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Unread 06-22-2013, 12:36 AM   #1
Rich Z
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Default MAF vs Speed Density tuning.

Now that some of this stuff is beginning to make sense to me, I figure I'll capture some of the writings that I've been finding most helpful for future reference.

Terminology and Confusion, part 1

A lot of big words get thrown around on all the forums, but how many people actually understand what they're saying? Once you actually read into their posts or problems, it quickly becomes obvious that they either don’t know what they're saying, or they just answered their own questions without knowing it.

The usual example: MAF tuning vs. SD tuning.
MAF tuning is not a _pure_ MAF tuning. During sudden changes in throttle input, or any other MAP jumps, the PCM prefers to refer to the VE table for airflow lookup/calculation. If you're not sure how VE table express airflow can, I highly recommend reading my 'How Speed Density Works' paper. If this was a "pure" MAF system, ALL requests would come from the MAF and MAF alone.

GM decided to make it into a hybrid system. Why would they do that, might you ask? MAF can deliver very precise, low noise signals, providing simple devices that can be easily calibratable to different applications, and have a reasonable range and resolution. But it also has a problem with not having the cleanest signal when not much airflow is going through the MAF sensor, or failing to deliver a smooth, universal airflow. Speed Density calculations however are just that--math. It's not dependent on physical conditions, thus not affected by the non-uniform airflow at lower MAF frequencies. As long as all the necessary sensors (RPM, IAT, MAP) are healthy, and all the lookup values (VE, displacement, IFR) are correct, the airflow numbers are going to calculated correctly, despite physical conditions like low, or reverse airflow. The PCM itself is very much airflow source agnostic and it uses whichever source is better suited, or at least yields less erroneous values. Another neat side-effect of having both Speed Density and MAF working together side by side, that if you detect MAF failure (DTC codes P0102 or P0103), the computer seamlessly falls back onto then pure Speed Density mode, so you can safely drive it to the mechanic.

So to all the MAF pundits: you can complain about SD all you want, but the truth is, you're running in SD at least part of the time you run your car, as there is no such thing as 'pure' MAF move on our PCMs.

Some smart guys saw it as strength, an advantage to this dual-source approach, and SD tuning became a reality. Turn off MAF, run in pure SD, and dial in your VE, so it precisely describes the breathing capability of your setup. What do we do with MAF then? After 4000rpm it's going to take over completely, and we're going to be ignoring our new perfect VE! That's when I figured out how to 'map' the airflow calculated from the VE table onto the MAF frequency-based scale. This way we have brought back the dual-mode capability to the system, just like the system is designed to work, but now it has new data, tailored to our application. Because of that single source of airflow data, if the PCM decides to jump from MAF to VE based airflow, the airflow numbers should be smooth fit, not causing wrong air mass readings (that's used to look up timing advance, which in effect can cause bucking), or airflow, which in effects causes knock, or at least hesitation, making for a terrible drivability.

An alternative, interesting approach was to use AFR%Error to manipulate MAF airflow numbers, to establish the new MAF calibration. While theoretically it should yield an identical result as it would with my Dynamic Airflow onto MAF frequency mapping, the reality is too fuzzy, and often yields discrepancies significant enough to cause the engine to get different numbers than it should have. While I do not recommend this method for MAF tuning, I _highly_ recommend using it to verify the VE tune, as well as to observe daily environmentally influenced fluctuations.

Another note to MAF pundits: If you claim that MAF is better purely on basis of not being able to get SD working correctly, you might want to watch out for your 'MAF' tune (I put it in quotes because it's still a hybrid with SD). Every time you get on the gas more vigorously, your untuned VE table will rear its ugly head, and give you an AFR spike, bucking, knock that's hard to reproduce, hesitation on takeoff and general unpleasantry. There is no escape from doing VE on these systems. Even the most hard-headed MAF tweakers out there have given up, and modify at least the idle areas of VE as without that, making the car idle is somewhere between difficult and impossible.

