MAF Resister trick.. whatcha think?

[Empire]

So I found this article that talked about ways to squeeze some more MPG out of your car. one of the things it suggested was a MAF/MAP "enhancer" which turns out to be a resister between the MAF and the ECU to trick the ECU into thinking that there is less air coming in, and therefore dumping less fuel.

Any thoughts on this?

almost seems like, given the way they suggest doing it, that it would be decently safe to do. I would feel much more comfy if I had a wideband afr to tell me how lean it was running to avoid any "red zone" types of situations. And I personally would want a to wire it up in such a way that I could have a bypass switch to put it back to stock on the fly. In case I needed to get to WOT for some reason, that bypass switch would eliminate any fears of going too lean. Right?

I'm actually picturing its use more for cruising speeds and light load situations.
Wouldn't want to worry about it as I blast to work, but on my leisurely ride home, it wouldn't be so bad.

The MAP MAF Enhancer Circuit

There's only 2 parts. Here's some Radio Shack part numbers that you can use:

Pot: 50K Linear Taper Potentiometer, Radio Shack #271-1716
Resistor: 47K Ohm Resistor, Radio Shack #271-1342 or #271-1131
Note: The resistor and pot values don't have to be exact. Just try to keep their values in roughly the same proportions as the ones I've shown. These values aren't particularly special. I wouldn't use less than about 10K on the pot, and the resistor should be approximately the same value as the pot. On some vehicles, this combination of pot and resistor can be too sensitive, such that very tiny movements on the pot make large changes in air/fuel ratio. You can make the device less sensitive by using a larger resistor. Double the resistor size or even triple it, and you will find the device is much less sensitive.

Cut the signal wire and connect the sensor side of the cut to one of the outside pot terminals. Connect the pot's center terminal to the computer side of the cut. See the diagram.
Note: If you have a preference for which way the pot turns when you are leaning the air/fuel mix, then you'll need to select which outside terminal to use by doing the following: Turn the pot all the way (until it stops) in the direction you want to be the least effect. In other words, if you want to turn the pot clockwise to make the mix leaner, then turn the pot counter-clockwise all the way. Now measure the resistance between the center terminal and the 2 outside terminals one at a time. One of these outside terminals will show no resistance to the center terminal. That will be the terminal you want to use on the signal wire.
The other outside terminal connects to ground, via the resistor. I recommend you use the ground that the sensor is using as per the diagram. You don't cut the ground wire. It must stay connected to the ECU. Instead, you'll tap into that circuit, as per the diagram.
That's all there is to it. To set the pot, first turn the pot all way towards least effect. There will be almost no resistance between the ECU and the sensor on the signal wire. This is the stock setting, with no change in the sensor's signal. Then start your car. You can now gradually turn the pot and you will eventually notice that the idle runs slower and eventually rougher. Back off on the pot until the engine is running smooth again. Also test the car on the road and make sure you're not losing power. If you are, then you need to back off the pot some more until these symptoms disappear.

[Loren]

It won't make a difference when you're cruising or driving "normally" because the ECU is in closed loop mode. No matter what the fuel table says, the actual pulse-width is going to be adjusted to achieve stoich according to the O2 sensor.

Under full-throttle or other open loop conditions, you can use a resistor in series with (or in place of) the IAT sensor and it will allow you to "lie" to the ECU, telling it that it's getting more or less air than it actually is, and it will fuel accordingly. You can probably make +/- 10% difference with this. But, it's only going to help when you're racing, not when you're driving for economy.

Now, if you really wanted to play this game, you could build a circuit that would intercept the O2 sensor signal. Then you could "lie" to the ECU and tell it that the engine is running richer than it really is, and it would lean the mixture.

They do the same thing in performance applications with an "O2 clamp" circuit. In that case, it allows your piggyback ECU or your rising rate FPR to make the engine run rich without interference from the stock ECU by intercepting the O2 signal and making it look okie-dokie even when it's rich. The opposite of what you'd want to to for an economy application.

Gotta be careful doing any of this, of course. Running too lean under load isn't healthy for your engine.

[Empire]

not arguing with you on this one, but the concept of the ECU not reading the MAF all of the time just seems so weird to me. I always thought the MAF was an "all of the time" thing.
Changing the way the ECU sees the intake temp could be interesting. I see how the domestic guys mess with that when they port out their MAF. Keeps them from running lean by making the ECU think it is cold all of the time. That seems like it could be potentially worse than just limiting the MAF signal a little bit. It wouldn't matter if it was 50, 80, or 100 degrees, the eCU would think it was 40 degress all of the time. Seems like a huge waste of fuel.
Being that my background is mid 90's nissan's, 240sx's mainly. seems like the o2 sensor isn't a "major" sensor. You could have a bad one and half of the time, not even know it. So assuming you had a wideband to manually monitor AFRs, you could keep the o2 sensor unplugged, and that would keep the ECU in Open loop, all of the time. Right?
and if that is the case, then it would have to rely solely on the MAF sensor to determine the fuel mix.

or am I thinking about that backwards?

