queston about adujsting the air flow meter(new idea)

[Silver Mine Motors]

Hey guys, I was searching to see how to adjust my fuel ratio on my 280zs air flow meter. I found this link which describes how the air flow meter works and also how to adjust it. Here is the link http://www.pbase.com/bronte/gallery/afm_adjustments

 

it basically says that the air flow meter sends an electronic signal in the form of volts to the cars computer. The more air passing through the air flow meter the more volts it sends to the computer.

 

So this is my idea: I started to think instead of opening the air flow meter and manually adjusting it to what we want, why cant we just put an adjustable resistor on the wire that sends the signal to the computer. The point of the adjustable resistor would be to adjust the number of volts the air flow meter sends to the computer.

 

This would be good because we would be able to adjust our fuel ratios when we are driving. All we do is put the adjustable resistor in the car.

 

So what do you guys think? would this work? Am I on to something?

[Silver Mine Motors]

so do you think this is possible?

[dig280z]

sounds do-able .

[RTz]

So this is my idea: I started to think instead of opening the air flow meter and manually adjusting it to what we want, why cant we just put an adjustable resistor on the wire that sends the signal to the computer. The point of the adjustable resistor would be to adjust the number of volts the air flow meter sends to the computer.

 

A series resistor won't do that. Resistors don't drop the voltage, they drop the current.

 

 

This would be good because we would be able to adjust our fuel ratios when we are driving. All we do is put the adjustable resistor in the car.

 

You can put a potentiometer in the water temp sender circuit and add fuel. In most cases, with mildy modded motors, a combination of adjusting the AFM and adding a potentiometer can get you pretty close.

[Silver Mine Motors]

so if i put a potentiometer on the temerature sender would it be possible to make it more lean?

 

 

also what if i put a potentiometer on the afm wire. would that let me adujst the ratio leaner.

[m4xwellmurd3r]

Putting a potentiometer on the Water temp sensor will ONLY make it richer, since it can only ADD resistance. HOWEVER. if you take off the plug going to the water temp sensor and use the potentiometer in place of the water temp sensor, as long as it has the correct range, then yes, you can tune it to be leaner or richer than what it normally would be. And I have no idea if you could do that to the AFM. I really doubt it though.

So basically, deleting the water temp sensor and using a potentiometer would do exactly what you want to, without touching the stock AFM. of course, you'd be wise to have a air/fuel gauge.

[Silver Mine Motors]

but if i did that the engine would not self adjust to temperature right? so i would have to play with it all the time.?

[MJLamberson]

but if i did that the engine would not self adjust to temperature right? so i would have to play with it all the time.?

 

Im not sure what you mean by that but the thermostat which adjust the flow of coolant works all by itself, the external sensors dont have anything to do with the flow of coolant, but like I said, this might not be what your talking about

[DuoWing]

I think he means that you wouldn't have the differentiated warm-up and then operating temps. You would have to adjust for the two settings. I think you'd just be better off opening up the AFM marking the default starting spot and little by little adjusting the spring tension to make it run more lean.

[JSM]

All things asside, it was a great thought.

[BobbyZ]

A series resistor won't do that. Resistors don't drop the voltage, they drop the current.

 

Ya they do. The voltage on one end of the resistor will be lower than the voltage on the other. Current will also drop with the addition of a resistor.

 

 

I think this idea might work fine. If you added a variable resistor to the line than the ECU is going to see less voltage and assume there is a smaller amount of air, adding a smaller amount of fuel. Tightening the AFM would correct for this and then you would have a variable resistor that increase or decrease the percieved amount of air that the ECU sees, changing the amount of fuel added.

 

I think it would work fine, might be hard to find the correct range of resistance values to use without destroying the engine though.

[RTz]

Ya they do. The voltage on one end of the resistor will be lower than the voltage on the other.

 

 

Please explain the two pictures below...

 

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The resistor is 8000 ohms. Voltage is the same, within one tenth. Series resistors are not efficient at dropping voltage.

[RTz]

also what if i put a potentiometer on the afm wire. would that let me adujst the ratio leaner.

