Question on MAF vs Speed Density

[mcglsr2]

Hey everyone, so I know the essential differences between a MAF setup versus a Speed Density set up. My car uses Speed Density (SD), which relies on the ECU having the VE to calculate the amount of air entering the engine. I always thought that changes to the car that affect air flow require a "re-tune" on a SD system due to the computer's inability to measure the additional airflow. In contrast, a MAF system measures the amount of air being ingested - meaning that a modification that increases this does not necessarily require a "re-tune" as the ECU can see the increase via the MAF.

However, after reading around on some other sites, it seems cars that use a MAF (specifically turbo cars), even after something a "simple" as a cold air intake, require a re-tune. What am I missing here?

Thanks in advance.

[turbovanman²]

Not sure what car's your talking about but most MAF car's can take considerable amounts of work before a retune is needed but it can also depend on how much room the OE left in the fuel tables. SD systems can take minor mods due to the OEM's letting them run on the rich side at WOT as a buffer.

[mcglsr2]

Hey Simon, I was trying to ask the question in general though I did have some specific cars in mind. I'm in the process of getting a new car, a 2011 STI - which uses a MAF setup. My "project" car is an 87 CSX, which is a SD setup. I had been doing reading/research on STI & WRX forums just to see what people were doing with the cars (though I don't plan on doing any engine mods as the car will be my DD) and it seems like people always say get a re-tune. Add a CAI or short intake, get a re-tune. Add a downpipe, get a re-tune. Look at the car funny, get a re-tune. I found myself wondering what was going on - after all, the MAF/computer is supposed to be able to adjust for increases/decreases in airflow (to a certain amount) - that was one of the "advantages" of a MAF. And those mods don't seem all that drastic. It's almost like those folks are treating the car like a SD setup, and that just went against my understanding so I am trying to figure out what it is I am missing

[OmniLuvr]

if im correct, i believe the maf system is a more exact way of tuning, and utalizes air/fuel tables at very exact points to the measured amount of air flow from the maf sensor. so to a point, the maf system can calculate how much fuel to deliver for the givin amount of air flowing into the engine, but it is also very dependant on the O2 sensor readings. when at WOT, it goes to more of a fixed table, and if you are increasing the amount of air in and out, you will deff be out of the range of that table. anything from an air filter, to intercooler, or a muffler will change the ve of an engine.

the subaru's are very finniccy with tuning, and something as much as moving the location of the MAF sensor will throw the calibration off. also, the subies dont have very stout bottom ends, and it only takes the cal to be off a little bit, to cost you big when them bottom end blows.

[turbovanman²]

Hey Simon, I was trying to ask the question in general though I did have some specific cars in mind. I'm in the process of getting a new car, a 2011 STI - which uses a MAF setup. My "project" car is an 87 CSX, which is a SD setup. I had been doing reading/research on STI & WRX forums just to see what people were doing with the cars (though I don't plan on doing any engine mods as the car will be my DD) and it seems like people always say get a re-tune. Add a CAI or short intake, get a re-tune. Add a downpipe, get a re-tune. Look at the car funny, get a re-tune. I found myself wondering what was going on - after all, the MAF/computer is supposed to be able to adjust for increases/decreases in airflow (to a certain amount) - that was one of the "advantages" of a MAF. And those mods don't seem all that drastic. It's almost like those folks are treating the car like a SD setup, and that just went against my understanding so I am trying to figure out what it is I am missing

Its possible the Subie's could be more sensitive but I would venture to guess is "the guys" you see posting , are probably either misinformed, brain washed or just eeking out every last HP, which a retune does. I read an import mag, I think its "Import Tuner", where they take a car, then do a bolt on, IE CAI, and dyno it, another bolt on, IE cat back, dyno it etc and to get a true effect and depending on which car it is, they sometimes do a retune, which of course, will maximize the bolt on's, but for the most part, most mods gain and don't need a retune. Of course, forced induction car's can need a retune as the bolt on mods increase boost and air flow-to max out said bolt on's.. I hope that's clear as mud, lol. : and I didn't repeat myself too much.

