Okay here is what my new update is going to look like. Please read and give feedback (positive or negative).

The System

Let's look at the limits to the air supply first and then look into the fuel supply later. To have enough background to understand the limits lets briefly look at how the system functions. Basically your engine acts as one big air pump... pulling in air and pushing it out. The amount of air needed is different for every rpm and such so you've got a varying amount of air entering the system at all times. The air fuel mixture needed for safe efficient combustion needs to be maintained accurately so you need some way of monitoring the air coming in to get the correct amount of fuel into air/fuel mixture. The system our cars use is a mass airflow system. This type of system monitors 'real time' intake ‘air-mass’ ( which is calculated by measuring the intake ‘air-volume’ and then adjusting for the temperature and barometric pressure) to determine the correct amount of fuel to deliver. Inside your air filter can and inside the filter it's self you will find your Mass Air Flow Sensor (MAF or MAS). All air entering the system passes through the air filter and then through the MAF. The MAF then counts air vortices it creates which are directly proportional to air volume and comes up with an accurate air-flow (volume) count for that moment in time. Every moment it recounts and passes the updated info along. It passes the air—flow (volume) count to the ECU via a signal that carries the intelligence in the form of a varying frequency (frequency modulation). The lower the frequency of the signal coming from the MAF the lower the air-flow. This is a real time signal that constantly accurately reflects the volume of air entering the MAF... well almost. In fact it only measures some of the air. If you've ever seen your MAF you'll have noticed two honeycomb type screens leading into two separate sections of the MAF. The upper section is called the metered section aptly because all air going through this section is counted. The lower section is the unmetered section and simply passes air without monitoring it at all. So the signal going to the ECU is actually only the volume of the metered section air. What good is that you ask... only measuring part of the air coming in??? Well with simple physics the ECU knows how much air is passing through the unmetered section. Since it is a given constant size it will pass a certain amount of air that is directly proportional to the volume of air in the metered section. So the ECU can take the accurate measurement of the metered section air volume (air-flow) and add to it an accurately calculated volume of air passing through the unmetered section to arrive at it's total air volume amount. So now it can calculate the fuel required right??? Wrong... not yet. You've got the volume but what is important is the oxygen in that volume... and air with a specific volume can have different amounts of oxygen in it depending on the density of the air. So the ECU has to make an adjustment due to the density of the air and what it ends up with is the air-mass. Well the two factors that dictate the density of air are the barometric pressure and temperature. At a higher barometric pressure (lower altitudes) the air is more dense (contains more oxygen per unit of volume) and thus needs to have more fuel supplied. Also when the air is colder it is more dense and needs more fuel supplied. So there is both a barometric sensor and intake air temperature sensor in our MAFs. They give the ECU 'real time' values of each of these air-mass effecting variables. So to make it's calculation on how much fuel to supply it takes it's total volume calculation, does an adjustment for barometric pressure and intake air temperature which gives the ECU it’s air-mass calculation. Then it takes this value and runs off to it's factory defined fuel map and inputs the value and gets the correct amount of fuel to add as an output. There are 2 limits that the ECU checks against during this process and we’ll get to those in a minute. Now this is all that happens during open loop mode (wide open throttle -wot). During closed loop mode the ECU actually does something else... it starts listening to the o2 (oxygen) sensor. What the heck is an o2 sensor??? Well it is a sensor that sits in your exhaust and measures the amount of oxygen (o2) left after combustion. This oxygen amount is directly proportional to the leanness/richness of the a/f mixture prior to combustion. The sensor measures the oxygen and outputs to the ECU a voltage that reflects this value. This is an accurate representation of the actual a/f ratio and so the ECU gives it priority over its calculations. It then adjusts the amount of fuel needed according to the o2 sensor voltage. If you have an a/f meter you'll notice the thing cycling all over the place... this is normal and is just the ECU readjusting the a/f ratio all the time to try and get the most efficient and safe value. But all of this does not matter in open loop mode because the ECU stops listening to the o2 sensor and just reads off its fuel maps. Read this page to better understand the closed and open loop modes of operation: http://members.home.net/costall/ECUprimer/index.html .
That's it! If you don't understand read this all again before continuing.

