Grams Per Second (AKA How to SUCC properly AKA MAF and intake thread AKA

Discussion in 'Tuning Knowledgebase' started by Enki, Dec 12, 2023.

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  1. Enki

    Enki Motorhead Platinum Member

    Feb 10, 2016
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    Yeah, so I'm shit at making titles. Whatever. Shutup. MOOOOOOoooooommmm....

    Anyways, plan is to use this thread to discuss a few things; everything will be in the OP but I'm not sure if that will happen all at once.

    Intakes and sizing
    So, you've got a stock intake. It's, what, 2.5" diameter? That's roughly 4.91 inches of flow area, give or take (there's a difference between piping and tubing, and the best way to learn about that is the hard way. Or Google). Let's say, you want more flow for more and/or bigger bangs to make the tires spinny harder. What size should you go to? Realistically speaking, as big as you can and that will fit without being a headache to tune and/or work on.

    An example of this would be running a 3.5" intake on, say, stock turbo. Overkill? Absolutely, but if you can use the same intake (with very minor modifications later) on a different turbo, it's arguably not a bad investment. It will fit, with some stuff moved out of the way, and lets you run a bigger filter (which has its own benefits; I'll touch on this later). Idle tuning isn't difficult on a 3.5", and with that big a pipe feeding that small a turbo, you'd see some efficiency increases. It wouldn't be much (maybe not even measurable depending on the tune), but it would happen. Also, swapping filters on a big piece of pipe is arguably easier than fiddling with the stock airbox and leaving a screwdriver in there or something. Airflow travel is smoothed out a bit, and you get wooshy noises.

    So, if 3.5" is good, wouldn't 4" or hell, even 5" be gooderer? Why not just stick the MAF wires DIRECTLY INTO THE TURBO! Well hold on to yer yammy jammies there bucko; here we run into our first issue: Mass airflow sensor resolution. The car likes to measure the airflow going into the engine so changing the size of the intake results in the mass airflow calibration going to shit when you put a bigger one in. Too big and the car winds up not running well or even at all at idle without turning up the idle speed or other potentially more drastic measures.

    So, just how bad is this pipe increase problem?

    Well, since baby Jesus invented spreadsheets and uppers, I can show you:

    upload_2023-12-11_20-28-52.png (this took like an hour I swear so praise me plz kthx)

    While the above is in square inches, the same kind of increases apply to non-freedom units. The math for this is as follows (thanks DuckDuckGo and OmniCalc!):

    upload_2023-12-11_20-30-44.png upload_2023-12-11_20-32-23.png

    So, as you can see, even adding half an inch (less than the width of your thumb) of diameter or double the radius (the across the edge measurement or 'd' in the above image), the available area for airflow increases by just about 44% when going from 2.5" to 3".

    MAF (Mass Air Flow) Sensor
    "44%! I've saved more by stealing cookies from Girl Scouts!" You say.


    However, when the ECU sees a voltage (and I know it checks more than that, this is the wildly inaccurate section still, so can it!), it equates that with a specific amount of airflow going into the intake. See, it can't tell how big the intake is. The MAF sensor only sees what it sees, and what it reports back is a voltage.

    On a stock tune, for a given voltage, the reported airflow will be roughly 44% higher, making the car inject 44% more fuel. So, now your car doesn't start. What do you do? Simple. Unplug the MAF sensor. While the ECU uses the MAF as its primary source of airflow truth, it has a fallback it can use called speed density that I won't be covering here at all because direct massflow measurements are superior and you'll never convince me otherwise and lets just ignore the fact that I put a 2 GPH meth nozzle in a K&N air filter because I was curious and made my car shut off mid WOT pull and...*Cough*

    Sorry about that. Sometimes, the science; it gives me the vapors.

    I did really do this though, and I still have that filter...Just in case.

    Now where was I? Oh, right.

    MAFCAL (Mass Air Flow CALibration)
    Basically, the ECU has a lookup table that translates voltages sent by the MAF sensor to actual airflow amounts. This is, really, a SUPER important step in tuning and should not be overlooked or under-appreciated. It's also the reason providing an ACCURATE list of modifications done to your car is super important for your tuner.