To farther prove my point about the MAF having very different characteristics on low vs. high airflow situations, let me demonstrate a typical spread of samples from < 5000Hz, and above 6000Hz.

As you can notice, the scatter is quite significantly higher in the lower frequency areas (look at R^2 values). I use different sets of parameters to filter out noise for these two distinct regions, because of their different characteristics, I use 5500Hz as the usual boundary between high and low noise areas. I had a very large smile on my face, when I downloaded LS2 and LS7 stock tunes, and saw not one, but two MAF tables, switching at 5800Hz! Apparently GM also has two different sets of filtering parameters for the two distinct characteristics of MAF and the signal quality based on airflow.

In the end, remember what SD or MAF is: a source for airflow figures. If the PCM doesn't care where it gets the numbers from, why would you?

Most of the arguments out there stem from lack of understanding of how they work, and what are their limitations and applications. There are a lot of myths out there, i.e. SD not being able to compensate for altitude or temperatures. This is just plainly not true, as that's precisely why MAP and IAT sensors are there for. MAF has problems with the small upper airflow limit, and reversion on some setups, so it's not perfect either. Understand both, use what works better for you.

Part 2 is going to be an Open and Closed Loop, should be up in few days.
Hope this helps,



Why tune VE?

This is a post in one of the forums, I usually don't like to duplicate information, but singular forums posts tend to get lost in the noise, so I'm posting it up here as this is something that should be very clear to everyone that's trying to tune their car.

The question was:
Does this (VE tuning) gain me anything performance wise?

This is a very good question, I'm glad someone is trying to understand what it is that VE tuning is actually about, not just how to do it.
  1. Without VE being perfect, you'll never be able to find out proper timing. When computer detects the tiniest tendency to run lean, it will be very trigger happy to pull timing with usually no good reason. Thus, if your VE is on the lean side (and it usually is, after all that's what adding better flowing parts is about--flowing more air) you will get a lot of knock in random spots, and no amount of pulling timing yourself will cure it, causing your car run like poo.
  2. Sudden transitions are hard to get right. Without VE being dead on, you are making it almost impossible to get right. Bad transitions cause knock, which lingers around, doesn't last just when going over the areas that aren't perfectly tuned. I've seen knock last over 5 secs. If you're a drag racer, that's diminished performance for half of your run. That's why it's also important to tune not just some of the VE, but ALL of it.
  3. When your car develops a problem, you will notice it. If your VE is well done. airflow numbers will be down, knock will appear, but you know it's not the fault of the bad tune, but a result of some hardware component failing.
  4. When VE is perfect, it is meaningful. If you add a part that supposed to improve engine's breathing, your VE will go up, and if you get it tuned perfectly before and after, you will know just how well the part works, and for what MAP/RPM range.
  5. With perfect VE, your airmass and airflow numbers will be meaningful as well. with their close correspondence to torque and horsepower respectively, you can optimize your powerband.
  6. Perfect VE enforces other tables to be meaningful as well. For example, to obtain the same proper fueling with wrong VE, you will have to hack either your IFR or PE numbers. With all of them perfect and meaningful, when you want a 12.9AFR, you can just command it in PE and it will happen, instead of taking stabs in the dark hoping that some arbitrary PE will make it happen by accident.
  7. Since VE dictates airmass and airflow, everything based off such tables will work better as well. Shift tables for automatics need to know how much power you're really making. If you're lying about VE, then this power estimation is also wrong, making the transmission misbehave.

In general, VE in itself is important. A lot of other things are derived by calculations based on numbers calculated from VE. It really ends up being a domino effect. If VE is meaningful and proper, then it forces other things to meaningful and proper as well. But if you botch/ignore VE, then the bad effects will propagate, making the entire tune a major hack, making the car drive horribly, and sending the tuner chasing his tail. You pick which domino effect you'd rather experience.

In the long run it's really just easier to do it Right.


