6Speeder

Please tell me I'm wrong.

If you tune a car in Speed density mode, and run it without a MAF it can't compensate for altitude changes when in WOT. Because of the O2 sensors it will be OK when in cruise (closed loop) mode, but will just read from tables (set at whatever altitude you tuned it at) when in WOT (open loop).

My concern: We tune a SD car at 7,000 feet DA and take it to sea level. When we go open loop the ECM will be giving the engine roughly 21% less fuel then it requires.

YES, NO?

redhardsupra

you're wrong

6Speeder

Very educational. Thanks a lot.

redhardsupra

You didn't ask for an explanation, you asked for a yes/no

To get to mass of air, you must have density and volume of it. Volume is physically constrained, so that's easy. Density can be figured out from pressure and temperature. The altitude will 'show up' in your pressure values, thus it will influence the density and mass.

Also: fueling correction modes are orthogonal to air predictive mechanisms. don't mix them up.

99NassauVette

Right... my goodness..


From my understanding when you tune in SD, it doesn't utilize the MAF any longer to adjust the fuel table.. It's goes directly off of what you define in the the tune itself. So by taking the car from one extreme to the other would definitely require you to re-tune the table.

6Speeder

That would be fine if the car had an actual barometric pressure gauge. It doesn't. It has a MAP sensor which is near zero, no matter the altitude, when at WOT.

6Speeder

That is what I believe also.

DSteck

That's not really how it works. Speed density is still an airflow model, just like the MAF. The MAF just measures airflow directly whereas speed density calculates the airflow based on an extremely close to actual model.

No offense, but it's pretty clear that you really don't understand what's going on.

I'll bet you a thousand dollars, or five thousand, or any sum of money you want that if you look at the reading from your MAP sensor at 7000ft, it'll be somewhere about 75-80kPa and not 100kPa.

Are you familiar with what the MAP is? It's MANIFOLD ABSOLUTE PRESSURE. That's a direct measurement of air pressure, the same thing a barometric pressure sensor reads. When at WOT, a naturally aspirated car is not going to see higher manifold absolute pressure than the outside barometric pressure. Saying that altitude doesn't affect the MAP reading is just ignorant. I'm actually trying to not be a complete jerk about this response even though I know you're sitting there thinking Marcin is brain dead and you are absolutely correct. Do you understand the difference between absolute pressure and gauge pressure? Do you know what the MAP is actually reading?



A proper speed density tune will be fine at various altitudes, even on factory compensation values which are external to the VE itself.

6Speeder

"Are you familiar with what the MAP is? It's MANIFOLD ABSOLUTE PRESSURE. That's a direct measurement of air pressure, the same thing a barometric pressure sensor reads. When at WOT, a naturally aspirated car is not going to see higher manifold absolute pressure than the outside barometric pressure. Saying that altitude doesn't affect the MAP reading is just ignorant. I'm actually trying to not be a complete jerk about this response even though I know you're sitting there thinking Marcin is brain dead and you are absolutely correct. Do you understand the difference between absolute pressure and gauge pressure? Do you know what the MAP is actually reading?"

That would be correct if the car isn't running. When it is, the engine is sucking in the air, under partial vacuum, and at WOT when the engine is sucking in as much as it can, the pressure is higher, but still less than ambient, no matter what the outside pressure is. Since you are such a tuning guy, and a jerk, why don't you paste up the table that the ECM uses to calculate the actual air mass from the MAP sensor readings at WOT.

BTW: I just used my handheld and collected data on the manifold absolute pressure sensor readings. Ignition on, engine off: 84kPa. Start the engine, at idle: 60kPa. What happened, did i just shoot up to 10,000 feet? Nope, still in my driveway! Cruising, could be anywhere from 28kPa to 60. Wot it read 83 at the highest.

THERE IS NO WAY YOU CAN CALCULATE AIR MASS FROM THE MAP DATA. Saying so is just ignorant. I guess you don't know as much about tuning as you think you do.

DSteck

Since you want to call me a jerk, let's just get it out of the way and make a public note that you're a clueless dumbass who obviously has no idea what's going on. I'm going to also assume you've never taken a thermodynamics class and consequently don't know anything about the Otto cycle or what volumetric efficiency even means.

Let's start with your first paragraph of dribble. Key on, engine off the MAP sensor will report barometric pressure. Go figure the ECMs that don't have a dedicated baro sensor (like the ZR1, CTS-V, and ZL1) actually sample and store this reading as barometric pressure when you first try to fire the engine. They will continue to resample looking for a higher pressure, and if it sees a higher pressure at WOT, it will reset the recorded baro reading.

