a slightly different path through the VE table when doing pulls at full throttle.

But on the other hand, you could hit exactly the same VE path by doing a pull at slightly-less-than-full-throttle back in Dallas. That would give you slightly lower manifold pressure, and the engine doesn't really care if the manifold pressure is due to full throttle at 5000 feet or 95% throttle at sea level. (Just for the record, that 95% figure is what's called a brown number... don't ask where I pulled it from.)

It could be that your tuner focused their attention on the VE cells you hit when driving around town, and the cells you hit during wide-open-throttle pulls, and guesstimated the rest of the table, and erred on the rich side.

Or it could be that something else pulled the LTFTs rich shortly before the dyno session (does your tune use MAF for idling and driving around? How accurate is the MAF scaling?), and the LTFTs were still pulling the open-loop AFR rich during the dyno session. You'd have to have more data to know for sure.

But what you describe isn't a problem inherent in SD tuning - it's a problem that can be solved with further tuning of the existing VE table. (Or MAF scaling, if that was the culprit.)

 

My thoughts... VE compensates for altitude since based on absolute... this means that cell may change per throttle position, but the airflow dynamics are the same. At WOT though may not have appropriate cells tuned if moving toward sea level.

I think trims are more for weather changes... humidity, seasons. Seems the IAT swings are much greater in summer vs winter, but I think this is more the sensor. And driving style... if more stop and go IAT is less accurate.

BMW does reference baro and if LS does I'd be curious how its used... maybe some scaling for idle.

And I'll always check my tune with a big weather change.

 

you said it: "based on absolute". lets not forget that not so long ago most cars ran sd. they had no problems being driven in the mountains.

does anybody have info on humidity? i believe i forgot more than i knew about it, but i seem to recollect that humdid air is actually lighter than dry air. this would compensate any effects to a great extend.

 

All the Dodge hemi’s are SD tunes. Even the Hellcat.
Ive tuned a lot of them.
They run fine.

Also, 1999 and 2000 operating systems use the stupid dual VE table setup.

Lastly, closed loop SD works dandy. I tuned my turbo LS Nova that way. I leave LTFT off.
Ron

 

Related question, might as well tack it on to this thread rather than starting a new one:

Is there any difference between the Z06 and non-Z06 ECUs in the 2003-2004 model years?

Thanks!

 

You need to quit learning things from the forum as 90% of the input is by un trained people-----
The VE table fuels the engine ONLY by the VE table--No 02 input NO MAF input---
It is strictly based on RPM and manifold pressure So the SD tune will compensate for differences between all P/T fuel and WOT fuel
HOWEVER there is NO COMPENSATION for fueling at P/T because there are no fuel trim corrections and NO target for P/T fuel AFR
The target P/T AFR is commanded strictly by what the tuner has tuned in -----At WOT again there is NO compensation held over from a rich fueling issue carried over to WOT from your fuel trims
Also at WOT there is no compensation for fueling at WOT based on air density or altitude using SD
A SD tune is best only used for racing applications where the racer has the software to re-tune his fuel curve based on air density -altitude or ambient temperatures----at each racing location-------

 

YOU ARE THE 90%!!! Please stop. For the 100th time, closed loop speed density will correct fuel trims. My God, listen to those who know what they are doing and stop posting incorrect information over and over and over again!!!!!!

:will y:

 

You're mixing up two different concepts:

1) closed-loop versus open-loop
2) speed-density versus MAF

Speed-density and MAF are two different ways to calculate the amount of air flowing through the engine.

Closed-loop means that the ECU periodically measures the AFR and adjusts ("trims") the fuel delivery to achieve the target AFR. This gives it a way to compensate for inaccuracies in the calculated airflow. (Inaccuracies which could stem from the sensor, sensor degradation over time, sensor drift caused by ambient heat, sensor calibration in the tune, VE calibration in the tune, etc, etc).

An ECU can use MAF with or without closed-loop corrections.

An ECU can use speed-density with or without closed-loop corrections.

The stuff I posted earlier will probably make more sense if you re-read it with that distinction in mind.

 

No SD does NOT adjust fueling EVER Fueling is based strictly on what the tuner has tuned in -- An Sd tune will adjust fuel for P/T and WOT only based on manifold KPA--- But those fueling changes are again based soley on the VE table----And there is NO correction to achieve stoich at P/T via your fuel trims as they are turned OFF as is your MAF inp

In order to run a true SD tune you must disable the MAF and the LTFT closed loop adjustments What you seem to describe is an OLSD tune and NOT a true SD tune---- I went to the EFILIVE training school years ago and was taught by the best in the business---This is what we learned and is common knowledge among pro tuners---
A race car ---- use an SD tune and be prepared to adjust the WOT AFR fueling at each track using a wideband---and maybe you will gain 5-10 RWHP
On a DD car or weekend warrior stick with the stock OL/CL tune and that will give you the best all round performance--

I will never listen to your un informed opinions----Go study some more--and don't take advice from the internet---- sorry
How many cars have you tuned ?? I am approaching 2500 tunes from boosted to turbo--- large cammed to SD to OLSD to stock CL/OL--------and what I explain works all the time--

From the EFILIVE website and I quote---

"The ECU is sealed and does not contain an internal baro pressure sensor. Because of this, the airflow equation does not assume that every engine will have a valid Baro sensor connected, only MAP is used for the engine load input (other inputs are RPM, VE, Engine Displacement, Air Temp, and Coolant Temp). If you use MAP as the y-axis input for the VETable, an engine tuned at sea level will probably have sensor readings of Baro=100kPa, MAP=100kPa at full throttle, and MAP=80kPa at part throttle. If that same vehicle competes at a race event at high elevation, the sensors may indicate Baro=80kPa, MAP=80kPa at full throttle, and MAP=60kPa at part throttle due to the decreased atmospheric pressure. "

The VE table tune then remains the same regardless of altitude as the VE table is strictly MAP /RPM BASED and NOT baro based--- So there is no accurate compensation for fueling using an SD tune as the VE table has NO baro compensation due to the decreased atmospheric pressure at high or different altitudes----------

So you are now not only arguing with me but now you are arguing and saying you know more about tuning then the company and people who actually developed and wrote their LS tuning software---You need to rethink your position on this issue

 

 

Tblu, about that paragraph you quoted from efilive... it describes a limitation of the physical PCM hardware. Which makes it equally applicable no matter what the PCM software is doing.

So basically that paragraph is saying that GM decided that barometric pressure compensation is not important enough to put a dedicated sensor in the car.

This does nothing to strengthen your case that SD is inferior to MAF.

(Note that what Joshboody wrote about checking manifold pressure before start - to get a one-time barometric pressure sample - would also be true for both MAF and SD. And it makes me wonder what that sample value could be used for.... maybe just a sanity check to determine whether the reading is within reasonable limits?)

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