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Okay, I'm pretty tired of explaining why S-AFCs reset, why CAIs can turn the CEL ON when they don't have air leaks, and things of that nature so I am going to attempt to share some love on how the fuel strategy works in most newer OBD2 cars. Bear in mind that this is a generalization, and can vary from manufacturer to manufacturer and model to model. Plus I'm only human and free information is liek anything else - you get what you pay for. That being said this may take a couple of posts to get it done right so please be patient :)

Lets start with the basics.

All gasoline powered cars currently being sold in the US (with the notable exception of Mazda Rotaries) operate on a 4 cycle combustions process. The power that is released from this process is derived from the chemical reaction of the HC in gasoline with the O2 in the atmosphere. The heat energy released in this reaction causes the gasses in a combustion chamber (remember air is only 21% oxygen - mostly its nitrogen with trace amounts of other gasses pretty much all of which are inert) to become super heated, and the natural tendancy towards the expansion of these gasses combined with the pressure from the combustion event is what drives a piston towards the crankshaft. I'm going to leave mechanical components alone for the moment, so take it on faith that an engine has a bunch of moving parts that allow air to get in and out of the combustion chamber and pistons that force a crankshaft to rotate, a rotation that eventually transfers its way through the driveline and causes the wheels to rotate as well.

This having been established, the relative power of an engine is obviously constrained mostly by the amount of air and fuel you can get into a cylinder to react. But its not just that simple either. You have to attempt to vaporize that fuel as fully as possible (liquid fuel is not very reactive) and you also need to try and make sure that you have enough fuel to get the most power out of the reaction without wasting too much. A lot of people think that this ratio is called stochiometry but it is not. Stochiometry actually refers to the ideal air fuel ratio to keep a catalytic converter operating at peak efficiency, ideal air fuel mixture for power is a bit richer than the touted 14.7:1. Anyways all major performace mods to a car are generally aimed at one of two things - either getting an engine to produce more power by getting more useable oxygen and fuel into the cylinder or to reduce the amount of parasitic loss to either the mass of the vehicle, mass of rotating driveline components, or even to aerodynamic inneficiencies in the vehicles shape. (Yes I know this doesn't take into account handling mods but thats a whole nother ball of wax)

Okay so we've established that we need to get oxygen and fuel into this combustion chamber thingy and that we need to cause a reaction between then and once that reaction creates work we probably ought to let the spent byproducts (exhaust) out of the cylinder to make room for new oxygen and fuel to repeat the process. This is accomplished using the 4 strokes of the 4 cycle engine. These strokes are termed Intake Compression Power and Exhaust (Or Suck Squeeze Bang and Blow in laymans terms). The intake stroke takes place during the downward motion on a piston in a cylinder. This creates an area of lower than atmospheric pressure in the cylinder. The intake valves are opened and the higher pressure air in the intake goes rushing in to fill the void. During this process fuel is sprayed into the inward rushing air stream. Eventually the piston reaches the bottom of its travel, the intake valves close and the piston begins to travel up again. This begins the compression stroke, so named because of the compression of the air fuel charge in the cylinder. The pressurization of the mixture is what makes getting so much force out of the reaction possible. Once the piston reaches the top of its stroke (or slighty before that) the spark plug fires and combustion takes place. This begins what is logically known as teh power stroke as that is where all the power is made. Eventually the piston gets back to the bottom of its stroke and its work is done. At this point we have a cylinder full of pretty much useless exhaust gasses, so we open some exhaust valves and allow teh gasses to start escaping. The upward force of the piston pushes the rest of the exhaust out.

Okay so there we have 4 strokes. Great! so thats pretty easy right... why the heck do we need all these stupid smog devices and catalytic converters and whatnot?