The ECU Limits

Well there are two different limits built into the ECUs memory that it uses to keep you and the car safe. The first one you will hit I will call Pre-Fuel Cut and the second is well known as Fuel Cut.

Pre-fuel cut is a limit to the air-flow value the ECU gets from the MAF. Remember that the MAF measures the air-flow and then sends that reading to the ECU via frequency modulation (a varying frequency). The ECU looks at the value it gets and before it does anything else it checks that value against this preprogrammed pre-fuel cut limit. If the actual air-flow value is greater then or equal to the limit the ECU will initiate the pre-fuel cut defense mechanism. The pre-fuel cut defense mechanism is to activate the boost control solenoid (BCS), thus closing the built in bleed of the BCS cutting the boost down about 4 psi. This is all that happens, the ECU does not, in this case, retard timing as it would in an excessive knock instance… it just activates the BCS. If you have a knock LED you will see it start flashing indicating the BCS is being activated. If the ECU sees that just pulsing the BCS on and off is not correcting the problem it may even send a continuous signal to the BCS to completely shut it down (the knock LED would go on solid). If you do not have the BCS connected in your boost control system then no worries... ECU can’t cut your boost. When do you hit this limit? Well you hit it pretty early on. In all the cases I’ve seen cars hit it with a cat-back, K&N, and about 15~16 psi of boost. This is the first limit you will hit with regards to the air entering the system.

The second limit built into the ECU is the infamous fuel cut. This limit is not to air-flow as the pre-fuel cut is... this one is to air-mass. So once the ECU has calculated the air-mass (see first paragraph) and before it goes to it’s fuel map to determine the amount of fuel to add it checks it’s calculated air-mass value against a preprogrammed limit. If the air-mass value is greater then that limit the ECU will initiate the fuel cut defense mechanism. The fuel cut defense mechanism is rather drastic... the ECU realizes there is a real threat to you and the car and tries to shut you down totally. What it does is it cuts fuel and spark completely for a second. This has been best described as hitting a brick wall. Imagine this... you’ve all had bad plugs before and been at wot and felt the power loss of a missing cylinder (25% power loss)... you can feel that kick... now imagine 100% power loss as the fuel and spark is cut from all cylinders... that’s fuel cut. The consensus for when you hit this seems to be from anywhere between 16~19 psi.

So how do you get around these two limits preprogrammed into the ECU? Well there are really 3 ways to do it:

1) If you are lucky enough to have an EPROM equipped ECU you can get Todd Day to reprogram those limits for you. He can increase those limits to some ridiculous value so the limits will never bet hit. He can also do a lot of other great stuff for you at the same time... like turn your stock boost gauge into a knock sum gauge. Unfortunately not all our cars are equipped with an EPROM ECU so those ones can not be reprogrammed. Go here for more info www.tmo.com/ .

2) Replace the stock air/fuel supply system with an aftermarket one like a Vein Pressure Controller (VPC) or PMS. These things replace the whole system and all it's limits. This is the ultimate way to go... but only if you have the cash. These systems are rather expensive and require a lot of understanding in order to run correctly. That's about all I can say about them since I've never even seen one.

3) You can modify the signal going to the ECU from the MAF to make it look like less air is entering the system then there really is. By lowering the frequency of the signal you make it look like less air is in the metered section and the ECU makes it's calculation inaccurately low. So you can actually have enough air in the system to be over the limits but the ECU still thinks it's lower then the limits.

