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Cold Air Intake MAF Sensor Placement – *KEY* Information

Flow of air inside pipe to illustrate the problem of poor cold air intake MAF sensor placement choice. IMAGE SOURCE: https://courses.ansys.com/index.php/courses/fluid-kinematics/lessons/simulation-examples-homework-and-quizzes-2/topic/homework-flow-through-a-90-degree-bend-pipe/

Flow of air inside pipe to illustrate the problem of poor cold air intake MAF sensor placement choice.

By the end of this article, you’ll be able to judge the quality of cold air intake MAF placement on aftermarket intakes from the likes of K&N, INJEN, S&B, AFE etc and also be able to choose optimum MAF sensor placement on a DIY cold air intake.

I’m going to take you through my story about how changing the filter type and cold air intake MAF Sensor placement on the Octavia VRS project solved some running problems. I’ll go through the problems I had, the changes I made and the effect these changes had on the drivability of the engine.


Key Takeaways

Filter medium type (cotton type vs paper) and locating a MAF sensor after a bend could cause inconsistent AFRs and unexpected driveability issues.


The Problems

Before I get into the datalogs and solutions, let’s look at the issues the car was experiencing.

Boost Control

The car had a “thing” of “bucking” after some extended periods of full throttle, say more than about 6 seconds at higher speeds. Easing off after one of these blasts, with a constant throttle at a 70mph ish cruise, you would see the boost switch between 0 & 5 PSI causing quite a bad jerking. Like something was broken. The power switching between two power levels. Not great. At all.

AFRs

All over, the motor runs rich. Low load & low RPM, high load & high RPM, the AFRs are around 11.0/12.0. At a steady cruise they are close to 14.7.

I thought the MAF sensor placement might be part of the issue because the MAF sensor was located after a 45 degree bend.

Priorities

The first issue I wanted to start getting sorted were the AFRs (and also the air filter).

If you have read the article I have done comparing performance filters vs OE type paper filters, you’ll have read that paper filters seem to flow almost as much as performance air filters while taking longer to get blocked than performance filters (OE paper filters generally have a much bigger dust capacity). OE type paper filters also have the added benefit of filtering out smaller particles.

In short, an OE type paper filter should make almost identical power to a “performance” air filter, while keeping the engine cleaner and hence, making the engine last longer.

“Original” Set Up

The Octavia was running a Greddy cone filter, not the worst but probably not as good as an OEM filter.

Cold air intake setup with 45 degree bend before MAF sensor. Greddy cone filter, cotton type filter media.
“Original” MAF sensor placement with Greddy air filter. I didn’t like it after a 45 degree bend.

With some measuring I found a nice sized OEM paper cone filter.

Cold air intake setup with 45 degree bend before MAF sensor. OE type paper filter
Greddy Cotton type filter replaced with OEM type paper filter. Bend before MAF sensor unchanged.

With the paper filter fitted, I wanted to have a look at the AFRs vs MAF (grams/sec) vs TPS vs RPM vs Short Term Fuel Trims by using the Car Scanner OBD2 app to datalog these channels.

The results of the datalog were interesting. Generally rich everywhere, as had been the case with the Greddy. AFRs better at cruise but things go awry at idle and at higher RPM ie things getting overly rich, like the Greddy.

Cold Air Intake MAF Sensor Placement

Now I had baseline numbers for the paper filter & Greddy filter with the “original” MAF sensor placement (after the 45 degree bend), I re-routed the intake piping by removing the 45 degree bend before the MAF sensor and by connecting the MAF sensor directly to the base of the new paper air filter.

Cold air intake setup without bend before MAF sensor. OE type paper filter
Bend removed before MAF sensor. MAF sensor mounted directly to air filter.

The air can now flow straight into the MAF sensor. The thinking here being the air flow through the MAF sensor (if the intake is straight before the sensor) should be consistent regardless of how fast the air is flowing through the pipe.

CFD animation of air flow through 45 degree bend
Flow through a 45 degree bend. All sorts of turbulence after the bend and for some distance. The big issue for MAF sensors after a bend is that the turbulence pattern will change according to the speed of the air ie RPM and throttle position making accurate scaling of the MAF sensor difficult/impossible. IMAGE SOURCE: Flow Through a 90-degree Bend Pipe | Ansys Innovation Courses

More Datalogging

I did some more datalogging with the new cold air intake MAF placement and this is what I found.