Well it turns out that combusiton in a cylinder is rarely perfect, and due to incomplete compbustion, lean and rich fuel mixtures and other factors the exhaust gasses actually contain some amonts of harmful gasses such as Carbon Monoxide, Hydrocarbons, Oxides of Nitrogen, Ozone, and Carbon Dioxide. Well no-one cared too much about them at first but in places like LA the effects of thoursands of cars making these gasses every day started to get to be pretty serious in terms of creating smog and causing other problems. To counteract this the government (through the EPA) and California (through CARB) demanded that auto manufacturers regulate teh amounts of these gasses exiting the vehicle. So the Manufacturers created Catalytic converters, air pumps, EGR systems, VVTi, EVAP systems, and other things to help. But the restrictions got greater and greater each year, and the fuel economy (or CAFE ratings) got tighter as well. So the manufacturers needed to do more than just attempt to neutralize these gasses once created, they needed to attempt to prevent their creation in the first place and to try and make every drop of fuel count. To this end electronic control systems were born.

Electronic controls started with electronically regulated carburetors, (which really blew goats to be honest) and soon moved into whats know as electronic fuel injection. The ability to precisely meter fuel through injectors was great, and manufacturers soon moved from simple throttle body based injection to port fuel injection (what we have today). In some countries where the fuel doesn't suck so bad they even have direct injection which allows super high pressure fuel to be sprayed directly into the cylinder. Each of these steps let the manufacturers more precsely control how much fuel went into each cylinder as well as improving the rate of vaporization and reducing the amount of fuel wasted as liquid. While fuel injection technilogy is in and of itself pretty damn cool, the computer cant just look at your foot and guess how much power you want, so a large variety of sensors were created to monitor the load demand you create on the engine. They also created other sensors to monitor the exhaust coming out of the engine in order to run the catalyst at peak efficiency and minimize waste. There have been many sensors used for different purposes over the year but lets stick with the Celica. Heres a list of the basic ones and what they do:

MAF - Mass Airflow Meter - Possibly the most important sensor, it measures the density and velocity of air entering the engine. It does this by inducing a current into a "hot wire" element and measuring to see how much heat the passing air can extract from it. The more dense the air moving past, or teh higher the velocity, the more heat that will be transfered. Hence this sensor gives the computer a pretty dang good idea of how much actual mass is passing through it. BUT the sensor is only measuring a sample of air - what portion of the total air charge measured in the sensor is directly related to the position of the sensor in the intake, and even so extensive calibration has to be done to determine what level of accuracy is delivered and what the ideal sensor position is. They also have to take into account the normal pulses that travel through the intake stream as a result of the constant starting and stopping of airflow created by opening and closing valves. This is why moving the sensor, coating the hotwire with oil, disrupting normal airflow, or introducing vacuum leaks into the intake closer to the engine cause problems. The precisely calibrated sample size is no longer quite so precise anymore. The number no longer represents an actual precentage of total airflow mass, and as a result this most important of sensors will lead the computer to make a false base air fuel ratio calculation. (in other wors, lets say it thinks that its getting 3 grams per second of airflow as opposed to the 4 it is actually getting. This will mean its initial guess as to how much fuel it needs will be enough to support 3 grams per second of airflow. This will eventually resuly in high positive fuel trims and possibly a P0171 system too lean code. If the opposite occurs and teh MAF reports a falsely high number you will see high negative fuel trims and possibly get a P0174 code. The MAF signal is what the S-AFC is attempting to modify to get the ECM to give the car more fuel. The problem is that to the ECM this is no different than a faulty MAF and it will correct the fuel strategy with negative fuel trim just like it would with a MAF that has a peice of thread, or a foreign object in the intake creating abnormally high air velocity past it.

TPS - Throttle Position Sensor - this sensor is of huge importance, as its position it the computers first and best indication that you are placing an increased load demand on the vehicle. It factors hugely in fuel control and load calculations. There is no real horsepower to be made here however, as lying about throttle position will only confuse the ECM into making a false load calculation. Most people who play with it find that changing it mostly just makes the car run like ****.