Okay let's look more carefully at the last possibility because I have no money for the VPC system and I have no EPROM in my ECU so I was stuck with solution number 3 as a default. I'm going to make up some fictitious units to give you the gist of the idea a bit clearer. Let 's say 6,000 gabs (fictitious unit) of air is flowing through the MAF into the system at wot. Let's say half is flowing through the metered section and half through the unmetered section. So 3,000 gabs of air is being counted in the metered section. This information is then passed on to the ECU from the MAF. The ECU then takes this air-flow value and adds to it what should be flowing through the unmetered section (3,000) makes some adjustments based on temp and barometric pressure (assume null) and comes to it's total air-mass of 6,000 gabs of air. Well let’s say the pre-fuel cut limit is 2,900. So when the ECU gets the air-flow value of 3,000 gabs from the MAF it sees it’s above the limit and initiates the pre-fuel cut defense mechanism… bad . So what if you reduce the air-flow signal going to the ECU and make it look like there’s only 2,800 gabs of air flowing through the metered section. The ECU then looks at the 2,800 and sees it’s below the 2,900 limit and so it assumes all is well. Now let’s look at fuel cut… let’s say the fuel cut limit was 5,900. So the computer checks it's 6,000 (unmodified) calculated air-mass against it's pre-programmed limit and sees that it is greater so it initiates fuel cut. Problem So again let’s manipulate the signal going to the ECU from the MAF to make it look like there is actually less air. Again you adjust the signal so it looks like 2,800 gabs of air are going through the metered section. The ECU in this case takes that number and adds what should be flowing through the unmetered section (2,800) and makes adjustments based on temperature and barometric pressure (assume null) and comes to a total air-mass of 5,600. It then checks the 5,600 against it's pre-programmed fuel cut limit of 5,900 and sees that it is less and so it does not recognize a problem and everything continues as it should (no fuel cut). Cool eh? So how do you make it look like there is less air-flow through metered section?

There are 2 ways I know of to manipulate the MAF to ECU signal. One way is with an AFC. It actually manipulates the air-flow frequency enroute to the ECU. So it can make the air-flow look higher or lower then actual. You control how much higher or lower it is up to +/- 30%. The best part is that you can have different variances for different rpm ranges (800, 2400, 4000, 5600, 7400). So for idle you can keep it stock (this is important and will be discussed later) and you can lower the air-flow readings for WOT rpms. Downsides to this method: well cash is one ($390 US) but not much of one and the other is that you'll still hit the MAF overrun limit which will be discussed later.

The other way is to open up the unmetered section of the MAF (the MAF mod). By opening it up more of the total air entering the system will flow there and thus less through the metered section. So of the 6,000 gabs of air entering say 2,800 will be flowing through the metered section and 3,200 through the unmetered section. The ECU has no idea the physical change you made to the MAF and calculates 5,600 (2,800 + 2,800) gabs of air total. The down side to this is that it is a mod that effects all rpm ranges. Why this is important? Well the physical decrease in volume happens though out the range which is fine during wot and during regular closed loop operations (regular driving). The place where it is not fine is during idle. That decrease in volume under this condition when there is very little volume to start with results in the ECU going substantially off it's fuel chart during idle. So your idle gets mucked up. The idle is pretty finicky and the volume has to be pretty close to stock in order for it to idle properly. So you have to physically make the volume stock at idle again. This is where that flapper door you've heard about comes in. This hangs down and blocks the extra airflow through the unmetered section during idle. So physically the flow goes back to stock (3,000 and 3,000). But when you tromp on the gas the door swings out of the way by the force of the extra air and it goes back to the 2,800/3,200 flow. So it now only looks cut at wot and uncut at idle. Any other time in between idle and wot is irrelevant because the ECU is in closed loop mode and looks at the o2 sensor voltage during those conditions and can adjust the amount of fuel based on that so no leaning occurs.

This problem with idle is not a problem for AFC owners. They can leave the airflow looking stock during idle and adjust the rest of the rpms ranges lower.

