Car Accessories and Performance Parts - part 2 - air metering and adjustments.

In this article of basic efi tuning, we're going to take a look at one of the major sensor values on any modern OBD-2 vehicle, their options for Performance Parts and Car Accessories that modify these values to reproduce changes in the performance of the vehicle.

To begin, we're going to discuss the 2 major air metering devices the engine uses to determine how much fuel to inject at any given moment. These would be the Mass Air Flow (MAF for short) and Manifold Absolute Pressure sensor (Map for short).

To begin with MAF sensors, the most barbaric of the bunch was the vane sensor. In more common terms, this was simply a paddle that set in the intake tract, and correlated how far it moved as the air moved over it and produced a 0-5v (0 volts meaning no air, 5 volts meaning the max the sensor could read) signal that the ecu could use to deterine how much air was coming into the motor. This was also calculated with an Intake Air Temperature sensor (IAT) since it did not actually know the mass of the air coming in, just it's flow rate. While these worked, having a paddle resting in the intake tract produced turbulence further back then desired in the intake tract, as well as moving parts with the possibility of failing. Most modern cars do not employ these anymore, I can't think of one to tell you the truth that I regularly work on, and even so, it would be beneficial and rather easy to find a comparable hot wire MAF to replace it.

The second MAF sensor, and most common in today's higher performance vehicles, is the Hot Wire MAF. This sensor is much more sensitive to actual air density with how it is designed and works. This sensor works by heating a suspended wire inside of the intact tract by a specific voltage. As the wire heats up, resistance is added to the wire which the computer correlates into a mass air flow signal. The more air, or the denser air flowing over the wire, absorbs some of the heat of the wire, thereby lowering it's overall temperature and reducing resistance. Most of these sensors have circuitry in the housing itself to convert the signal to a 0-5v response for the computer, though some manufacturers just read the resistance. On vehicles only looking at resistance, corrosion of the wiring over time can cause severe driveability problems, and is the main reason why most manufacturers moved away from that system.

The third sensor to look at in calculating air flow is the MAP sensor. These are very common in your Chryslers and Hondas, to where as the MAF is seen on Fords and Chevrolets mostly. These sensors, instead of resting in the intake path of the motor, reside on the intake manifold, or routed to a source from the intake manifold. The computer uses this sensor to find out the air pressure of the vehicle, and then other sensors and values to determine the amount of air in the motor. A simple way to describe how a computer looks at this is as follows:

Vehicle is at 7hg of vacuum - Throttle position sensor - 50% - correlation table of 50% load - medium injector duty

or

Vehicle is at 16psi of boost - Throttle position sensor - 100% - correlation table of 100% load - maximum injector duty

or

Vehicle is at 7psi of boost - Throttle position sensor - 45% - correlation table of 50% load - medium injector duty

The correlation tables are basically pre-programmed load grids, which take into account many other sensors as well too, such as ambient air temp, intake air temp, throttle position, throttle inlet pressure and anything else the engineers can think of and throw into them. The figures above are just made up truthfully, so don't repeat them to your friends as an actual cars load tables, but merely a simple explanation of how they work. I couldn't put it into words any better then doing that.

If you're still reading then that's good, you're about to get into what these really mean and how to use them to your advantage. If you feel a wall of text smacked you in the face, then that's also good. Simply put, I can't stand ignorance in the slightest, and if I'm going to give you information on how parts change your car and what to do with them, I want you to understand the impact and exactly how they work. I hate posers, and retards that really have no clue what they're talking about. Now onto the good stuff

Performance Parts
Quite a few car accessories can help you modify these sensors to gain additional horsepower and torque from your vehicle safely. These include some common ones such as Apex'i's S-AFC, S-AFC II, and S-AFC Neo, Greddy e-manage, TurboXS D-tec, and even something as simple as a voltage clamp. They all do the same exact thing, and that's change the voltage signal or resistance that the air metering sensor provides and making the computer THINK there is either less or more air in the motor so that it can adjust it's fuel maps.

The simplest of all Performance Parts, the voltage clamp. A small circuitry to limit maximum voltage on a line, which in our case is the 0-5v sensor signal to the engine or powertrain control module (ECM or PCM depending on vehicle). As an example we'll use a Dodge SRT-4, a highly powerful map sensor based turbocharged 4 banger from the boys at Chrysler's PVO team. Stock the vehicle us a 2.25bar map sensor (bar = 14.5psi roughly), meaning it can read from 29hg of vacuum to about 18psi of boost (with 15psi being the stock target). The computer uses the voltage signal of the map sensor to determine how much pressure or vacuum is in the intake manifold and adjusts fuel to it, obviously giving more fuel for more air being in the chambers then less, to give it an optimal air/fuel ratio. While working on numerous SRT-4's, from the basic car accessories, to the larger performance parts upgrades, one thing has been constant across the board, and that was that these cars are rich, pig rich, and could gain a healthy amount of horsepower by leaning them out.

