Bench testing a 1990's hot wire MAF and ECU

[RIP]

Intake tract design before the MAF can make a massive difference in it's reading, even clocking the MAF 90⁰ can alter the reading.

Can I ask how you know this? Is it an assumption or something you heard or have you seen the readings go wrong?

The MAF measures (well, deduces) volume of air going past it. Changes in intake design would indeed change the volume. But the MAF would read the changed volume. Or have I missed something.

 

[DanZac]

But the MAF would read the changed volume. Or have I missed something.

By my understanding (which is hazy at best) the MAF will be reading the airspeed (Air Velocity passing over it) and then that figure will be used by the ECU to calculate air volume using the known area of the intake duct. (Volume = Air Velocity x Cross section area of duct).

If you have changed the cross section area of the duct the ECU will still be making the calculations based on the original duct area.

Unless you can tell the ECU that the duct area has changed the system won't be able to make the correct calculations?

 

[RIP]

By my understanding (which is hazy at best) the MAF will be reading the airspeed (Air Velocity passing over it) and then that figure will be used by the ECU to calculate air volume using the known area of the intake duct. (Volume = Air Velocity x Cross section area of duct).

If you have changed the cross section area of the duct the ECU will still be making the calculations based on the original duct area.

Unless you can tell the ECU that the duct area has changed the system won't be able to make the correct calculations?

But the MAF is the duct, it hasn't changed area, so the speed x area calculation gives the same result. If other parts of the intake have changed diameter then the air speed in those areas will change to keep the volume constant, but the volume of air moving is still the same as it is at the MAF.

I can believe that changes to the air intake downstream from the MAF can have an impact as the friction looses could be different, making the ECU's fuel calculations wrong.

 

[Keithmac]

Can I ask how you know this? Is it an assumption or something you heard or have you seen the readings go wrong?

The MAF measures (well, deduces) volume of air going past it. Changes in intake design would indeed change the volume. But the MAF would read the changed volume. Or have I missed something.

I have a Mitsubishi GTO-TT, originally came equipped with a Karman HZ MAF and then after many iterations I converted it over to a Hot Wire GM Maf.

Removing or altering the screens, moving it's position, altering the inlet tract all scew the readings as I've seen it personally when road tuning / datalogging the ECU.

My Golf is on Speed Density and that is far more forgiving to any change before the throttle body.

 

[Keithmac]

MAF Accuracy....when intake changes...

Question: Since the MAF, by definition measures airflow, when testing an intake /cai etc. Higher Hz should mean better airflow correct? Esp since g/s, lb/min even cyl airmass is all calculated by Hz. Certainly there will be some variation on different days/temp/humidity, but say you test...

forum.hptuners.com

 

[Keithmac]

If you have no way of seeing what the PCM is seeing sensor wise and now way of knowing what the AFR of the engine is at any given state then you are peeing in the wind really.

For me a minimum on any project that extensively alters the original engine setup would require a Wideband AFR meter at the least. I have use Zeitronix ZT-2's with great results, very reliable sensors.

Hopefully you can get it sorted out but for me, on the bikes and cars I like to see a good live data stream of all sensors.

There is a rule of thumb for Air Mass but if you can't see what the PCM is reading you can't tell if it's plausible or not imho.

Have a good read on here and other tuning forums, this will gine you an overview of setting a system up from scratch and what to look for. If you have an understanding of how it all works it's much easier to fault find.

Megasquirt Support Forum (MSEXTRA) - Index page

 

[Keithmac]

But the MAF is the duct, it hasn't changed area, so the speed x area calculation gives the same result. If other parts of the intake have changed diameter then the air speed in those areas will change to keep the volume constant, but the volume of air moving is still the same as it is at the MAF.

I can believe that changes to the air intake downstream from the MAF can have an impact as the friction looses could be different, making the ECU's fuel calculations wrong.

Intake tract alters the Velocity through the MAF and this screws the readings, you can't just take the sensor itself in a MAF based system.

Any sharp bends before the MAF can cause real problems, any turbulence can cause metering problems.

The inlet track on a MAF car needs to be a known quantity to the PCM, or calibrated.

On a Speed Densit system none of that matters, just Manifold Pressure, Charge Air Temperature, Coolant Temperature and Battery Voltage Compensation. Much more robust system.

 

[RIP]

I have full Tech2 access to the ECU readings. A wideband fuel-air meter is £200 and might be worth buying. I've done some outside reading this morning and read that changing intake system can cause turbulence which messes up the MAF readings.

My immediate problem is why I've had three MAF failures in a year. My oscilloscope tests show everything should be ok. I've found a new quality MAF, so hopefully this has just been bad luck. Agreed that checking the fuel-air ratio is a good idea and maybe in the future I need to redesign the intake system to reduce turbulence.

 

[Keithmac]

What MAFs have you been using?, any aftermarket is a none starter really imho.

I would take a good second hand OEM MAF over any new aftermarket one.

Hopefully you have just been unluck if there was no chance of oil contamination or foreign debris damage.

 

[RIP]

What MAFs have you been using?, any aftermarket is a none starter really imho.

I've found a Nippart which I'll order when I get back from holiday. I had a Meat & Doria but it didn't work at all (bad one?). After the two original used OEM MAFs had failed I fitted a cheap £30 MAF which has been working and the buggy has now had six days out, though some poor running. I didn't want to fit a better MAF until I'd got rid of the oiled filter. The two original MAFs failed while the buggy was still being built and no filter was fitted, just short filter-less test runs in the garage.

The link you gave it to a forum rather than an actual post on intake systems. From what I've read there needs to be a 15-25cm length of smooth tube before the MAF to reduce turbulence.

Are there piggyback solutions to calibrate a MAF? I'm really not ready to fit a Megasquirt as the build has other important things that need doing first.

 

[Hotstuff]

Did you manage to make any progress with your MAF problem?

 

[RIP]

Did you manage to make any progress with your MAF problem?

The Nippart MAF works but I haven't tested it much as I've been working on some other parts of the buggy.

I've ordered parts to improve the air intake to get laminar flow for the MAF. Hopefully that will sort the poor running.

I'll post an update when it's done. I'm also hoping to have some body panels on the buggy soon so I can post some photos of the whole build soon.

 

[Hotstuff]

Looking forward to seeing what you’ve been up to.

 

[RIP]

Looking forward to seeing what you’ve been up to.

I repositioned the air box so that I could have a long hose before the MAF to stabilise the air flow. I also serviced the engine, replaced the HT leads and dizzy which were pretty bad, and fitted the Nippart MAF. The engine runs better now.

I've put off buying an AF gauge due to mounting costs of this build. My diagnostics dongle shows a stable reading from the MAF.

 

[Hotstuff]

Thanks for sharing @rcx132.

Glad to see that she is now running well and the pattern MAF sensor works.

From the little I could see in the photo, it looks like a multi link suspension setup on the front.

I bet your quite excited to get her finished.