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Generalizing Cap's MAF Mod

1934 Views 10 Replies 6 Participants Last post by  pattielipp
From what I've seen, it sounds like no one has properly figured out how exactly the MAF mod should be preformed if you already have an aftermarket intake on the car. I figured I'd attempt to run some calculations and see if I could figure out a model that could be used to calculate spacer height as a function of the diameter of your intake at the location of the MAF sensor.

Based on the simplified calculations I did, I found that a 0.25" spacer increases the free area of the factory intake by ~5%. If I understand subsonic compressible fluid dynamics correctly, that should lead to an increase in airflow of 5%, assuming a constant velocity.

Here's my measurements and math:
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This should mean that to maintain the same AFR, we need to increase the fuel flow by ~5%. In the MAF/Injector Mod, this is achieved by replacing the stock green injectors with yellow 2ZZ injectors after increasing the air flow.

The problem is, that I'm not sure I'm even on the right track here. I'm not an Engine Expert, or even a Mechanical Engineer, I'm just an Astronomer using his extremely basic understanding of engines and subsonic compressible fluid dynamics to make an estimate. I'm not sure that the readings on the MAF aren't changing due to it's relocation, on top of the increase in area for the air to pass through. The 5% number may be a red herring, and not applicable to a generalized model for the MAF mod. (If it is applicable though, then this mod may work on other cars as well. I could see the Miata crowd going nuts for this)

So, can anyone help me double check my theory?

If the theory isn't way out in the wrong direction, then I'll post the rest of my equations for calculating spacer thickness based on a few measurements taken of a generic intake.

By the way, if the theory is correct, then anyone who did the mod by tuning the ECU or using a piggy back ECU instead of the yellow injectors (or by adjusting the fuel pressure regulator) should have needed to increase the fuel flow by close to 5%. Though I'm not sure if that would be across the board, or if it would be only in the low-mid range.

Thanks,
~Catachan
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So my System is Naturally adding 310/280 or 10% more fuel.. and I'm fooling the ECU into thinking it's getting about 15% less air, so I will get what ever advance I can to give the 'Ooomph' that is missing in the Mid Range.. that gives my Formula a -7 fuel trim.
I must have missed this post by Cap in the original thread.Cap's Post

This raises questions.

I'm Fooling the ECU into thinking it's getting about 15% less air
How does this work? The flux area increased by only 5%, so is this indicative of the MAF reading differently because it is closer to the center of the air flow? I'm not sure why that would be the case, since I would have thought we were far enough from the walls of the tube to avoid boundary layer affects. Does more of the MAF actually take part in the measurements than the area around the "ring" and the little sensor on the side? I would have expected that the ECU to think it was getting 5% less air than it actually is, not 15%. Or is Cap saying that he is fooling the ECU into thinking that it is getting 15% less air than it was prior to the mod?

that gives my Formula a -7 fuel trim.
What do these fuel trim numbers even mean? Are they %, volts, ccs? Does anyone know?
Ah crud, I put this in the wrong forum, I think it should be up one level.
The yellow injectors are 310-320. The green injectors are 260. This is not a 5% difference. Your theory needs some refinement.
MAF MOD is not just about swapping injectors by yellow 310cc injectors and installing a spacer. It about cutting two rectangles that are between the airbox and MAF. They are initially (I think) there to give turbulent air (for a better airflow reading I suppose).

By example, the PPE CAi just have intake tube (smaller outlet than the throttle body) and relocated MAF. Without anything inside the intake tube, only side of the temperature sensor (in contact to the air) is dusty. The other side (throttle body side) is clean as new. If I remember right when I had the stock airbox, both side of the temperature sensor was dusty.
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What do these fuel trim numbers even mean? Are they %, volts, ccs? Does anyone know?
In general, in Toyota ECU's, the fuel trims are with reference to the injector opening time. The injector opening time is modified by the fuel trim,

inj time (trimmed) = (1+FT/100)*inj time (base).

FT stands for the net fuel trim, sum of LTFT + STFT.

LTFT is a value stored in a 3-d table with load and rpm as the axes. STFT is determined instantaneously based on O2 feedback.