Comparison of engine air flow with different cold air intake MAF locations.
The tables above plot RPM on the x-axis, TPS on the y-axis and the cells show MAF g/sec.
The colours show the standard deviation for each cell ie how far the data for each cell deviates from the average.
Ideally we want every cell to red, red being the best, blue being the worse. The vertical rainbow bars show the deviation from the mean. We want the colours as close to the bottom as possible.

Top Left: Paper filter after 45 degree bend.
-Top Right: Paper filter, no bend before MAF
-Bottom Right: Original setup, Greddy filter, MAF after 45 degree bend.

45 Degree Bend, Paper (Top Left) vs Greddy with 45 degree bend (Bottom Right)

Down low (low RPM, low TPS) the Greddy looks to be more consistent (air flow readings).

At high load & higher RPM the paper filter looks to be more consistent.

Driving the car, the power seemed more stable with the paper filter over the Greddy.

Greddy wins low load, load RPM. Paper wins high load, higher RPM.

45 Degree Bend Paper Filter (Top Left) vs No bend before MAF, Paper Filter (Top Right)

At idle, no-bend edges the bend and also edges the Greddy.

Lifting off throttle at higher RPMs, no-bend much more stable than Greddy. Low throttle openings, higher RPMs no-bend is more consistent.

At high load, high RPM, no-bend more stable than bend up to 3200RPM. Above 3200RPM at high load, perhaps bend edges no-bend. And both paper filter tests beat the Greddy in this area.

No-bend edges bend because the no-bend log has more reds and orange cells vs bend.

Summary of Datalogs

No massive differences between the setups but looking at the amount of red and orange cells, the no-bend, paper filter seems slightly better than the other two setups.

Changes in Driveability

I’ll confess, these were not blind tests, I knew what intake setup was on the car when I was doing the testing so my judgement could be affected. When I went into this testing I did think the no-bend, paper filter would be best. It’s possible I am seeing what I want to see.

But Assuming I’m Managed to be Impartial

In general driving, both setups with the paper filter seemed to be smoother and less “boosty” than with the Greddy filter. It seems like the throttle is easier to modulate with the paper filter.

With regards to noticing a difference between the paper filter with the bend and without the bend, I’m not sure if I could tell any difference. It’s possible there is an extra layer of smoothness without the bend but I can’t be sure.

Idle seems to be better (smoother) with both paper filter setups (but the logs do NOT back this up).

Idle AFRs with both paper filter setups is leaner. The Greddy had an average AFR of 12.59 at idle whereas the paper filters have 14.56 (bend) and 14.01 (no-bend).

Generally, AFRs between the bend Greddy setup and the bend paper filter setup are pretty close above 2100RPM and 11% throttle. And, perhaps the AFRs with the bend paper filter are slightly more stable at high load, high RPM over the Greddy.

However, the paper filter without the bend I think is noticeable leaner (less rich) than the other two setups.

Comparison of AFRs on different cold air intake MAF locations.
Average AFRs compared between the setups. Top right (no-bend) paper seems to be noticeably leaner (less rich) than the other two.

Consistency of the AFRs also seem to be slightly better for the no-bend paper vs the other two setups.

Do these differences make themselves known when driving the car?

Again, I’d say the only difference I can feel is in modulating the throttle, it seems the throttle is more granular with the paper filters (which is a good thing).

A Summary of the Three Setups

In short, I think the rearranging of the MAF sensor (so the intake air hits it in a straight line) has definitely not made things worse and it seems like the air flow going through the MAF sensor is more consistent, the MAF sensor is measuring more accurately, more of the time.

The amount of difference in idle AFRs was unexpected. Whereas before I was considering rescaling the MAF sensor or tuning the MAF sensor signal, especially for idle, to make things leaner, now I am relatively happy with the idle AFRs. I’ll be leaving the setup as is for now (no-bend, paper filter), I’m almost satisfied with the AFRs. But for sure, I want idle AFRs to be 14.7. Sorting the AFRs has just moved down the priority list for now.

Unexpected Benefits

I mentioned at the start about the apparent boost control issues and how the car would be jerky after a bit of a blast.