[Loren]

The ECU always reads the MAF. It's the IAT that you're original post is wanting to mess with. (and the ECU always reads that, too... but it's not too disastrous if you fake that signal)

As so often happens in this "economy" forum, you're getting "performance" mods all mixed up with "economy". The people who fake their IAT to keep from running lean DO NOT CARE that they're running rich and wasting fuel.

Don't kid yourself about the O2 sensor not being a "major" sensor. While an early 90's car (or later) will run with a bad one, if the O2 output is bad and the ECU knows it (trips a code, turns on the CEL), it runs in "open loop" mode all the time. It can't run closed loop without the O2 feedback. So, yes, the car will run... just like it runs when it's cold or when you're at WOT. It runs richer than it needs to, and it's not unsafe for the engine (though it may be bad for your catalytic convertor over time), but it will run just fine... just use more fuel than necessary.

If you drive at full-throttle all the time, then the O2 sensor doesn't matter at all. But, if you're daily driving with any concern about fuel economy or not killing your cat, the O2 sensor is very important.

[Empire]

see, that's what I was getting at.
if the ECU is in open loop all of the time, then couldn't you modify that MAF signal to lean the mix back out instead of running at 12:1 (which I believe nissan's default to)

[Loren]

I suppose you could. But, I wouldn't recommend it.

Ever seen the AFR plot from a stock car on a dyno? At WOT, fueling from the table with no O2 feedback, the AFR is anything but constant. Yeah, they'll hit somewhere around 12-12.5, but that's not constant. Can be anywhere from 10.5 to 14 (I have a chart here from my Miata with a stock ECU that shows about 10.8 to 13 at WOT from 3-7k rpm). If you were to trim that to adjust "12" to "15", you'd end up with a range that could go as far as 17:1 or more. That's potentially dangerously lean for anything other than a light cruise. At best, it would run like a car with a poorly tuned carburettor.

[Empire]

well yeah, i hear ya on that, but I wouldn't want to lean it out for WOT or anything like that.
I'm talking strictly light load, light accel, and cruising speeds.
Like, way way way under 50% throttle... I'd say a cap of 20% throttle.

Shouldn't be so dangerous then right?

Just deactivate the "hack" when you want to get on it more.

[Loren]

Yeah, I suppose that could work.

Assuming you can find a fixed value of resistance to substitute for the IAT sensor for "economy mode", you could get fancy with it. Put a limit switch on the throttle that you can use to flip a relay when TP is > 20%. When the switch is off, it uses your fixed value (reducing fuel consumption and power), when the switch is on, it switches the stock IAT sensor back into the circuit to allow normal operation.

Transitions from cruise to accel would be awkward, but otherwise it would probably be okay for puttering around.

[2TurboImports]

I used an apexi afc. When I calibrated WOT, I told it WOT was actually like 70% so no overriding would normally take place. Then I could adjust fuel tabling at way less than WOT. Seemed to work fairly well, for a power standpoint anyhow.

What really upped the MPG for city driving was a 7lbs flywheel. However, on the interstate you had to keep your shit pegged to stay at 80 since there was less mass keeping the engine spinning at higher speeds.

[Empire]

by adjusting the MAF signal, telling the ecu there is less air coming in, and modifying the o2 sensor so the ECU doesn't see the lean condition, the goal is the same as the AFC. The difference being, a AFC costs, used, 100 or more dollars. A few resisters and switches are less than 20 bucks. That's the basic appeal to doing it this way is the over all cost.
When trying to stretch every mile out of every gallon, one of the key points is the cost of it. the break even point. We could build a MPG monster, get 80 mpg but if it costs 50 grand, then what's the point? by the time you broke even on that 50 grand, chances are, that MPG monster would need repair.
Unless of course the goal is just bragging rights to say you got a 300zx to get 35 mpg.

[Loren]

Light flywheels are an interesting thing when it comes to fuel economy. While it does take less power (thus less fuel) to spin them up, you also get a lot less in-gear coasting with one. If you're hardcore into fuel economy, you know all about DFCO and pulse-and-glide... a lightweight flywheel won't "glide" anywhere near as far as a heavier one will.

I wasn't into the whole hypermiling thing when I had my Miata, but one of the first things I notice when I put a 7-pound flywheel on it (stock one is 1 was that it lost speed VERY quickly when I lifted off throttle. Pretty cool from a performance driving perspective, your right foot feels a lot more in control of the car and the whole car feels more responsive.