Terminology and Confusion, part 2 (OLvsCL)

Open Loop vs Closed loop tuning is another huge source of misunderstanding. Partially because it's just few terms out of a huge body of Control Theory (PID also comes from the same area), without understanding the rest of the principles and theory behind it. Another reason is because of how people use it, it's almost always referred to as 'OLSD', as if it was one thing, which it is not.

Open Loop and Closed Loop are just a methods of control of fueling. OL is basically a system with no feedback. Think of a sprinkler system that sprays the lawn whether it needs it or not. To contrast that, you have CL--a system which takes the output if its own operation as in input for the next round of calculations. In practical terms, it would be a sprinkler system with a ground wetness sensor, and only activating the sprinkler system if the ground is dry. The good part is not wasting water when the lawn doesn't need any more. The bad part is that we actually need sensors, threshold levels, hysteresis models, and other scientific junk, just to keep the damn lawn from drying out. This is definitely a place to consider effort vs benefit.

So what does it mean for a car? The main benefit of OL control is the direct relationship between what you tell it to do and what it does. It will do exactly what you tell it to, which is good if you tell it the right thing, and potentially catastrophic if you don't. That's why most tuning is done in OL--you want to see exactly how much airflow (MAF or VE) and which commanded AFR (OLFA or PE table) yields a particular AFR. This is the entire logic behind tuning--once commanded and resulting PE agree 100%, you can back calculate the airflow from displacement, pulse widths, injector flow rate, RPM, MAP, IAT and AFR. This is how you obtain airflow characteristics of an engine, no matter if it's with MAF or SD approaches.

Once you obtained that airflow characteristic, you could continue running in OL, and all the environmental changes would show up as change in airflow numbers. In SD, VE table is calibrated in what I call GMVE units, which take temperature and barometric pressure into account. This means that if that pressure or temperature changes, it is easily recalculated to current conditions. In MAF mode it's even simpler, more airmass cools the hot element of the MAF sensor better, automatically giving you a new, adjusted reading. Both models work just in any condition. (this is an answer to all the 'do I have to retune for weather?' questions that show up at least 3-4 times a week on forums)

So if it works so well, then why would we ever need CL one might ask? Doing math for all these models is great, everything agrees, but in practice, things like airflow measurement, or air fuel ratio measurement are an inherently difficult problem. Tuners drive around and scan and know what to adjust when. Normal people dont do that, they hop in and just want it to work, without scanning, analysis, and reflashing their car's computer. Thus, CL became that automatic tuner. It looks at data from different sensors, and if it consistently points at a new better setting, it adjusts. It's a perpetual feedback loop, not so commonly refered to as the Closed Loop. This model of course has its limits. While it will adjust to things like weather changes, or driving through the Rockies, it will not adjust for racing camshafts, huge heads, changes of displacement, and other significant changes to the airflow. Car's computer is willing to adjust, but also must be able to detect hardware failures. To a computer, airflow reading way out of its usual range is flagged as an abnormal event that should be looked at, while to a human it just might mean we put some heads on it. Computer has no way of knowing which one it is, we must tell it.

If you read and understood the last two paragraphs, you might have noticed, that a human tuner, and CL mechanisms (fuel trims) have the same function: to observe and adjust airflow changes. If you think about it, what we usually call the OL tuning method, is really CL--except that the mechanisms doing the adjustments are not automatic and computerized, but human, and done outside of the system.

This brings me to conclusions: in part 1 of this writeup we learned that MAF mode doesn't really work off MAF alone, and now we learned that Open Loop is a human powered Closed Loop.
I think what happened here is that we got lost somewhere between lack of technical understanding, and the traditional American tendency to polarize and zealotize (is this even a word?) concepts. This isn't your usual Coke vs Pepsi, Chevy vs Ford, Republicans vs Democrats war of ideologies. Reality is complex, and simple models are just too simple to describe it. That's why when we want a flexible system we end up doing hybrids, as there usually is no 'one size fits all' solution.