 

You said "adjustable resistor", I said "potentiometer". They're the same thing. Go back and read my first post, with that in mind and you'll have your answer

 

Taking a step back... as I said, a series resistor won't work. However, a voltage dividing circuit can be built out of two or more resistors. This is how the AFM works in the first place... it creats a voltage dividing circuit (as well as the temp sensors, TPS's, etc.) Google voltage divider... its one of the simpliest and most used circuits in electronics. I have not made an effort to figure out how do-able this would be in this case. There a few reasons, but I'll give you two to think about...

 

1) It would not only change the voltage at a given position, but it would also change the AFM's voltage curve.

 

2) The AFM 'tops out' before you reach peak RPM. In a stock NA L6, this happens somewhere around 4000 RPM, give or take. This means, at some point, the ECU is no longer measuring additional air. Any drastic changes you make to the AFM will make this transition very difficult.

 

I like your thinking... creativity is the spice. If you want to play with this stuff, I encourage it, but I strongly recommend using an O2 meter (preferably a wideband). This will help you understand the effect of your changes.

[Silver Mine Motors]

ok so now we know that resistors don't drop the volts in a current.

 

so in order for this to work what has to be changed in the current. volts?

[RTz]

I'm going to attempt to remove the size 13 from my mouth...

 

Post 12 is accurate. What I said about voltage dividers is accurate. However, I didn't stop to think that adding a resistor in line with the AFM would effect the existing voltage dividing network of the AFM. In retrospect, it should influence it.

 

I have never tried it because I think its the wrong choice, for the two reasons I mentioned in post 13.

 

Sorry for the confusion.

[BobbyZ]

I have no idea what that picture is showing. It looks like you are applying 14.6 volts across the lead. And then 14.6 volts across the resistor. What the display is showing is just the voltage you are applying.

 

When you have the 14.6 volts across the resistor, one lead is at 14.6 volts and the other lead is at 0 volts. The resistor has dropped 14.6 volts at about 1.825 mA.

 

Watch me! Its harder to see in my pictures but you get the idea. I am applying 5V to a series of 3 resistors current is 0.77mA. Watch the voltage drop (the little pointy things are the probes for the fluke meter)

 

Voltage is about 5V (resistance in the wires has caused a voltage drop)

 

 

 

After the first resistor the voltage has dropped about a volt.

 

 

Then a large amount of voltage is then dropped through the second resistor

 

That last resistor drops the final 0.62 volts, if I had probed the last point I would have gotten zero.

 

Two can play at this game...

 

Dont forget Ohms law. V = IR (The voltage drop across a resistor is equal to the current through it multiplied by the resistance) Resistors drop voltage just fine.

 

I still think that the variable resistor could work fine. Like you said, the AFR works off of a voltage divider. You are just adding adjustment to the divider. Might be hard to get right but it seems like it could work.

[BobbyZ]

Sorry I may have gone a little overboard with the pictures. Any chance to play with the Keithley sitting on my desk. That thing is so cool

[RTz]

Top of the page... http://www.doctronics.co.uk/resistor.htm

 

"What do resistors do?

Resistors limit current."

 

 

Top of the page... http://www.kpsec.freeuk.com/components/resist.htm

 

"Function:

Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED."

 

No reference to dropping voltage.

 

I have no idea what that picture is showing. It looks like you are applying 14.6 volts across the lead. And then 14.6 volts across the resistor. What the display is showing is just the voltage you are applying.

 

 

This is essentially what my picture above is measuring...

 

 

 

 

You can put as many resistors in series as you like and the voltage won't drop...

 

I'm having trouble seeing what's in your pictures, but it looks like its a voltage divider. Voltage dividers require two or more resistors arranged in a specific manor.

 

Like this...

 

Why does the voltage drop? Because you 'shorted' the circuit to ground. That's the way I understand it, anyway.

 

 

Sorry I may have gone a little overboard with the pictures. Any chance to play with the Keithley sitting on my desk. That thing is so cool

 

No apology... I like pictures

[BRAAP]

My first question is why do you want to alter the AFM for tuning purposes? Our tinkering over the years and testing on the Dyno revealed many things, and one is that the AFM is not really the place to perform electrical alterations unless you are trying to tune out/in something within the range it functions, which to date, we still have not found a situation for.

 

 

With that said, here is what we have found.

