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Discussion Starter · #1 ·
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:
Handwriting Whiteboard Rectangle Font Line


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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Discussion Starter · #2 ·
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?
 

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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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test
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.
 

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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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test


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.
 

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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.
 

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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.
 
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