Do the twist... the vnt twist

Tinker

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Aug 21, 2000
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So with the results that DW found from T.S.'s engine (BTW excellent info Drivebiwire), how many people are going to do the untwist? Or do you still feel comfortable with that mod?

By the looks of it too much boost = destructive results.
 

car54

theGAME
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Dec 5, 2000
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Woodbridge VA
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2002 Jetta
i think the problem was uneducated modifying of the maf sensor. the vnt twist didnt cause ts's failure.
 
M

mickey

Guest
I don't understand how modifying the MAF sensor could be a problem. It cannot supply more than 5.0 volts to the ECU, since that's all it gets FROM the ECU in the first place.

-mickey
 

Davin

Top Post Dawg
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L.A.
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2001 Golf GLS 5spd blk/blk
mickey,

Putting the MAF in a larger enclosure will decrease the voltage for a given airflow from the stock condition. Going above 5V isn't an issue...

-davin
 

VelvetFoot

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Sand Lake, NY
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NB, 2000, Yellow
I also say that ts' big gulp abominator intake did not cause the overboost.

But I am not willing to experiment with it on my car.
 

GotDiesel?

Top Post Dawg
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2001 Jetta GLS
Maybe this will make it easier to understand why the big gulp mod may well have been the straw that broke Ol Silver's back.

What the MAF really measures is air velocity. Air moving past it at varying speeds cools it to varying degrees and the circuitry measures how much current it takes to keep it at its operating temp and reports that to the ECU.

The MAF values are calculated based on air moving through a pipe of a certain size. Let's say for sake of easy math that the pipe has a radius of 1 unit.

The cross section of the pipe has an area of 3.14 square units (pi r squared).

Increase the radius of the pipe to 2 units and the cross section grows to 12.56 square units. A doubling of the width of the manifold tubing results in a QUADRUPLING of the area.

So what you say? Well, think about it... That means you have to flow FOUR times as much air through the big pipe as you do through the little one for the air velocity to cool the MAF to the same extent.

The turbo is not going to be told to back off its boost until it's pumping four times the normal peak volume.

As I understand it, the turbo is always trying to pump as much air as it can. The ECU reins it in based on where all the values of the various sensors fall on the map.

I don't know the exact measurements of TS's mod but the formuala still applies... the airflow increases at the square of the pipe size increase.
 

msauve

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Nov 8, 1999
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Hamburg, MI
GD, the MAF measures the actual mass of the air moving though it. It does this by heating a wire (older technology) or thin film (newer tech), and measuring how much current is needed to maintain a consistent temperature above ambient. This has the effect of measuring both velocity and density, which along with knowledge of the cross sectional area of the intake, allows the ECU to determine the actual mass of air flowing into the engine.
 

weedeater

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<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> The turbo is not going to be told to back off its boost until it's pumping four times the normal peak volume. <hr></blockquote>

I hope the ECU pays attention to the MAP sensor during all this...
 

GotDiesel?

Top Post Dawg
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Pacific NW
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2001 Jetta GLS
I stand corrected.

Your comments lead me to another question. I know that cold air is denser than warm air. But I'm wondering if there is some compensation for the actual temperature of the air moving past the MAF.

The actual temperature of the air is going to have some effect on the amount of heat loss the MAF deals with, beyond the fact that the cooler air is more dense to begin with, won't it? And perhaps humidity also comes into play?

It's amazing all this stuff even works at all.
 

84ZZ4

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Morrisville, NC
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2005 Jetta Wagon GLS TDI
Where did those pictures of Steve's MAF mod go? I saw them around her earlier...

Did the modification allow air into the engine that did not go through the MAF?
 

Davin

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L.A.
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2001 Golf GLS 5spd blk/blk
weedeater: it does! That's why the car went into limp mode when the charge pressure spiked.

GD?: you stand corrected on what? i think that msuave's account agrees with our explanation other than the fact that the sensor deals with density on it's own (I assumed it used the BARO, but it self-compensates).

In terms of the air temperature... I'm not sure that the "constant temperature" that the system keeps the wire or film at is really constant... it may be a reference that's set to a specific number of degrees above ambient temperature.

Also, as for heat transfer, convection heat transfer (what you get from moving air) is proportional to delta T, so if the wire is kept at a temperature which is a constant value above ambient, the heat transfer properties should remain the same regardless of the actual ambient temperature.

It's been a long time since I took heat transfer.
But I'm 95% sure this is right!

-davin
 
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