So you are making the ECU think there is less air in the system then there really is to get around pre-fuel cut and fuel cut. So 6,000 gabs of air are actually there but the ECU is supplying only enough fuel for the 5,600 it calculated as being there. Hmmm... that's a problem. It means your air/fuel ratio is gonna be lean. Well this is and isn't a problem. Our stock fuel maps create rather rich a/f ratios from the factory. So there is quite some room to lean things out... and leaning out is great because it means more power. BUT there is a safe limit to how far the a/f ration can be leaned out due to detonation. So I would not do any of this without some way of monitoring the a/f ratio. Get and a/f gauge or voltmeter hooked up to the o2 sensor so that you can see if you're leaning out too much. I personally don't run below .85V on the o2 meter at WOT on pump gas... in fact in the summer I usually keep it around .88V. So you can reduce that MAF to ECU signal frequency lower to postpone fuel cut but don't lean it out too much. To do this just adjust the high end rpm settings on the AFC to a leaner setting. Keep the low ones where they are or you'll screw up your idle. If you don't have an AFC and you cut out so much that you are running too lean there is a way to compensate. You can add variable resistor (pot) into the air temp line. The intake air temperature sensor has a resistance value that varies on temperature. A colder temp results in more resistance. Remember also that the colder the air the more dense it is and the ECU adjusts the fuel supply accordingly. So if you are too lean you can increase the resistance in that line as much as is needed to reach a safe value again. Only do as much as required to stay as close to the fuel map as possible. I currently run this configuration.

So that's it... you've postponed fuel cut and pre-fuel cut so that you are no longer hitting them. Now you just need to make sure the fuel supply is there and start upping that boost :-)

The Other Limit - MAF Overrun

You will hit another limit to the stock and that limit lies in the MAF. As to when this limit is hit I’m not 100% sure. Some people with dataloggers say they’ve seen it before fuel cut while others tell me it’s after fuel cut. Until I get a datalogger and do my own experiments I’ll have to leave this unresolved. But Todd Day lists on one of his pages that it does get hit until you have a 20G at 19 psi. Anyway, back to it... as the air-flow increases due to the boost increase the velocity of that air also increases. Unfortunately at a certain velocity and above the metered section no longer can create and read the vortices correctly... it starts missing counts. So a low and erratic count takes place and that info is passed to the ECU. This causes a low air-mass calculation and thus a leaning of the mixture. Again since it's an erratic signal the injector pulse width and thus a/f ratio become erratic too. Overall it is lean and will lead to knock and thus an increase in the knock sum value. The ECU will see all this and retard timing at any knock sum greater then 7. If the knock sum gets up to max (43) then the ECU will also start activating the BCS to attempt to cut your boost and relieve the conditions causing the knock. That really messes up your top end bad. Not only that but detonation is dangerous and can kill your car. So you have to find a way to reduce the velocity of air in the metered section back down to a point where it can still be read accurately. If you've already hacked your MAF to open up the unmetered section then you've already got rid of this problem. By reducing the physical volume of air in the metered section you have already reduced the velocity. So the MAF hacker won't hit this limit.

However the AFC boys will still hit it. You don't actually change the physical volume with the AFC you just manipulate the signal on it's way to the ECU. So the AFC boys will have to hack the lower section now too in order to postpone this limit. Then extra leanness and problem with idle can all be compensated for with the AFC settings (as long as the unmetered section isn't open too much) so there’s no need to add the flapper door and pot.

Also Todds mods will not protect you from this limit. So with his mods once you hit this limit you'll have to do the whole MAF mod (cutting, flapper, and adjustment or cutting and AFC).

Modding your MAF correctly gets rid of all three limits at once... and it’s the cheapest solution. If you are interested in this email.

That's it for this topic... give me feedback or ask questions.

A special thanks to Mark Hessler and Todd Day for their inputs and experience.


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Mike J

-Quadcylla-
92 Laser RS-T, FWD

Vortex Racing
http://www3.sympatico.ca/robin.jackson2/HomePage.htm

[This message has been edited by Mike Jackson (edited February 15, 2000).]

[This message has been edited by Mike Jackson (edited February 15, 2000).]

[This message has been edited by Mike Jackson (edited February 22, 2000).]

That was almost longer that my awd system explanation.

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Michel
it's going to be ready tomorrow, I promise
http://www.azur.qc.ca/rallye