So as we learned before, the map sensor gives a varied voltage between 0-5v to the computer based on air pressure in the intake manifold. Our main goal is to lean the car out at max boost, to bring it up from it's current 10.2:1 Air/Fuel ratio. Bypassing the vehicle's factory boost control (which also reads off of the MAP sensor) with a simple manual boost controller, and our simple map clamp wired into the map sensor, we can now begin testing the vehicle to find the optimum clamp point for Wide Open Throttle (WOT) under full boost. So, as we begin, prior to adjustments, the car at 15psi is seeing roughly 4.5 volts at the sensor. We're going to decrease this maximum voltage, limiting it's highest value to about 4.1 volts via the map clamp (every car is different, do not use these values without accurately testing the vehicle with an air/fuel monitoring device to find your cars specific values and reactions). Effectively, we have made the computer think that the car is only running 12psi of boost now, so it adds fuel for that, when in reality the car is running at it's 15psi target still. This has had the net result of leaning the vehicle out at max boost, but has changed nothing for it's partial boost regular driving. Most of the times this was good for a solid +10whp on all cars, and a map clamp costs about 30 bucks, or 5 if you make it yourself. Simple changes like this though do have adverse effects on some computer systems, which are described near the end of the article.

This same exact principal works on MAF as well too. Both styles of sensors give the car the same basic information, limit their value, you limit what the computer sees in terms of air being available to the motor.

Other car accessories in this field are just more advanced, and definitely fall into the dedicated performance parts section. Each one is basically an evolution of the simple map clamp, though with the ability to increase the sensor value as well as decrease it across many points in the rpm range.

The Apex'i AFC's are by far the most common of the bunch, and have been in use for years. Again, nothing more then an advanced map clamp, but now it lets you change it across a broader spectrum of the range, and also take into account other values such as throttle position. AFC's have 2 basic maps, low throttle and hi throttle, and then scale in between. Before we get too deep into this one, I really suggest reading my very first lesson if you haven't, Car accessories and performance parts - part 1, the basics of fuel pressure. I'm going to stress again the most important part, if you're adding or removing at least 5% across the entire band, a fuel pressure change is needed more then MAF or MAP adjustment. If your fuel system has been increased so much that you've lowered fuel pressure past acceptable limits, and still pulling fuel, then you my friend need to stop playing with piggybacks and put a real standalone EMS system in the car. Limiting your sensors range is inviting problems in driveability and ultimately your car's computer from being able to adjust correctly to further changes from performance parts and various accessories.

AFC's, and the highly more configurable D-tec and e-manage allow you to smooth out those air/fuel ratio inconsistencies in your vehicles maps. For the novice with a modified fuel system that really needs some tweaking, I actually think the d-tec and e-manage are far more user friendly, though they aren't the most popular or normally people's first pick over the AFC. The AFC was one of the first, easy to use, digital controllers in that performance part segment, and the other car accessories like it have been growing up in it's shadow. One of the main features I personally like about the dtec is it's interface with a gameboy, and the ability to incorporate a wide-band air/fuel sensor into it. All of these parts work off of throttle position and rpm correlation, but the d-tec has a nice interface that makes it easy to find that part of the table, realtime, or afterwards in a log file of the run. Air/fuel dips in a certain area, go back to the log, find the highlighted box in the table, and make your adjustments to it. Very easy, very simple.

As for further walkthroughs you will have to look elsewhere for those. At this point I'm not going to do a step by step tuning guide for each and every piggy-back on the market, but jump into the problems that arrise when you're using these performance parts on most modern vehicles.

Most common problem I run into, especially on my earlier example of the SRT-4, is the fact that some people try to ADD fuel with a piggy-back. This can and cannot work in some circumstances. The main reason why it fails to work, is that most people are trying to add fuel at the top of the sensors value range already. All ECM's and PCM's have a value that is not supposed to be crossed, this is referred to as the limit easily enough. The most common rookie mistake, and one that I fix weekly from competitor shops in the area that don't know better, is taking a car that's slightly lean, but with a smooth fuel curve, and adding 5-10% to the map or maf value. The control module sees this as a problem, and possibly an open short to the 5v line or loss of resistance in the sensor, sets a check engine code, and ultimately puts the car into a 'limp mode' meaning it's running off of a pre-determined map and no longer taking said sensor into account for critical calculations.

Another common problem with increasing fuel is the computer trying to protect itself. When manual control of the boost has been taken over, and you push a value far too high through the air metering sensor, the computer will cut fuel to protect itself. No fuel = no combustion, and highly against what we're trying to accomplish here with tuning the car for more power.