So the lesson from this is to learn, explore, and never be afraid to look at an alternative solution, as in more cases than not, you'll both be right and wrong at the same time, just for different set of parameters. There are very few absolute rights and wrongs, but if you are comfortable with all the alternatives, then at least you have a good chance of picking the best solution for your application, your purpose, your environment. If you're a tuner that always wants to run on the rugged edge and get as close as possible to 100% of potential, you probably want OL-SD. For a daily driver that doesn't get scanned too often, CL-MAF or CL-SD are the way to go. If you're bracket racer and you want as much consistency and control as possible, OL-MAF will probably yield you the desired effect.

Don't be a close minded zealot--just because a buddy with a fast ride told you something, doesn't mean it's going to work for you.


Last edited by Rich Z; 06-22-2013 at 01:11 AM.
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Unread 10-31-2013, 01:55 AM   #2
Rich Z
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Well, OK. I've learned a lot since I started poking around with this tuning stuff, but it seems like people are missing something important when talking about a closed loop system. Heck, even an open loop system has issues that I have yet seen anyone address.

First off let's describe how a typical closed loop MAF based system, like we normally see on our later model Corvettes operates. You have a MAF sensor that determines the air charge based on a frequency generated by the air flow passing through the sensor. This tells the PCM what charge of fuel to spit into whatever cylinder that will be firing when that air charge hits the cylinder. Then afterwards, the oxygen sensor in the exhaust detects the proportion of oxygen in the exhaust from that cylinder firing and reports back to the PCM about whether or not that fuel charge was correct to produce the desired air/fuel ratio. Pretty over simplified, I think, as there is a devil in the details.

Let's start with the MAF. There is a conversion table used by the PCM to convert the frequency detected to an actual measurement the PCM can use for airflow. This table can be modified via aftermarket tuning, and obviously in a stock tune it is an average of all the vehicles that GM did the development of the tuning with. Fine tuning, in an aftermarket sense, can tailor this table to be more realistically relevant to the engine in YOUR car at THAT time. Maybe a more finely tuned MAF table will better enable the PCM to more accurately determine what that aircharge is going to be in the engine's cylinders. But then again, maybe your engine is just average, and tuning will not improve this at all.

So here's part of a problem. When the MAF detects the airflow THAT air charge is still in the MAF, and not in the engine. So there is a delay that has to have been programmed into the PCM (and as far as I know, there is no way for aftermarket tuning packages to change that) that tells the PCM, "hey, get ready, the aircharge that is going to be in the cylinders is XX.XX sometime in the near future." And, of course, the VELOCITY of that air will change with engine speed, along with volume. And this is a complication because velocity and volume can have the same effect as far as the frequency of the signal detected. The MAF works via a heated element that is cooled by the passage of air over that element and generates a proportional frequency. A heated element can have the temperature changed by both volume and speed of the airflow. But I'm sure this has all been calculated out by GM during the tuning development using a STOCK air intake system. So here's the rub. Change ANYTHING in your air intake that changes either volume, air speed, or both, and you throw those calculations right out the window. Change the DISTANCE between the MAF and the engine cylinders and you've really thrown a wrinkle into things. Now that charge of air that the MAF has told the PCM is heading towards the engine cylinders might get there before or after the PCM thinks it SHOULD be there. So the fuel charge might be completely off because the calculated fuel charge doesn't apply to the air charge that is actually in the engine cylinders at that time.

So let's muddy the waters even further. In a CL-MAF (closed loop) system, there are the O2 sensors on each of both of the exhaust pipes that are reporting back to the PCM what the oxygen content of the exhaust is so the PCM can determine how close it's calculation of the proper fuel charge to apply to the air charge detected by the MAF was. So if the calculation was incorrect because of the before mentioned CHANGES in the airflow the MAF is detecting, then the PCM is going to be jumping around trying to give an AFR calculation that satisfies the oxygen sensors. Which, in reality, it might NEVER be able to do, because it doesn't know that it is basing it's calculations of a fuel charge on the wrong air charge. Granted, at an engine speed that doesn't change, this would not be an issue, since the air charge should be stable, and correspondingly, the fuel charge should be as well. But during rapid changes of engine speed, well, things could be changing faster than the the system can realistically work with. How fast is TOO fast? Beats me. Maybe it's really not that much of a problem because the changes can't be drastic enough to be very significant.