ECT - Engine Coolant Temperatue sensor - this is important because the amount of fuel needed to sustain a reaction and produce power varies by how much latent heat is in an engine. Until it begins to warm, an engine needs more fuel, and once warm it needs less. This is due to changes in the vaporization rate of the fuel and viscocity of the oil, both of which work against you in a cold engine. The cool air vaporizes les fuel making teh fuel that is sprayed less reactive. The higher oil viscosity (thicker oil) can make it harder for components to turn due to friction. Also the lack of suffucient amounts of oil in some areas provides additional drag. This sensor, while important, can and will be over-ridden by the oxygen sensor to a large extent once the vehicle enters closed loop fuel monitoring. The important thing to remeber is that while the MAF is very important it is also preliminary- the O2 sensor is ultimately the judge jury and executioner when it comes to determining exactly how much fuel will be used.

IAT - Intake Air Temperature Sensor - this sensor measures the temperature of the air in the intake. Mostly this sensor is used to assist with the amount of fuel used during a cold start. Once things are running normally the MAF operation eliminates the need to use the IAT for fuel calculations and it becomes what is mainly a diagnostic reference. The fact that so little weight is placed on the value being reported is what makes the famous "E-Bay Mod" so worthless. The little bit of change you can actually make is completely over-ruled by the Oxygen sensor during closed loop fuel control.

Oxygen sensor (or air fuel Ratio sensor in some cars) - a sensor placed in the exhaust stream to determing just how much oxygen is coming out with the exhaust. Its pretty smart when you think about it - if we sprayed enough fuel we will have almost no oxygen, too little fuel and we will have a lot of left over oxygen that has not been used, too much we have no oxygen at all. The sensor closest to the engine is considered the primary or number one sensor. It is the only sensor that is as or more important than the MAF to fuel control. All of the "fuel Trim" you will see in an engine is the result of what is reported by the oxygen sensors. The secondary sensor, while not used to modify fuel control for performance reasons, does monitor the condition of the air leaving the catalyst. It does this for two reasons. The main one is to confirm that the catalyst is still working. If not the CEL comes on and you get a P0420 code. The other reason is to monitor the exhaust for catalyst damaging exhaust levels. If it sees a bunch of oxygen it may determine that the catalyst needs more fuel to run right and increase fuel trim. The main thing to take away here though is that the Primary O2 sensor is God in an OBD2 car. No matter what bull**** you feed the computer about other sensors, it will still be able to bring fuel back to pretty much right where it wants it to be with the O2 sensor readings anf appropriate fuel trims.

Crankshaft Position Sensor - Hugely important sensor - it is used to determine engine position and speed (rpm). It is key to proper timing of ignition and fuel delivery. It is also used to watch for misfire.

Camshaft Position Sensor - These days it is used to measure position of the intake camshaft relative to the rest of the engine which is how the computer is able to control it with the VVTi system. (A control system has to know where a thing is to know how much more to move it and to make sure it is where it should be)

Knock Sensor - Measures for spark knock (a very dangerous condition) this sensor enables the computer to retard timing and adjust fuel strategy to protect itsel form damage when you have bad fuel or other contions like it.

The Great Unknown - Fuel - The ECM does not monitor fuel quality or pressure in any way. It simply assumes that it always has a correct pressure and flow rate of high quality fuel of the appropriate octane rating. It bases injector duration on this supposed constant. Low pressure can lead to a lean code and high pressure to a rich code. This is why it is critical to use a high quality fuel of the appropriate octane rating. One potential danger in fuel is Sulfur - it can clog injectors, leave deposits on just about everything, and makes your exhaust gas smell like rotten eggs. Additional problems can be cause by tar, coke, carbon, and other elements that have not been refined out of the fuel. All of these things can lead to nasty carbon buidup in cobmustion chambers and on valves and its all just bad news. I strongly urge you to support any efforts you hear about to regulate fuel quality or mandate fuel testign and public posting of resluts - the fuel quality in other countries with similar tech bases is way higher, especially Japan. Better fuel means more high precision parts and less problems down the road for your car.