 

1) Water Temp sensor; In altering the resistance at the water temp sender offers a linear AFR change across the entire RPM range under all conditions. As stated above, the only way to get “leaner” with the OE EFI is to use the resistor/potentiometer in place of the water temp sensor itself, but then you loose the coolant temperature compensation. I’m sure a savvy electrical engineer could build an electronic doodad that reads the water temp sender and then alters that return signal to the ECU both above AND below that of the water temp sender resistance.

 

2) TPS; The OE TPS is like a single pole double throw switch. Idle is one circuit closed, WOT closes the other circuit, (actually kicks in at approx ¾ throttle opening), everything in between idle and ¾ throttle is “open circuit”. I tried a FIAT TPS years ago that closed the WOT circuit only at WOT, so at above 3/4 throttle position, the engine surged and had flat spots till the WOT switch was closed, at WOT. Discovering that demanded playing with a manual switch in the cockpit hooked up the TPS to see how the ECU uses that TPS switch, i.e. the throttle no longer actuated the TPS I did with a switch.

a) The idle circuit is a richer mixture and will only allow the engine to rev up to 3200 RPM, then as the RPMS drop, the EFI kicks back on at 2800 RPM. This is a VERY violent rev limiter! Many of you have found this after washing your engine bay and the TPS connector gets wet. This is incorporated for high RPM when you drop the throttle to closed, under deceleration conditions, the ECU shuts off fuel delivery to the engine completely for emissions. If you have a very free flowing exhaust, and depending on how rich your AFR’s are to begin with, will notice a slight to moderate ”POP” in the exhaust as the fuel is turned back on as the RPMS drop down through 2800 RPM, but only at closed throttle during decel.

b) The WOT switch alters how much influence the AFM has on the fuel map. Open circuit, mimicking part throttle cruise, the AFM has tremendous influence on the injector pulse widths. At WOT, it has substantially less influence and the pulse width is mostly based on the ECU dedicated map, though the AFM does still have an influence, remember, it is substantially less of an influence at WOT than it does at part throttle. Also, the when the WOT switch is closed, the base fuel map is fattened as well.

 

3) Thermo time switch AND cold start injector; The Thermo time switch function is to allow the cold start injector to dump more fuel into the intake via the cold start injector during start up only and only for a couple seconds, when it really really cold outside and the engine is really really cold. Without the cold start injector and/or Thermo time switch, if the EFI is in good tune, the out side temp is REALLY cold, (around the freezing point) and the engine is also cold, the engine may take a few more revolutions to fire off during cranking. That is the only side affect of eliminating the cold start injector and Thermo time switch.

 

4) Inlet Air Temp sensor; The IAT is in the front of the AFM, looks like little white pencil eraser of sorts. It reacts to VERY quickly to temp change, (tested a few of them from freezer to heat gun using an OHM meter). The ECU uses that to adjust for air density based on Temperature. The IAT functions just the same as the water temp sensor and by altering its resistance has the same effect on AFR as the water temp sensor, just on a much finer scale, not as drastic as the water temp sensors affect.

 

5) AFM; This complex electro-mechanical device is a well engineered, durable, intricate piece of antiquated equipment, and performs its intended job VERY well! It functions is tell the ECU how much “volume” of air the engine is ingesting, hence the name Air Flow Meter, (MAF stands for Mass Air Flow sensor and MAF’s measure the MASS of the air the engine is ingesting, which is more accurate as the engine actually needs fuel added based on the mass of the air, not the volume). At WOT, the AFM actually tops out, full open between 4000-4500 RPM! After that, all fuel delivered to the engine is based on RPM, water temp, and air temp. The AFM has NO affect on the injector pulse width above 4500 RPM at WOT, (which is the main reason why I question wanting to “tune” the resistances within the AFM) The air flow flap opens as the “volume” of air pushes it open against the return spring and then passes around it on its way into the combustion chamber. Attached to that flap at 90 degrees to it, (see pic below), is the damper flap which is cushioned by the air space above it. Engine acceleration enrichment is delivered by the “over swing” of the flap in the AFM. The flap is also counterweighted. Altering the damper flap by drilling a small hole in the upper flap and/or removing mass from the counter weight, will allow the flap to over swing further, thereby fattening the acceleration enrichment. Loosening the AFM return spring also has the same effect, but also richens the mixture within the operating range of the AFM. Of all the L-series OE EFI tuning I have ever done, I have not found a need to alter that acceleration enrichment of the OE EFI above and beyond what adjusting the return spring for good cruise AFR has delivered.

