MAF vs Speed Density, why do we prefer which one?

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MAF vs Speed Density, why do we prefer which one?

Post by AndreyB »

Say we have a green turbo-charged Volkswagen. Would MAF-based fuel control be preferred of MAP-based Speed Density any most importantly why? Is there a place for a combined strategy of some sort?

Would this be different if same green Volkswagen was naturally aspirated?
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Re: MAF vs Speed Density, why do we prefer which one?

Post by Simon@FutureProof »

Its a 100% MAP/speed density job for the turbo vw.

MAF is good but tuning MAF for boost is much harder.

Edit - for clarity MAP and speed density is generally better for turbo cars as it captures the increased pressure better than a MAF system.

MAF is better at getting an accurate picture of the amount of air entering an engine but its generally a lot harder to make it work well with a turbo.

The ideal system is a hybrid that uses both maf and map to build a better picture of the airflow but we don't have one of those. (Yet)
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

OrchardPerformance wrote:
Sun Sep 26, 2021 11:06 pm


The ideal system is a hybrid that uses both maf and map to build a better picture of the airflow but we don't have one of those. (Yet)
I think the ideal PERFORMANCE system includes the MAF in the dumpster....just sayin.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by blundar »

If I had to pick a MAF or a MAP sensor, I'd take a MAF any day of the week. MAF is inherently far more accurate for metering airflow in conditions where you can achieve or nearly achieve laminar flow. Choose your MAF wisely, mount it wisely, filter it as little as necessary. You know airflow. No guessing. No VE.

MAP sensors deal better with reversion. MAP sensors are far faster responding. MAP sensors are easier for the unwashed masses to wrap their heads around for some reason.

One of the things I've been hammering on slack about is generating models where MAF and MAP can be used interchangeably (hell, also with TPS/Alpha-N) so that you don't have to pick one method.

Using MAP/VE and having a MAF present to check against would probably be my choice for your vehicle in question.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

blundar wrote:
Mon Sep 27, 2021 3:31 am
If I had to pick a MAF or a MAP sensor, I'd take a MAF any day of the week.
Funny, I always go MAP when I can and MAF as a last resort afterboth speed density and alpha-n, so I never use it :) I love the concept of the MAF and I think on a modern stockish or stock other than higher boost it probably is easier to stick with the stock MAF setup but I just never mess with stuff like that so MAF just never makes a lot of sense to bother with and its always a flow restriction so removing them is a quick but generally small HP win on NA stuff. But stock VW turbo just adding more boost, sure, keeping the MAF is probably the easiest.


MAP....it gives a pretty direct indication of cylinder pressure which is vey helpful knowing where to set timing. Like VE it pretty universal engine to engine so again I can just open the lambda table and get straight to work because I know what numbers I expect to see. It also doesn't much care how you design the intake track, what cam you use or anything else really. Its a challenge on ITBs but other than that most any engine can be tuned nicely with a MAP signal. This is my go-to on any performance engine because I know it will work well and yield as much HP as is possible.

VE is pretty universal, no new numbers to learn engine to engine, just open the table and get to work but to you're point needing to do this is a lot of extra work that the MAF eliminates if its working right....but as I said I just don't work on stuff MAFs are very happy with so a necessary evil.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by blundar »

mk e wrote:
Mon Sep 27, 2021 12:06 pm
[MAP....it gives a pretty direct indication of cylinder pressure which is vey helpful knowing where to set timing.
that's the thing... it really DOESN'T tell you cylinder pressure. grams/cylinder tells you cylinder pressure. MAP tells you what you have prior to the intake valve opening. grams/cylinder is what happens after the valve opens. MAP / pressure / boost is a measure of RESTRICTION, of RESISTANCE not a measure of flow. Variable cam engines can illustrate this particularly clearly - it's entirely possible to have a situation where at a given RPM (to be fair) a higher MAP reading actually ends up meaning lower g/cyl (and therefore cylinder pressure) than a cam position where MAP is lower and g/cyl is higher.

I realize I'm being a bit of a detail oriented stickler here but I think it's a really important distinction to wrap your head around.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

blundar wrote:
Mon Sep 27, 2021 1:10 pm
mk e wrote:
Mon Sep 27, 2021 12:06 pm
[MAP....it gives a pretty direct indication of cylinder pressure which is vey helpful knowing where to set timing.
grams/cylinder tells you cylinder pressure.
No it doesn't...it implies it depending on cylinder size and IAT, so you care about a different value for every engine size and IAT. MAF is a direct flow reading so the most accurate flow number....on well mannered engines at least. You are not wrong that it can be converted to a pressure, but its not a pressure and having an MAF sensor does cause a pressure drop so HP drop.

MAP is pressure, so it is the most direct reading available but you are also not wrong that cylinder pressure is different from MAP.....hence the need to pair it with a VE table.

