MAF (Mass Air Flow) versus Speed Density, Discussion & Debate for ALL

[Ross]

There are pluses and minuses to each system. When it is an option I prefer a maf based system or a blended set up. But there are applications were the maf becomes a bigger headache to deal with then speed density. If that becomes the case speed density will be the way to go. At the end of the day I feel it comes down to what is right for the particular vehicle more then anything else. I do not feel there is a set in stone point were if you make x amount of power or plan to use your vehicle in this manner you have to choose speed density of maf based calculation.

[Td-d]

Originally Posted by DJCarbine

I would say that virtually everyone with FI(as far as I know) is running the maf as a "blow through" setup

Perhaps on the BRZ - but not necessarily on other models (e.g. the STI). And this discussion is stated in the general.

[DeliciousTuning]

Originally Posted by DJCarbine

I would say that virtually everyone with FI(as far as I know) is running the maf as a "blow through" setup, so it really doesn't matter.
The vented air is unmetered and will not cause fueling issues.

I personally feel "blow-through" systems to be a very poor solution for any vehicle. If leaving the system as a Mass Air Flow and only relocating the MAF sensor to a charge pipe is incorrectly using the sensor. Setting up a vehicle this way can cause problems as you are using a airflow sensor to determine quantity of air based on the quantity of air defined by air flow, atmospheric pressure sensor and intake temperatures near or at the air filter.

Placing this sensor in the air flow of a pipe that has a continuously changing pressure is a very bad idea. Because the MAF sensor is basing it's calculations on most importantly atmospheric pressures, the air flow values will be incorrect almost all the time because it can not adjust to the pressure changes in the pipe. Because of this, the vehicle can literally go through the same point in any Load/RPM based table (say fueling or timing) with different boost levels.

Example "Blow-Through" Problems:

Scenario 1: You are tuning for partial throttle say 10 PSI of boost and set your timing values accordingly for more torque. Now under you hit 20 PSI and set your timing values accordingly. Now in these conditions, partial throttle MAF Voltages are 3 volts (air flow) and 4 volts (air flow) respectively.

Scenario 2: Partial throttle climbing a hill under boost (20 PSI) and because of the lack or airflow from the throttle being only 50% open the MAF voltage are around 3 volts. Because of this the timing values will be far too aggressive and will cause detonation. Of course you can tune for this, but then your partial throttle becomes sluggish.

Now with if the MAF sensor would have been in the proper place it would have registered all the air flow properly in order for the FI system to maintain that boost level in those pipes. Or if Speed Density would have been enabled then the tune would have worked properly also.

So "Blow-Through" is just a bad idea and not something that needs to be discussed on this thread.

Cheers,
William Knose

[jamesm]

i don't agree with the above at all. i always use blow-through mafs on turbo conversions and they always work fine. you may be right about the specific issues you point out, but we could do the same for any setup. they all have their particular issues.

to be honest i don't really follow your example. i also don't understand how a heated element airflow sensor is affected by pressure, but then i'm no expert on how the things work internally either. all i know is i've had lots of cars with them and they all run just as well as the cars i've had with draw-through or hybrid maf/sd setups.

[DeliciousTuning]

Originally Posted by jamesm

i don't agree with the above at all. i always use blow-through mafs on turbo conversions and they always work fine. you may be right about the specific issues you point out, but we could do the same for any setup. they all have their particular issues.

to be honest i don't really follow your example. i also don't understand how a heated element airflow sensor is affected by pressure, but then i'm no expert on how the things work internally either. all i know is i've had lots of cars with them and they all run just as well as the cars i've had with draw-through or hybrid maf/sd setups.

It is a little hard to explain but like you stated, the MAF sensor is not affected by the pressure differences. That is the exact point, a mass air flow sensor does not know that the pressure has changed, only that the air flow has slowed or increased which translates into a final load value.

So you can have single points in the tables/maps being crossed by vastly different boost pressures because the MAF sensor (which determines load) sees the same speed of air under varying boost pressures. As you have seen, it can be made to work, but I have found it to not be ideal and something I prefer not to tune.

Cheers,
William Knose

[DJCarbine]

Originally Posted by endrswrd

This is only true if the leak is before the maf sensor. You could be leaking out your IM/tgv/block gasket or injectors or any of the IM ports that see vacuum / boost and it would be leaking metered air. Blow through does prevent a majority of the boost leaks you would see from boost tubes / couplers however.

Leaking metered air is always a possibility with MAFs.

Just to clarify, I was only speaking about a bypass/blowoff valve not vacuum leaks at the throttle body/manifold or even an idle control circuit. I've dealt with that enough to know that it can drive you absolutely mad trying to find them.