So back to the oxygen sensors. They are also trying to detect a particular charge of air and fuel based on the amount of TIME it takes for that charge to get from the cylinder(s) producing the exhaust to the sensor itself. So then you go an put headers on, that will generally move the position of the oxygen sensors further away from those firing cylinders. So what does this mean? Are those sensors now detecting a charge of air and fuel that is NOT the charge or air and fuel that the PCM thinks it is getting feedback from? So, again, the PCM could be thrown akilter trying to compensate for either a too lean or too rich mixture based on this feedback that is not accurately produced by what the PCM calculated at all, because the detected exhaust is NOT of that charge the PCM thought it was.

So yeah, if you go and change your intake to a cold air intake, put in a larger throttle body, put on a bigger MAF, and put on headers, you just threw a monkey wrench in all those signals that the PCM is counting on in order to create the proper calculation for the air/fuel ratio. So heck, in my case, I go and put a 427 cid engine right in the middle of this, which is going to have different air movement needs altogether from the LS6 engine, and you've got a real mess on your hands.

There are tables in the PCM that are able to be edited to tell the PCM some things about such changes, but so far I haven't seen anything that can touch the TIMING between the events needed in a Closed Loop MAF based system. So it seems to me that using such a tuning system in a moderately to heavily modified car will be difficult, at best, to get a perfect tune for any longer.

The reality of it, from my naive and novice knowledge so far, is that an Open Loop Speed Density tune is the ONLY way to realistically go. This sort of tune relies solely on the VE (volumetric efficiency) table for the air/fuel ratio calculations the PCM does. Don't use the MAF at all. The air charge is a calculated value from the VE table based on the vacuum or boost of the engine and the speed of the engine AT THE TIME that this information is needed by the PCM. Being Open Loop, the PCM doesn't care what the oxygen sensors are telling it. It is presuming that the values in the VE table are correct and that is all it needs.

Of course, there is still a wrinkle in this. When we are trying to tune the VE table, we are still dependent on feedback from a wideband O2 sensor. And the same issues apply with trying to determine if the air charge that we are getting feedback from at the wideband is actually applicable to the VE table entry that the PCM is using. This can be compensated for pretty handily, though, by spending a lot of time doing steady state tuning, meaning, gathering sampling data from the engine at consistent engine speeds over the entire range of combinations of engine vacuum/boost and engine speeds. If you have those values as close to perfect as you can get, without any oddball steep transitions from one cell to the next, that will be as close to a perfect tune as you can expect to get. At least as far as the basic air/fuel ratio the PCM calculates using the VE table. Of course, there are other types of modifiers that the PCM uses (coolant temp, incoming air temp, etc.) but they can be managed much more easily if the basic values used by the PCM are accurate.

Anyway, that sort of stuff has been bothering me about this tuning stuff. It may not be really all that significant, but I read so many accounts of people doing tunes on cars where they put the car on a dyno, make a few pulls, make some changes to the tune under wide open throttle, and call it a day. Seriously, if you do ANY changes that changes the airflow characteristics of your engine, it is my opinion that you are getting short changed. Especially if you are still using a basically stock tuning methodology using the MAF and oxygen sensors in a Closed Loop configuration. Quite likely, when you are not driving in WOT, your PCM is working overtime trying to compensate for values that are making it swing widely from producing lean and rich fuel ratios. A good way to check this is to disconnect your battery for a while, and then after reconnecting it, take the car for a drive. Does it run rough for a short time and then eventually smooth out? Well, your tune is screwed up, and is getting smoothed over by the short and long term trims. THEY are compensating for a tune that is not even close to being optimal for your car.

IMHO, of course.

I'll proof read this some other time. It was tough enough just typing it in...
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