Both work, I just find it much easier to work MAP and VE because they apply the same to every engine, nothing new to learn.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

mk e wrote:
Mon Sep 27, 2021 2:52 pm


Both work, I just find it much easier to work MAP and VE because they apply the same to every engine, nothing new to learn.
I'll add this, I would like MAF a lot better if it were normalized....MAF/rev/cc or something like that so the numbers become universal Like MAP and VE numbers are. But even then I can't imagine ever installing an MAF sensor on a race engine on purpose.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by Simon@FutureProof »

Our maf is done in a way that the number in the table is equivalent to a correction %.

The maf algorithm calculates cylinder filling and the table is only there to be able to calibrate out resonance, inaccuracies and other effects.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

OrchardPerformance wrote:
Mon Sep 27, 2021 4:16 pm
Our maf is done in a way that the number in the table is equivalent to a correction %.

The maf algorithm calculates cylinder filling and the table is only there to be able to calibrate out resonance, inaccuracies and other effects.
Nice. I've not seen that before.

What are the units on the axis? That is what generally bothers me with MAF vs MAP. With MAP you basically know where you expect to see what but with MAF there is no way to look at the table and know what the numbers are without playing with it...at least on the ones I've seen.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by Simon@FutureProof »

https://github.com/rusefi/rusefi/wiki/MAF

All the detail you could want in that link (maybe).
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Re: MAF vs Speed Density, why do we prefer which one?

Post by blundar »

mk e wrote:
Mon Sep 27, 2021 2:52 pm
No it doesn't...it implies it depending on cylinder size and IAT, so you care about a different value for every engine size and IAT.
I'm going to disagree with you there. *grams / cylinder* is the ultimate measure of cylinder pressure. The end. When the valves close and the compression stroke starts, grams/cylinder will tell you your cylinder pressure at the end.

grams / cylinder is very, very, very precise. You can derive grams/cylinder using a MAF. You can derive grams/cylinder using VE. Hell, you could even derive g/cyl using alpha-N. At the end of the day, the more accurate your g/cyl measure the more accurate your cylinder pressure estimation is. Most modern OEMs still model g'/cyl even when they are using speed-density to do it. Doesn't matter if you're talking about a race engine or a street engine or a emissions oriented econobox. g/cyl or normalized (where 1.0 = complete filling at sea level) FTW.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

Interesting, thank you for the link. I do like it but I honestly never really thought through the thought that having a normalized axis would kind of require VE to come back in or maybe that is just to make the integration easier to deal with, either way its an interesting implementation.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

blundar wrote:
Mon Sep 27, 2021 7:00 pm

I'm going to disagree with you there. *grams / cylinder* is the ultimate measure of cylinder pressure.
Hmmm....what are the units of grams/cylinder.....grams right? Just sayin mass is not pressure ;)

Sure, you can calculate it though and that is my point.....its just harder to human read a mass flow and have any idea what that means to pressure. Most anyone can look at a MAP number and understand about where the cylinder fill/pressure is.

Yes, most implementations of speed-density and alpha-n do calculate air mass flow to calculate fuel flow theses days so measuring air mass flow directly is a more straight forward way to control fuel flow. No argument there.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by kstuart »

mk e wrote:
Mon Sep 27, 2021 2:09 am
OrchardPerformance wrote:
Sun Sep 26, 2021 11:06 pm


The ideal system is a hybrid that uses both maf and map to build a better picture of the airflow but we don't have one of those. (Yet)
I think the ideal PERFORMANCE system includes the MAF in the dumpster....just sayin.
I suppose if you are 1985 general motors....

Trying using speed density on something like a gasoline engine with a variable geometry turbo. You know when the engine can have drastically varying ve at a constant manifold pressure/rpm.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

kstuart wrote:
Thu Dec 02, 2021 8:01 am


Trying using speed density on something like a gasoline engine with a variable geometry turbo. You know when the engine can have drastically varying ve at a constant manifold pressure/rpm.
I've never run into that setup. I was talking about NA where even small pressure drops in the intake matter so MAF is suboptimal, but that isn't true with boosted setups so sure other paths may be preferred in some situations. In the one you describe it sounds like adding an exhaust pressure would sort it just as adding cam position sorts is on variable cam applications. But having MAF on a turbo seems fine too.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by Bluepower »

guys, how about MAD, or Manifold Air Density? pounds of air, instead of pressure(resistance) or flow(g/s)
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

Bluepower wrote:
Thu Dec 02, 2021 3:39 pm
guys, how about MAD, or Manifold Air Density? pounds of air, instead of pressure(resistance) or flow(g/s)
hmmm....if you know the IAT and MAP you can calculate MAD and that is what is done for speed-density math but that is not enough as you need to know the mass of air in the cylinder, so then you multiply the density by VE to get trapped mass. With MAF you get a flow rate for the entire engine and multiply by cycle time then divide by number of cylinders to again arrive at trapped mass and could then easily calculate its density but that term is not needed unless you are calculating cylinder pressures.