At first I was thinking it was a wastegate issue, perhaps it was damaged. Then I read an article on S4wiki which talked about the problems of putting a stronger spring in a VAG wastegate actuator. If I read it correctly, Motronic 7 ECUs use the N75 valve and the DBW throttle to modulate boost. After reading that I thought perhaps the wastegate had been modified by the previous owner (car is running a K04-064 turbo). The next was for me was going to be disconnecting the N75 valve and see what boost the engine made on the wastegate spring only.

But…

But during the testing of the MAF sensor placement and the new air filter, the boost control issue seems to have disappeared. It’s still not perfect, there is still some surging on acceleration if the throttle is not pushed all the way down, but the boost/power switching that would happen after a blast and cruising seems to have stopped. I’m not going to count my chickens just yet but somehow, at the moment at least, the combo of paper filter and straight path to the MAF seems to have helped in some way.

Hypothesis

Going back to the RPM vs TPS vs MAF tables above, there was quite a big variation in the MAF signal at closed throttle and high RPM ie high vacuum/lifting condition eg where the jerking problem would surface. It’s possible the jerking was due to turbulent air passing through the MAF.

I’m not going to say the problem is solved. It’s just I have experienced these drivability issues yet with either of the two paper filter setups.

If the problem does happen with the new no-bend, paper air filter setup in the future, I’ll leave a comment at the bottom of this article.

I’ll be honest, one of the mechanics at TurboSpeed said problems with the AFRs would be more down to the filter type than the bend before the MAF. I discounted the filter medium playing a part but having changed the filters, it looks like the filter material might have some effect of MAF sensor readings. At least in this case.

Cold Air Intake MAF Sensor Placement – Summary

From the testing I have done so far it seems like MAF sensor placement after a bend:

  • Could cause unusual driveability problems (the jerking).
  • Could cause more inconsistent AFRs for the same airflow and RPM, at least at idle and at least in this case.
  • The type of filter element could also cause inconsistent readings, the cotton type filter medium of the Greddy could be causing more turbulent flow than the paper type filter, in this case.

Things To Aim For

In my opinion, after this testing, I personally would recommend fitting the MAF sensor as close to the filter as possible and in case, without any bends upstream of the filter. Car manufacturers seem to mount their MAF sensors there so there is a probably a good reason for it.

And if mounting the MAF sensor after a bend was unavoidable, I think I would try to have at least 30cm of straight pipe before the MAF sensor.

Wrapping Up

As I said at the start, hopefully after reading this article you will be able to look at an aftermarket cold air intake and assess whether the MAF sensor placement is good or not. And if you are making a DIY cold air intake, you have a good idea of where to place the MAF sensor and some of the problems that can occur if the placement is not optimal.

Which Setup Am I Going To Use?

It’s probably obvious by now, I’m going to use the no-bend paper filter setup.

I prefer OE type paper filters (assuming they are big enough) over aftermarket cotton/foam/metal mesh filters, primarily because they filter out more dirt from the engine (less wear on engine) and also because they flow almost as well as “performance” air filters.

And no-bend because it seems the MAF is reading more consistently without it. The fact that the surging problem appears to have disappeared and driveability in general seems to be better is the icing on the cake.


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MK5 Supra Cold Air Intake Comparison

With the info gleaned from this testing, let’s have a look at some cold air intakes for the MK5 Supra

HKS – Proper job here. MAF mounted close to the filter in a perfectly straight pipe with MAF running parallel to flow. Don’t think MAF placement could be better.

DinanLooks problematic. MAF sensor mounted just after a 90 degree bend. From the CFD model above this could be the most turbulent part of the intake.

AMS – MAF mounted just after a very slight bend. HKS placement probably better but AMS placement looks fine.

Armaspeed – Close to filter but for some reason leading edge of sensor is raised, slightly. Probably fine.

Mishimoto – Mounted close to filter, in straight section but diameter of the pipe where MAF located looks larger. MAF under reading air? Deliberate?

Injen – MAF mounted after what looks like a 30 degree-ish bend. Not ideal.

Purely from MAF placement location, the HKS looks to be the best. But I personally have questions marks about the filtration quality of HKS air filters.

Other Links

Improve your factory intake without changing the filter

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This page was last modified Nov 15, 2023 @ 1:57 pm

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