LTFT is learned over time, by some obscure algorithm based in feedback control theory.
This is a nice idea, but I suspect that the actual realities of three dimensional fluid flow would render it a fool's errand. The local cross section area may be a good first approximation, but I suspect that the MAF is too short for it to be accurate.
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In general, in Toyota ECU's, the fuel trims are with reference to the injector opening time. The injector opening time is modified by the fuel trim,

inj time (trimmed) = (1+FT/100)*inj time (base).

FT stands for the net fuel trim, sum of LTFT + STFT.

LTFT is a value stored in a 3-d table with load and rpm as the axes. STFT is determined instantaneously based on O2 feedback.

LTFT is learned over time, by some obscure algorithm based in feedback control theory.
This.

Also, as a standard for OBD2, fuel trims are easy enough to understand as a percentage of added or removed fuel from the base table. You add the LTFT and STFT to see where the engine is happy at the moment. Most ecu's can handle up to about 25% before no longer being capable of making the adjustments to keep things safe. Lean and rich codes will set typically around 15-20%.

So for CAP to say that it is a -7 means that it is running 7% richer than optimal of 0%. Altitude and air density play into this as well, so you'll need to know about what it is typical to see the difference afterwards.
So for CAP to say that it is a -7 means that it is running 7% richer than optimal of 0%.
This is either a misconception or miswording. The engine is not running richer. The ECU performs a 7% fueling correction to the base fueling to make the engine run at the target.
Right, and showing a negative number means it is pulling fuel. A positive number would mean it is adding fuel. It's been this way on every OBD2 engine I've repaired.
Ok.. a little late to this game, but I spent some time discussing ecus, fuel trims, and the maf mod with cap Years ago.. you are WAY over thinking things… at one point cap(or someone else) ran the stock intake through a flow bench and found that the amount that the stock intake could flow is well above what is required for a stock 1zz, so your calculations of more airflow based on less obstructions, for all in tens I’ve purposes are null and void in this case. The math and theory of it is awesome though!
the part, and magic, that you’re missing is that of the MAF sensor itself. The MAF sensor reads the volumetric airflow based on a “hot wire”. The more airflow running overtop of that wire, the more it is cooled, and the sensor registers this and provides a voltage from 0-5v. The veins in the stock intake direct air over that Hotwire, and the ecu uses that reading And it’s internal calibration to create a short term fuel trim. That short term fuel trim along with an O2 reading is used to create an internal map, or correction to the fuel trims and creates a long term fuel trim.

now, the ecus internal map using rpm and maf voltage to pull fuel trims, ignition timing,vtc, etc out of each “load cell”.

well, what happens if you reduce the maf voltage being sent to the ecu? It will think it’s under less load and will read the data from a lower load cell.. what’s in a lower load cell you ask? Well increased timing because at lower load levels, knock is less of an issue amongst other things.. on the flip side, whats also in lower load areas? Less fuel.

less fuel and increased timing beans the ecu has to compensate for due or throw a cel.. if the fuel trim ends up being greater than X then it can’t fully compensate and it will throw a CEL.

so, we need to counteract the fuel issues, or the fuel trims will be way off. How do we do that? we add larger injectors or increase the fuel pressure, so now the ecu pulls fuel out and creates new fuel trims. The difference is now we’re within the allowed short term fuel trim to create a long term fuel trim..

So now we’re left with a lower voltage maf voltage because our maf is no longer receiving as much airflow on the Hotwire. Out ecu thinks we’re in a lower load cell causing increased timing, but less fuel, so we add more fuel by larger injectors.

hopefully this clears some stuff up….
Oh.. in terms of an aftermarket intake, what I would recommend doing is finding out the surface area of the maf sensor and then subtract that surface area from the area of a slice of the intake pipe…. Calculate the potential flow/voltage registered by the maf based on the area/surface area/flow over the hot wire based on it being in the are/pulled out of the air, etc.. a 2.5” intake vs a 2.6,2.7, etc. will all register different voltages and will cause a different load cel to be read and reflect different timing, fueling, vtc, etc.
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