Was simply saying it depends on the platform and maf placement

[DJCarbine]

Originally Posted by DeliciousTuning

It is a little hard to explain but like you stated, the MAF sensor is not affected by the pressure differences. That is the exact point, a mass air flow sensor does not know that the pressure has changed, only that the air flow has slowed or increased which translates into a final load value.

So you can have single points in the tables/maps being crossed by vastly different boost pressures because the MAF sensor (which determines load) sees the same speed of air under varying boost pressures. As you have seen, it can be made to work, but I have found it to not be ideal and something I prefer not to tune.

Cheers,
William Knose

I would have thought that the maf would "see" more air depending on the density of the charge air so that it would see more air as boost is turned up.... I've only tuned SD so my grasp on MAF tuning is limited.

[Luckrider]

Can someone explain to me why EcuTeK can do SD? Or, if nobody is willing to pierce the veil of mystery behind the platform that @EcuTek engineered, why can't OFT/BRZEdit/EcuFlash tune a BRZ/FRS with SD?

[Rombinhood@OpenFlash]

Originally Posted by DJCarbine

I would have thought that the maf would "see" more air depending on the density of the charge air so that it would see more air as boost is turned up.... I've only tuned SD so my grasp on MAF tuning is limited.

You are correct. Hence the term mass airflow meter. It does indeed read airflow mass in pressurized conditions. But one issue I've seen in blow-thru MAF applications is oil contamination from oil blow by from turbos. This is why every MAF referenced OEM turbocharged car I can think of (GT-R, Porsche Turbo, etc,.) uses draw-thru MAFs. It's also no surprised that they use bypass valves that recirculate discharged air back into their induction system (downstream of the MAF). Turns out what works for MAF accuracy also works for emissions and noise regulations. Of course, it's important to have a leak-free induction system in MAF referenced systems. But I think this is reasonably easy to ensure with quality clamps and couplers.

[Rombinhood@OpenFlash]

Originally Posted by Luckrider

Can someone explain to me why EcuTeK can do SD? Or, if nobody is willing to pierce the veil of mystery behind the platform that @EcuTek engineered, why can't OFT/BRZEdit/EcuFlash tune a BRZ/FRS with SD?

I don't believe there has been much of a push for open-source development of Speed Density support. But if the current development state of other Subaru ECUs is any indication, SD (and hybrid MAF/SD) will most likely be supported in the future. Ecutek is usually the first to do things like this since that is what they are very good at (and they are monetized to do so).

But I think much of this MAF vs SD trade-off discussion doesn't even apply to 99% of users who don't plan on making more than 350-400whp. And I suspect even that whp range is conservative.

[jamesm]

Originally Posted by Luckrider

Can someone explain to me why EcuTeK can do SD? Or, if nobody is willing to pierce the veil of mystery behind the platform that @EcuTek engineered, why can't OFT/BRZEdit/EcuFlash tune a BRZ/FRS with SD?

they've disassembled the rom in something like IDA and added new logic, i'd guess directly in assembly (if that's chinese to you, suffice to say it's a rather low-level way to make a computer do stuff, and not super readable or friendly to work with). probably using techniques along the same lines as commercial software cracking. i've never done it myself so i'm not the guy to ask about it, but i think that's general idea.

open source speed density exists for lots of other platforms, just not ours yet.

[RickyB]

One drawback of hot wire MAF sensors is that for the first couple seconds of operation, the wire is drawing a lot of current while heating up to operating temperature, which just happens to look to the sensor as if the engine is ingesting huge amounts of air. For this reason, manufacturers often add at least a 1bar (barometric) sensor and intake air temperature sensor to handle fueling using SD or pseudo-SD during the first few seconds of operation because many folks turn the key straight from off past ACC and ON to START when they get into their car. For race application, it's not an issue since you can teach a racer to turn ignition power on and wait five seconds before cranking the engine. For street applications, the extra sensors also allow SD operation in cases where the MAF fails and also allows the ECU to potentially detect MAF failure or the use of a piggyback unit to alter the input signals.

Both SD and hot wire MAF can be made to run very well in almost all situations provided that the sensors are properly situated and calibrated and that the ECU/EMS is well implemented and expertly tuned. Even alpha-n can be made to work well on NA engines when appropriate barometric and air temperature compensations are applied.

There is another air flow system called a karman vortex MAF which also has its plusses and minuses.

[arghx7]

A lot of the practical concerns have already been discussed here. To summarize: the MAF sensor was never designed for very high airflow, and the ECU software as we understand it was never really designed for a full speed density implementation. So you run into hardware and measurements issues with high power MAF-based builds. With speed density, you have the headache of shoe-horning in a control system the engine was never designed for in terms of software and hardware.