Well theres a lot more to talk about but its getting late and I'm tired. I'll try to answer any questions anyone has as best as I can.

a Quick couple of words about some of the problems we see and why:

CAIs - The theory behind CAIs is taht they supposedly deliver a cooler (denser) intake air charge with a lower restriction than a traditional intake, which is generally true. The main problem is stuff gets left loose creating vacuum leaks. Remember any air that does nto pass by the MAF sensor is considered "unmetered" and hence is not known to the ECM. It will eventually make up for it in fuel trim with the O2 sensor, but there may be some lag, rough idle, and CEL issues (most likely with a P0171) The other problem is one of positioning and design. Remeber how critical I told youMAF positioning and tuning was? Well aftermarket manufacturers don't know or car about that for the most part because they know that if they make something shiny and loud people will buy it. Some companies (Like AEM) seem to have learned from their mistakes and are finally coming around to knowing what to expect and positioning the MAF to realize a minimum of negative consequences. As for WHY teh problems happen, typically they wind up mounting the MAF so that the percentage of air flowing through relative to the total percentage entering the engine is not the same. While air is considered a child of fluid dynamics, it IS compressible and its flow characteristics can be a real bitch sometimes. Most notably air velocity as it comes around a bendin a pipe, or turbulence created by imperfections in the pipe itself will have an effect. Having the sensor out of parallel to the direction of airflow will also reduce the amount of air going through it can cause issues.

S-AFC - same deal as above. It basically tells the computer you are getting more air than you really are by changing the MAF signal. But as I mentioned in the O2 sensor segment, the ECM will adapt to the actual circumstance of what is coming out teh exhaust valve and eventually fuel trim will negate any positive effects from the S-AFC. Some folks argue that this is not true at WOT (wide open throttle) simply because the ECM drops out of Closed Loop fuel control and supposedly continues based purely on MAF and TPS for maxium power. The problem with this assertion is that any fuel trim that has been established at the lower RPMs will carry over to WOT, so the effect will still be diminished if not completely eliminated.

IAT EBAy Mod - I think I covered this well enough in the normal IAT section but basically its a resistor thats supposed to make your car think its always cold so you always get your "cold start enriched" fuel curve. Its a neat idea, but the IAT is such a minor factor in a MAF vehicle that the change is minimal and it is compensated for by the O2 sensor.

Gause or Foam Type oil coated high flow filter - These really do work great, they reduce the amount of resistance to air trying to enter the engine. One big problem - if you oil them too much teh oil gets on the MAF wires and plays havoc with the sensor. Please read the directions on servicing them carefully - they need a LOT less oil than you might think, and they require a good long time to sit and let capilary action distribute the oil evenly across the filter element.

Nitrous Oxide. - Essentially its more oxygen. Nitrous is something liek 35% oxygen as opposed to normal air which is 21%. Its also stored under really high pressure. These 2 factors combine to let you cram a lot more potential energy into the cylinder than you can with a normal air charge. the bad news is its not a self renewing resource, and it requires an additional amount of fuel relative to the additional amount of oxygen it puts in. The ECM cannot control this so it winds up being loosely controlled by the nitrous kit's nozzle design. The REALLY bad news is if you screw up ist very easy to go dangerously lean, which can cause very high cylinder temperatures, detonation, and all manner of nastiness.

Turbos / superchargers - simply put they compress air in order to be able to cram more of it in to the cylinder. Like nitrous it obviously requires more fuel. the good news is that you can control that fuel with a standalone system or a clever enough piggyback (so I'm told - I have no personal positive experiences with a piggyback) The stock ECM is not capable of giving enough fuel to compensate, and it does not know how to compensate for the higher pressure of the intake charge. Boosted induction is anything but simple - a LOT of thought and design work goes into a good setup and there are a billion things that can go wrong. Like camming a car properly its kind of an art form in and of itself.