MAF lets you use direct measurements which is generally preferred unless the act of measuring alters the result which is usually the case with a MAF sensor as there is a pressure drop across the sensor. When you are running boost you can tolerate a pretty large pressure drop but on an NA engine any pressure drop is a HP drop and best avoided if HP is your goal if accuracy is the goal (say to meet emissions regulations) then MAF is the way to go.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by Zeiss »

Personally, I would always prefer a MAF sensor over a MAP sensor. Precisely because I get directly, always and under all operations, the air mass that the engine is currently taking in. I do not have to correct, adjust or anything else, just directly calculate the injection quantity for the desired AFR.

The data sheets for MAF sensors are also no problem. At least at Bosch you get them very easily and quickly.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by mk e »

Zeiss wrote:
Fri Dec 03, 2021 9:09 pm
Personally, I would always prefer a MAF sensor over a MAP sensor. Precisely because I get directly, always and under all operations, the air mass that the engine is currently taking in. I do not have to correct, adjust or anything else, just directly calculate the injection quantity for the desired AFR.

The data sheets for MAF sensors are also no problem. At least at Bosch you get them very easily and quickly.
That is just not true on an NA engine with decent overlap in the cams. These setups pretty much always have reversion in the intake track so the MAF pretty much always read incorrectly on top of being a flow restriction. Then to fix the reversion you end up adding more flow restrictions and you're down 5%+ on HP from where no MAF put you....been there, done that. But mild cams and/or HP a secondary concern, sure MAF is quite simple.
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Re: MAF vs Speed Density, why do we prefer which one?

Post by ssmith »

Zeiss wrote:
Fri Dec 03, 2021 9:09 pm
Personally, I would always prefer a MAF sensor over a MAP sensor. Precisely because I get directly, always and under all operations, the air mass that the engine is currently taking in. I do not have to correct, adjust or anything else, just directly calculate the injection quantity for the desired AFR.

The data sheets for MAF sensors are also no problem. At least at Bosch you get them very easily and quickly.
I thought there's lag with a MAF on throttle transients?

FWIW BMW N20 engines have a MAF before the turbo, but it's just used for long term trim to the calculated air mass using air temp, MAP, RPM, and valvetronic position (and probably other values).
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Re: MAF vs Speed Density, why do we prefer which one?

Post by sepp2gl »

Hi guys and happy new year!
just looking through some of your discussion about speed-density (SPD) versus mass-airflow (MAF).
For the scope of stoichometric combustion (lambda=1) both have some pros and cons.
MAF pros:
- direct and precise massflow into the engine (at least for steady-state engine operation)
- especially stoichometric combustion systems with EGR benefit from steady-state accuracy.
MAF cons:
- highly expensive
- sensitive to air-pulsation, in extreme cases part of the airflow is measured twice
- Package constraint; with a compromised package a beding air-tube closely before and after the MAF-sensor, accuracy might be highly degraded
- poor accuracy in transient engine operation; rapid opening of the throttle leads to enhanced filling of the intake manifold, which does not correctly reflect trapped airmass of the engine. So this needs to be compensated by an intake-filling modell.
- with turbocharged engines a MAP-sensor is needed in addition for boost-pressure control.

SPD pros:
- MAP+IAT lower in cost compared to MAF
- for turbos mandatory aniway
- precise indication of cylinder-trapped airmass in transient conditions
- steady state inaccuracies can be compensated by AFR-control
- easy package; MAP and IAT can be integrated into one device
SPD cons:
- model-based calculation of trapped airmass (was an important factor with 8bit-µC in the 1980/90s, NOT TODAY)
- IAT is only a mixed-up information of the actual intake air-temperature and the manifold-temperature, a reason for steady state inaccuracy of SPD
- For today's engines with extreme VVL or VVT like Miller/Atkinsson the model for trapped airmass can be very complex

Historically both MAF and SPD came in for electronic fuel injection in the 1980s.
That time had relaxed emissions regulation (compered to today) and 8-bit microcontroller. MAF was used for high-end, less cost critical applications, whereas SPD was used for low-end application where product-cost mattered much.
The overestimated transient air-mass in transient operation was used for acceleration enrichment (in those days this wasn't an issue for emission legislation).
The missing model accuracy of SPD due to 8bit controller was compensated by lambda-control and adaptation and didn't matter for emission legislation either.
Today (since ~20years) model-based estimation allows for almost any kind of "virtual sensors" in engine operation, and the µCs are powerful and highly affordable. So today SPD would be my choice.

If I skipped any pro/con in my above overview, I beg your pardon. Please just let me know.

T2US, sepp2gl
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