Found online while reading some FAQ’s on the HP Tuners forum
[learn_more caption=”Engine Modes”] Engine Modes There is a bit of confusion over the different modes the computer can enter so im going to try and clear this up for everyone.
- MAF Closed Loop-MAF is Active, Main fueling O2 sensors are Active Most Stock LS1’s run in this mode corrections are made by the MAF, VE table & O2 sensors at part throttle & from 4000 rpms the MAF takes control of fueling based on your PE, OLFA, COT & any associated adder tables
- MAF Open Loop-MAF is Active, Main fueling O2 sensors are Inactive or removed Many use this mode when the o2 sensors cannot be trusted for whatever reason ie Big cam, LT’s, etc.
- Speed Density Closed Loop-Maf is inactive, removed or Failed, Main fueling O2 sensors are Active Many revert to this mode after doing VE tuning in SDOL mode so the computer can make some corrections for fueling Unless using an HPTuners Custom Operating system you will be stuck in the Low Octane Table Unless using an HPTuners Custom Operating system you will be stuck in the Secondary VE Table(where applicable)
- Speed Density Open Loop(OLSD or SDOL)-Maf is inactive, removed or Failed, Main fueling O2 sensors are Inactive Most will say this is best for VE tuning because you have complete control over what the engine wants to do. Setup your OLFA tables as you’d like to drive around town Setup your PE tables as you’d like to drive around town Just remember the computer will always command whatever table is richest so keep an eye on any adder/multiplyer tables that may be in effect such as Cat Over Temp. Unless using an HPTuners Custom Operating system you will be stuck in the Low Octane Table Unless using an HPTuners Custom Operating system you will be stuck in the Secondary VE Table(where applicable) Bottom Line is to remember Is Speed Density mode doesnt dictate if your in Open Loop or Closed Loop. [/learn_more]
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.
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.