Theres more to say and my typing probably sucked but too bad - I'll go through the post tomorrow and try to clean it up. Nite all.
 

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Damn dude, how long did it take you to write that $hit. :) Great info, though. I'm going to need to spend more time reading it.

Curt
 

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Cliff notes version? Just kidding. I'm still reading.
 

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Move your low throttle settings to zero and tune for wfo with a high - high throttle setting (80%) is what I use and it works fine on a Spyder.

You should be able to figure out when the ecu goes into open loop with a pocket logger and simply make no corrections below that point. That will keep the ecu from realizing there should be corrections and the s-afc will work just fine.

BTW according to my data logger there is zero fuel correction in open loop. MAF and ict is it. Again this is on a spyder. Does the celi ecu show a fuel correction % in open loop?

On another note if you want to richen up your mixture for cheap have another 02 bung welded further down stream on the exhaust tract. The 02 will up the fuel trim to read the same voltage but it doesn't know the egt has lost some heat through the exhaust. This works on a spyder btw. :) YRMV on a celi.
 

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LilRocketSpyder said:
Move your low throttle settings to zero and tune for wfo with a high - high throttle setting (80%) is what I use and it works fine on a Spyder.

You should be able to figure out when the ecu goes into open loop with a pocket logger and simply make no corrections below that point. That will keep the ecu from realizing there should be corrections and the s-afc will work just fine.

BTW according to my data logger there is zero fuel correction in open loop. MAF and ict is it. Again this is on a spyder. Does the celi ecu show a fuel correction % in open loop?

On another note if you want to richen up your mixture for cheap have another 02 bung welded further down stream on the exhaust tract. The 02 will up the fuel trim to read the same voltage but it doesn't know the egt has lost some heat through the exhaust. This works on a spyder btw. :) YRMV on a celi.
Hate to say it but Celica has more strengent programming in the ECU, it behaves differently from Spyder. And this difference allow S-AFC to work on Spyder and not Celica :(
 

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cool2miketlu said:


Hate to say it but Celica has more strengent programming in the ECU, it behaves differently from Spyder. And this difference allow S-AFC to work on Spyder and not Celica :(
Come to think of it they must be different. How many 02 sensors does the celi 1zz have? The spyder has 3.

I am a hard headed arse so I am going to run my own test when I get the 2zz in since the s-afc is already installed. The good news is if it doesn't work I can sell it and not be out any additional money for the whole swap. :) Nothing like 50 h/p for free :).
 

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Discussion Starter · #16 ·
LilRocketSpyder said:


Come to think of it they must be different. How many 02 sensors does the celi 1zz have? The spyder has 3.

I am a hard headed arse so I am going to run my own test when I get the 2zz in since the s-afc is already installed. The good news is if it doesn't work I can sell it and not be out any additional money for the whole swap. :) Nothing like 50 h/p for free :).
They are very different. the differeces in the fuel control logic stem from the fact that the Celica meets a much tighter emissions standard than the MR2 does. Due to the low volume of cars imported the MR2's focus was kept purely on performance. I think the MR2 is a LEV as compared to the Celica which is ULEV
 

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Discussion Starter · #17 ·
SQ GT-S said:
Damn dude, how long did it take you to write that $hit. :) Great info, though. I'm going to need to spend more time reading it.

Curt
Couple hours I guess wasn't really paying attention.
 

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Griffin said:


They are very different. the differeces in the fuel control logic stem from the fact that the Celica meets a much tighter emissions standard than the MR2 does. Due to the low volume of cars imported the MR2's focus was kept purely on performance. I think the MR2 is a LEV as compared to the Celica which is ULEV
The spyder is also a ulev. Anybody wanna buy a spyder 1zz ecu LOL?
 
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