That's not it for problems, oh no, not when using piggybacks. Here's 2 more that people almost never even glance at when installing car accessories and performance parts. Long-term and Short-term fuel trims, and ignition timing. Oh boy lets look at ignition timing first, as this one can be a killer when not monitored.

Vehicles use their air metering devices to calculate load on the motor. When these values are decreased, the engine simply thinks it's not working as hard, and some, but not all decreased injector duty (yay what you wanted to lean the car out) and then raised ignition timing for better combustion and performance at lighter loads. As we get into further articles, you will definitely learn, that as horsepower goes up, timing needs to go down to prevent detonation which will damage pistons and other portions of the rotating assembly on the motor. Many OBD-2 monitoring devices can datalog these crucial values so that you can see the changes as the air metering device has been altered. This is just one more reason that when installing car accessories and performance parts, you should always go back to the most basic adjustment first, fuel pressure and get it as close as possible with just that alone.

We're not done yet though. While not as deadly as the ignition changes to the motor, the short term and long term fuel trim changes can wreak some major havoc as well too. You may THINK you have the perfect tune, but be miles away from it. These 2 values are actually very self explanatory here but I'll do it anyways for somebody not following. The short-term fuel trim monitors the o2 sensors on the vehicle, and make adjustments based on their readings to accomodate for things such as clogged injectors, changes to the fuel system, or anything along those lines. This value alone can add or subtract 25% injector duty in most modern cars. If this value stays in a certain range for long enough, the vehicle then converts some of that to long-term fuel trim, which gives an additional plus or minus 25%. So lets get to an example here to make this easy to follow.

You've just installed your new car accessory, this fancy performance part is letting you make all kinds of changes to your fuel delivery so you're driving around, correctly might I add, with a wideband air/fuel sensor to monitor your changes. After about 20 minutes of driving you feel like you have it, that the tune is finished and you're all set. Here's what 99%, yes I'm saying 99% considering how many cars I've ran into this on never checked, is your STFT and LTFT. When you started your car was extremely rich, so it ran for a bit as you made fuel pressure adjustments and got it close, correctly might I add, then you started working in your AFC while driving some more. This ENTIRE time the car was rich, and the computer knew this and was making adjustments of it's own. It pulled all 25% of your STFT on first startup to get the car to an acceptable level, and then needed more as you tinkered with it, so it started to pull LTFT as well too. By the time you were done, in REALITY, and I'm not making this up, your vehicle pulled 30% of the fuel and you did 5-10%. Don't believe me, I dare you to run the same situation on a fresh tune and not get any STFT and LTFT during it.

Once your tune is complete in your head, you should ALWAYS go back and read the obd-2 data to see where the factory computer is resting at. It is very easy to fix this, and does not take much work other then the fact that there is more time involved. If your car is resting with negative fuel trim values across the driving range, then help it along some by pulling just a tad bit more with your AFC. These values do not react as fast as your performance part does, so you may have to drive for 5-10 minutes to see a change in LTFT, though STFT normally reacts very fast, damn near on the fly. Also remember that STFT is taking into account LTFT already, so if you see STFT at 0, but your LTFT is at -20%, the car is still pulling -20% at that point, the short-term just doesn't feel the need to do anymore adjustment.

Ideally when all is said and done, your LTFT and STFT should be within +/- 5% of 0, and your new performance part should only be pulling at max 10% in some areas, and not across the entire band. If you're obviously out of this value range, regardless of how good you think you are, something was done wrong. Sometimes it isn't necessarily the tuning, but certain performance parts just don't work with the application as well as other accessories do (i.e a larger set of fuel injectors, higher flow fuel pump, rising rate fuel pressure regulator compared to static). All those items can drastically alter the fuel curve in the vehicle, and if not matched properly, is very hard to overcome.

If you're still a dedicated reader from part 1, you'll notice I said piggyback systems suck, and that's mainly because of the misinformation floating around about what they can and cannot do. In the grand scheme of things, for lightly modified cars, they can do wonders, and even if you use one yourself on a modern efi vehicle, trying to avoid cockiness here, I'm sure 95% of you never even knew to look at these other values and just assumed all was good. Another reason why I don't put all faith into them as that they're limited, and some cars just can't use them as they pull from too many different sensors and revert changes you make (or adapt, however you want to look at it).

Again, use the comments section to let me hear your stories, nightmares, or triumphs with these systems, or if you found this article helpful and informative, even if it is a long read.

In part 3 of Car Accessories and Performance Parts, I want to touch on some of the myths of various power adders. What they can and cannot do for your car, I'm even going to break into some of the ebay power chips, how some of them really can benefit you in the power department, but why to still avoid them with the real facts as to how they work.