[mcglsr2]

Okay, I went back and re-read through one of my books, and it had this to say about it:

Because it [the MAF] is measuring the actual flow of air into an engine, it lends itself to modified engines. Porting the intake tract and cylinder head, for instance, will increase an engine's volumetric efficiency. This means that the engine breathes more freely and create more power. The AFM [airflow meter in the MAF] will simply see this for what it is: more airflow. Consequently the appropriate amount of extra fuel will be injected. As long as the injectors are sized sufficiently to handle the increased power production [really, to handle the increased fuel needs to match the increased airflow], an MAF control system will accomodate the increased airflow

The above is from Street Turbocharging by Mark Warner, PE; pages 110 & 111 - with my comments in bold brackets.

So the gist of the above is that a MAF seems like it should be able to keep up with moderate changes to the airflow of engine, as the ECU will adjust for it, as long as the change doesn't max out the injectors. This has always been my understanding of the "benefit" of MAFs. Now, perhaps the Subies are at the harry edge and therefore don't take well to airflow changes, but I'm not sure I believe that (granted, I have no data that says one way or the other). I have a hard time thinking that the engineers would cut things that close. So what I'm wondering is whether the cars just don't take well to mods, or perhaps that the folks over there are a bit exhuberant with their re-tunes. A very popular (and some say 1st) mod is an Access Port which basically lets you put new maps on the ECU, so perhaps it's a matter of "re-tune because it's so easy so why not."

I think I'll post on some of their forums and ask what's going on with the whole MAF thing, see what they say.

Now, if we are talking Open Loop system like Speed Density, then a head porting will screw up fueling mixture, that I know.

[zin]

I think you have a pretty good understanding of the capabilities and drawbacks of the two methods... AND I think Simon has a pretty good insight into why you are hearing about re-tuning after a spark plug change!

Re-tuning after a minor mod on a MAF car is like chicken soup when you're sick... "It couldn't hurt!"*. *assumes a competent tuner, otherwise all bets are off!

It is also true that the Japs style of engineering isn't the same as we are used to here in the US, at least as far as fuel systems go. It seems like if the car is designated to make 200HP, you get a 205HP fuel system!

The other limitation could be in the factory tune itself. Often the factory will "clamp" the amount of adjustment allowed to the adaptives, so, while the fuel system may be capable of adjusting to your new mod, the operating system may not allow it to do so, and a re-tune would fix that issue.

Personally, MAF is the way to go. With today's sensors and computers, they are much more adaptable/accurate. That is part of the reason almost all new cars are MAF, not SD, though that has more to do with emissions than performance.

Heck! IIRC, someone was offering an aftermarket fuel injection system that had GM's ECU at it's core, pretty much a tunable version of stock LS-1 controller!

Anyway, that's my .02 worth!

Mike

[turbovanman²]

I am actually still shocked that most or all the aftermarket systems are all SD, and especially now when you can pick up a high flowing MAF sensor for peanuts.

[zin]

I am actually still shocked that most or all the aftermarket systems are all SD, and especially now when you can pick up a high flowing MAF sensor for peanuts.

I think part of it is there is a perception of the MAF sensor being a restriction and/or a point of failure... Also, big cams will give both a MAF or SD system a hard time, which is why many use alpha N, at least at idle/low RPM...

Other than that I have to agree. Especially when you consider that only MAF systems can account for humidity... But then racers can be like old dogs and new tricks!

Mike

[thedon809]

Everyone uses SD so you can vent the BOV and sound cool .

[mcglsr2]

Thanks for the replies, guys! It's funny you guys say that about MAFs - I understand why all new cars are MAF, it provides more control for the computers to adjust to conditions. However, I am a SD man myself - I actually prefer the SD systems. I'm not sure I have a good reason for that, it could just be my ignorance. But I know one thing is true about the two systems (well, at least it used to be true, maybe not any more), is that SD systems, specifically in turbo'd applications, are not at all sensative to the length of pipe runs to get the charge air around, whereas the MAF systems are a little more sensative to it. I think this has something to do with lag, and I read it once in the Corky Bell book, but essentially the trick with the MAF systems was to keep the pipes as short as possible. With a SD system, not necessary. Of course, that may be a lame reason to prefer SD over MAF

It will be interesting to see once I get the STI - I'll have a SD car and a MAF car, though I won't be chaging much on the MAF setup. This might be a good learning opportunity for me

[risen]

Just something that no one has mentioned yet re: subies. Some of their newer systems will use both a map and a maf. Exactly which is used when or how, I don't know, but I had someone point it out to me on a wrx a few years back. But he pointed out to me that he was able to vent his bov to the atmosphere without doing anything to the car and it didn't stumble when it vented. That dual sensor setup might be part of the reason the subie guys are refining their tunes so often, though that's pure speculation.