 

 

As an aside. Back in the mid ‘90’s, I had the wild idea of eliminating the AFM from the air stream altogether. I sacrificed an AFM and cut up the body, connected a bicycle brake cable to the damper flap, bolted this AFM to my intake manifold plenum and connected the other end of the cable to my throttle linkage. The idea was to use the AFM electrically, but actuate it with the throttle, not the air stream! Also had the TPS switch in the cockpit so I could manually trip the WOT circuit during this testing. WOW what a learning exercise that was! To drive the car without the engine stalling completely or at least drive it somewhat smoothly, the throttle pedal had to follow the RPM. With the TPS in cruise condition, (open circuit), that range of pedal position to RPM was a very narrow range. With the WOT circuit closed, I had much more leeway in the throttle position to RPM range before the engine would let me know it didn’t like it, hence the WOT circuit not relying on the AFM so much for calculating fuel delivery. Also, just to drive the car I had to accelerate the same at the same rate all the time and finding that constant cruise condition to maintain a given speed that the engine would like was, oh so difficult. I even added holes along the length of the flap to alter the rate at which it was being actuated.

 

In short, I found the AFM works as originally designed and works INCREDIBLY well, and due to the narrow range that it functions, which is only below 4500 RPM and mostly under cruise conditions, the only tuning or alterations I perform to the AFM is to adjust the spring tension. Outside of that, tuning the AFR’s is done with water temp sensor. Oh, one other point. My particular L-28, with the water temp resistance first set for max WOT performance, then the AFM adjusted for best cruise conditions, while at idle, the CO adjuster would allow the flap to close enough to allow a lean enough mixture at idle. So I used Dremel tool and ground a small trough in the floor of the AFM just under where the flap rested at idle to allow that air to enter the engine and not be registered by the AFM, (just as the CO adjusting screw does) there by allowing the AFM flap to close a little more leaning out the mixture, but only at idle. One other down side to that mod is if the fuel switch is in the AFM, (early EFI), sometimes the AFM would close just enough at idle to turn off the fuel pump. Fuel pump triggering had to be hard wired to the ignition.

 

 

[Chris86NA2T]

The concept of altering AFM voltage is basically what the super afc does. While it may work well on certain cars with different EFI systems, its a terrible idea on a 280 or 300zx efi computer. The efi computer uses AFM voltage as a major part of the load calculation. By reducing this voltage you effectively reduce the calculated load value that the computer is using to choose the correct cell on the fuel/timing maps. Now this can lead to nothing or it can lead to disaster. It all depends on which cell is selected with the new voltage. For instance, suppose you have an AFM output of 3.5 volts while under full load. now you reduce that voltage to 3 volts, the load scales in the maps shift backwards (left). Take a look at this picture..

This is a stock timing map from a z31 (i forget which year exactly).. The rpm is obviously vertical, and the top numbers represent engine load. This is a calculated value. If you are interested in the exact formulas its described very well here: http://ztechz.net/id10.html

The cell values are actual degrees of ignition timing, lets look at the cell that corresponds to a load of 80, and 4400 rpm.. At this point you are in the middle of a full throttle run. If you were to drop the afm voltage then you would effectively moved the calculated cell to the left. Suppose the voltage drop at the afm shifted you 3 cells over, look at the timing you would be at.. You jump from 24 to 35 degrees.

 

Regarding the change in air fuel ratio, take a look at a stock fuel map:

As you can see, the same thing applies here. A reduction in voltage does not always correlate to a richer AFR.

 

I used to work on a kids r32 skyline with an RB26. He had more money than brains and decided to try and make the car fast. He took your idea to the extreem and used a safc to "compensate" for 720cc injectors. It took me an hour of math to explain to him that because of the changes he made he was running 14-1 AFR's and over 40 degrees of advance which resulted in the holes in his pistons.

 

This is an extreem example. He was basically cutting the afm voltage in half and screwing everything up. Small changes to AFM voltage that only shift the load scal by one cell will generally not hurt anything. It also will not help anything as it is a completly unpredictable alteration.

 

Your best bet is to spend the 400 bucks on a nistune daughterboard (need to have a z31 ecu conversion). This will give you full control over the stock ecu with complete predictability and repeatability.