Neither set of problems are insurmountable; they're just the nature of pushing a car far beyond what it was originally designed to do in terms of hardware or software.

Now let me address the below question:

Originally Posted by Luckrider

Can someone explain to me why EcuTeK can do SD? Or, if nobody is willing to pierce the veil of mystery behind the platform that @EcuTek engineered, why can't OFT/BRZEdit/EcuFlash tune a BRZ/FRS with SD?

The short answer is, you make a set of lookup tables to bypass the MAF signal in the calculation. It's already available for older turbo Subarus. It's a code hack, but it can get the job done. Now let me give a more detailed answer.

So, here's a history lesson. The first speed density system worth noting is the Bosch D Jetronic system. D stands for "Druck," which is German for pressure. That was on the Porsche 914 back in the 1970s and just used a MAP sensor. Airflow based control, called the Bosch L Jetronic (L for "Luft" or air) system, was introduced later (Porsche 924).

Early airflow-based control used a moving door (vane/flap type) to measure volume of incoming air. A Mark III Supra nonturbo had this. A Karman vortex meter was another type of volume airflow sensor using airflow disturbances for measurement. The Mark III Supra turbo had this, and so did many Mitsubishi engines up until the Evo X era.

There are two families of mass airflow sensors. One is the hot wire type and one is the hot film type. The hot wire type, which is what's usually used on Subarus, produces a voltage. The hot film type, which you'll find on say a GM LS1 engine, produces a frequency signal. All the airflow sensors may be packaged with temperature and other sensors.

Speed Density and MAF hybrid

Since the late 90s to early 2000s though many/most control systems are actually both. They're a hybrid from the factory. Most of the widely reverse-engineered Japanese cars don't seem to work that way, but basically everything American and German is hybrid combining speed density and MAF. Do some internet searches on tuning a GM V8 and you'll read all about what's involved. With the right software settings you can basically turn the two systems on or off. Even the modern purely speed density systems are still calculating mass airflow.

Now take a look at a simplified torque model and simplified airflow model to see how this works. This is from the old Bosch Motronic 7 you'd find in something like a late 90s VW.



At the upper left you have the driver's request from the electronic throttle. This converts to a torque request. The torque request figures out how to make the torque using fuel AFR, spark timing, and airflow. Then the airflow model determines how much air is needed to achieve the target torque. How does the ECU know what the torque is though? It uses a torque model.




So when we talk about speed density and MAF, we're only talking about the cylinder charge estimation. That's one portion of the control system. Then if we break it down to some physics [ame="http://en.wikipedia.org/wiki/Ideal_gas_law"]Ideal gas law - Wikipedia, the free encyclopedia[/ame] you'll see why speed density and MAF are in some ways the same thing and can be substituted.



What all this above says is that if I have pressures and temperatures I can model mass airflow, and if I have mass airflow I can model temperatures and pressures. If there's enough information there in the form of accurate look-up tables or real measurements, you can do either one. It gets more tricky when the engine is in a transient condition like accelerating (rather than cruise control at 70mph on flat road). That's where the modeling has to be made to work well, or you have to have a bunch of look-up tables calibrated well enough to get the job done. When variable valve timing and lift gets involved, it's even more complicated.

When you use a speed density software hack, you're replacing a measured value from a sensor with a value from a lookup table and a bunch of correction factors applied to it. The "hybrid" speed density that's been referred to in this thread is more like flipping the software hack on and off based on some conditions. It's not fully like the more advanced modern systems--it's not combining a measured value from an airflow sensor with a modeled value in the same way that cars do from the factory.

But guess what--those OEM systems took a lab full of engine dynos and a staff of engineers and software people to be tuned properly. These aftermarket speed density conversions are a straightforward implementation.

There are a lot of hardware and software limitations to aftermarket tuning of heavily modified engines, but if you put enough time and expertise into it you can make them work well enough to get the job done. You're still going to make sacrifices though, in terms of tuning time and maybe the final driveability.

[u/Josh]

Originally Posted by DeliciousTuning

It is a little hard to explain but like you stated, the MAF sensor is not affected by the pressure differences. That is the exact point, a mass air flow sensor does not know that the pressure has changed, only that the air flow has slowed or increased which translates into a final load value.

This is not true. A hot wire MAF sensor, which I believe our cars have, works by measuring the heat loss from a wire which is in the way of the air flow. It then uses this measured heat loss to calculate an air mass flow rate. The amount of heat lost is a function of incoming air temperature, pressure, and flow rate (and other things). Higher pressure (at the same temperature and velocity) would mean more heat loss which the sensor would read (correctly) as more mass air flow. I'm don't know a ton about convective heat transfer so I can't say how large an effect the changing pressure will have on the reading, but it will absolutely make a difference.