[BadAssPerformance]

I am actually still shocked that most or all the aftermarket systems are all SD, and especially now when you can pick up a high flowing MAF sensor for peanuts.

SD is easier (less varables) for correctly tuning, especially a boosted application, so the MAF really doesn't offer any advantage on an aftermarket tuneable system other than adding complexity.

---------- Post added at 07:20 PM ---------- Previous post was at 07:14 PM ----------

Just something that no one has mentioned yet re: subies. Some of their newer systems will use both a map and a maf. Exactly which is used when or how, I don't know, but I had someone point it out to me on a wrx a few years back. But he pointed out to me that he was able to vent his bov to the atmosphere without doing anything to the car and it didn't stumble when it vented. That dual sensor setup might be part of the reason the subie guys are refining their tunes so often, though that's pure speculation.

Boosted MAF systems usually run a MAP too to get an actual measurement input of intake pressure.

Chrysler still hasn't used a MAF yet in production have they? ...well other than the DSM stuff? many ways to skin a cat

[cordes]

SD is easier (less varables) for correctly tuning, especially a boosted application, so the MAF really doesn't offer any advantage on an aftermarket tuneable system other than adding complexity.

---------- Post added at 07:20 PM ---------- Previous post was at 07:14 PM ----------



Boosted MAF systems usually run a MAP too to get an actual measurement input of intake pressure.

Chrysler still hasn't used a MAF yet in production have they? ...well other than the DSM stuff? many ways to skin a cat

I totally threw out the reason why he said so, but 5digits stated that the SD systems are good for him and good for us. They require a lot of engineering and calibration for initial setup (good for him) and are easier to tune and get right for us after the fact. As I say, I don't recall what all was easier than MAF systems. I filed that under don't care because I don't use it. Now I wish I hadn't.

[thedon809]

Maf sensors are finnicky too. Any little dirt or anything on the heated part that measures air and it can mess it up.

[RattFink]

Speed Density FTW! I don't think there's anything wrong with SD. MAF sensors are pricey when they go bad, and are finicky when they get dirty as mentioned above. Most MAF systems have a MAP or Baro sensor also, so it just seems a lot more complicated to be a little more accurate.

[roachjuice]

My parents have an 09 Sti. Perrin stage 1 tune and an intake. Runs real good for 2 mods. Don't know why it requires a re tune after just an intake. That's subi for ya lol. I beat my friends 03 350z 20-110mph with all bolt ons.

[dodgeshadowchik]

SD is easier to deal with. While it is true that the MAF reads the actual air volume rather than basing it off of an algorithm, they are much more difficult to deal with. The MAF still needs to be re-tuned per mods like intake size and such, since the sensor needs to be able to accommodate for the change in size. And getting this "re-tune" correct tends to be a pain in the butt. I know with the 3" GM MAF on the Talon, it was quite a trip trying to get it to read correctly EVEN with a laptop tunable ECU. The newer stuff is probably easier, of course.

Cordes is right; getting that initial SD calibration is quite an undertaking, but there should be enough out there now in which you can find a good base to start from. right?

[roachjuice]

He must have had a duel bov. Like half vented and half recirculated. Perrin has them on their site. I'm personally am SD guy. KISS Keep It Stupid Simple. That's what I to by.

[cordes]

SD is easier to deal with. While it is true that the MAF reads the actual air volume rather than basing it off of an algorithm, they are much more difficult to deal with. The MAF still needs to be re-tuned per mods like intake size and such, since the sensor needs to be able to accommodate for the change in size. And getting this "re-tune" correct tends to be a pain in the butt. I know with the 3" GM MAF on the Talon, it was quite a trip trying to get it to read correctly EVEN with a laptop tunable ECU. The newer stuff is probably easier, of course.

Cordes is right; getting that initial SD calibration is quite an undertaking, but there should be enough out there now in which you can find a good base to start from. right?

I think one can get very wild with an 8v and get some terrific drivability in an afternoon with an understanding of how the ECU works.