But you want to see tables for how the ECM calculates what to do at WOT... So let's start with the basics, which I'm sure you don't know since you're asking me to tell you.

Gen IV ECMs use "virtual" volumetric efficiency which is a table dictated by some contraints and broken into 30 zones, all with their own set of six coefficients (for a total of 180). The zones are broken up based on engine speed and MAP, and the coefficients are:

MAP -> A
MAP² -> B
RPM -> C
RPM² -> D
MAP•RPM -> E
Constant -> F

They are used as follows (where MAP represents the current MAP reading and RPM represents the current RPM reading):

GMVE = A•MAP + B•MAP² +C•RPM + D•RPM² + E•MAP•RPM + F



I'm sure this doesn't make a lick of sense to you, so here's what it looks like when converted to a table:



Your next thought is "so what". Well here's what:

Cylinder Airmass = GMVE•MAP / MAT

MAT = manifold air temperature = IAT + ECT•bias

So you see... The ECM is VERY capable of calculating an airmass (which is actually what matters as this is what's used to calculate the injector pulsewidth, not airflow) in speed density. This is exactly what gets used to calculate what to do at wide open throttle, part throttle, idle, etc.

Now let's address the scenario of what your datalogger told you.

Key on, engine off = 84kPa. I'd assume this puts you somewhere around 5500 feet in elevation. Idling showed 60kPa (that's not very good idle vacuum, so I am betting you have an aftermarket cam). Cruising varies (obviously) because of varying throttle positions. Your WOT reading of 83kPa (1kPa lower than barometric) indicates that you have a very slight intake restriction.

Obviously your car's elevation didn't shoot up to 10,000 feet while idling. It's called throttling. This isn't a diesel that's essentially WOT all the time (although newer diesels actually use throttles). I'm trying to figure out why you're even mentioning the various MAP readings, which are totally normal, but I assume it stems from not understanding how speed density actually works. I hope you understand that a lower MAP reading represents stronger vacuum (again, this is where knowing the difference between absolute and gauge pressure is a must).

Saying that you CAN'T calculate final fueling based on a MAP reading (in conjunction with RPM) is what's ignorant. You've proven that you really just don't grasp what's going on. This isn't Alpha-N which is probably what you're thinking. Insinuating that I don't know what I'm talking about just makes you look very dumb and completely out of touch with this topic of conversation. I am quite familiar with how the ECM works and arrives at final values. It would appear that you are not. Let's not forget that YOU are the one asking the question here, not me.

So how does the ECM calculate fueling at WOT for different elevations? You already answered it in your post. Your barometric pressure is 84kPA and your WOT reading was 83kPa. So, the ECM runs it's routine samping RPM, MAP, and MAT to calculate the cylinder airmass. Using that in conjunction with the injector calibration, it knows what injector pulsewidth to use for that combustion cycle in a given cylinder. If the car is moved from 5500 feet to sea level, where barometric pressure is 100kPa, then the ECM just sees the higher pressure, calculates the higher GMVE, and appropriately injects more fuel.

I made something for you:

redhardsupra

@DSteck, does he sound like he's worth wasting perfectly good linear algebra on? Obviously he knows better, that's why he's asking, right...?

99NassauVette

DSteck,

Even though this guy tried to troll you... You've managed to turn this into a learning thread for many of us... Thanks for that!

6Speeder

DSteck: Now that's better. I ask a question, based on my understanding as a non-tuner, and get a stupid answer and insults. So I responded with insults. If you had come on with your last response the first time (without the additional insults) we would all have learned something, which is what I wanted in the first place, and moved on.

Yes, I live at 6,000 feet but it was a good DA day. Yes I have a relatively big aftermarket cam which doesn't produce much vacuum. Makes pretty good power though.

DSteck

I almost didn't want to respond in the first place.

I do what I can. But I don't know what I'm talking about.

I never insulted you. You started in with that. So, I maintain your dumbass crown. All I said was that you don't really get it, and tried to give an abbreviated response that wouldn't go over your head. I know exactly what you were thinking when you replied to Marcin about a barometric pressure sensor. One thing I can't stand is when people get an idea in their head, don't let it go, and then try to crap on the extremely knowledgable crowd about it. That's what your reply to Marcin did, and that is why I responded the way I did (with complete disregard for your feelings).

I'm out.

6Speeder

I believe if you reread your original post and if it was directed at you, you would find it insulting also. Yes, I took (and passed) thermodynamics. It was a requirement for my BSEE. Yes I was able to follow your equations.

Now if you or Marcin had just answered my question with out his dumbass trite "You're wrong" or your extremely condescending tone, this would have been a much better thread. Have a nice day.