    So, what does a MAFCAL look like? It's basically a row of voltage numbers from 0-5v and a row of airflow numbers either in grams per second (g/s) or pounds per minute (lbs/min), or some other MASS over TIME measurement.

    While I can show snippets of what different calibrations look like on VersaTuner, I don't have AccessTunerRace to compare with; it's also worth noting that different generations of cars and even years can have changes in the voltage ranges used for the calibrations; why does this matter? Well, it matters because trying to copy/paste some other cars calibration could cause serious issues, if not the outright destruction of your engine.

    Now, I have a custom made 3.5" intake on my 2009 Mazdaspeed 3, and this is what the 2 to 2.5v range looks like for me:

    How does this compare to bone stock? Let's see:

    Well, that's a little bit of a change, innit? What you can't actually see here, is that this increase is *not* linear. To show this, I gotta go back to the spreadsheets; disregard the crunchy bits:


    This is from my calibration spreadsheet that made for messing around with this stuff. Represented in Red is a MAFCAL from someone's car that was running terribly, and blue is the interpolated and smoothed output I was able to generate using that base calibration. The math needed for this is, put delicately, fucking insane so I won't be discussing or sharing it (y'all dummies gonna blow your shit up and I don't wanna be fingered at as the cause); the main thing to look at here is the shape of the curve. It's a gradual but rapidly ascending curve.

    Cool. Pretty. Matches my eyes and my aura on drunk gaming night. Neato.

    Anyways, the curve shape changes more aggressively the larger the intake. At 5.0 volts, a 4" intake may push 800 g/s, while an easy breathing 3" one can't quite touch 500.

    Those of you that are tuned know that getting that MAFCAL going can be tricky, and may take several revisions to get to a point where the car starts and runs well without being all over the place on fuel trims.

    Ah, yes.

    Fuel Trims
    This is the real most, juicy, dripping wet sciencey nerd stuff you wanna mainline right here.
    What are they? Well, the long and the short of it is the long and the short.

    No, really. Long Term Fuel Trim (LTFT) and Short Term Fuel Trim (STFT). These are important to note, but not something to obsess over. The barebones gist of how they work is basically they calculate the difference between what Air Fuel Ratio (AFR) the ECU is trying to hit vs what the MAFCAL says is going through the engine.

    OK that's easy enough, but if they are important, why shouldn't I pay super close attention to them?

    Well, that's because, and I cannot stress this enough: EVERY LITTLE FUCKING THING AFFECTS THEM.

    Engine coolant temperature? Yep.
    Air temperature? Yep.
    Barometric pressure and humidity? Yep (so yeah, basically the weather in general).
    Quantity of ethanol and/or water in the fuel? Yep.
    If you drive past a cattle farm and the cow farts blow your way? Hella yep.

    "Well, shit. How do you even account for all these variables?" You ask; I answer, but first the definitions:

    Short Term Fuel Trim (STFT):

    • The instant recorded difference between expected and actual Air Fuel Ratio (AFR). This value is not saved anywhere.
    Long Term Fuel Trim (LTFT):
    • An average of the STFTs recorded over time and stored in the ECU by specified MAFCAL range.
    Wait...Specified MAFCAL range? Yep. That's what these partitions are in the tune:

    First, let's get the ECT (engine coolant temp) one out of the way, as it's relevant but not entirely worth discussing here:

    Speaks for itself, really; a base trim to apply to all fuel calcs until the engine warms up. Those of you on full E85 in cold climates having issues with the car starting on the first crank in the winter: Go have a peep at the warmup enrichment tables and thank me later.

    Back on topic:




    Note that in stock form, 4, 5, and 6 are all the same 200g/s number, but they do work if you change them to other numbers (each one needs to be larger than the previous for it to work properly though).

    So, what do these numbers do? They are storage breakpoints. Any recorded LTFT values between 0 and 5 g/s are stored in partition 1. Anything between 5 and 18 g/s in partition 2, etc. This is why LTFTs can change drastically and seemingly at random.

    There was a time when I messed with these in hopes that it would help me get closer to 0, but that was essentially a fools errand (your tuner probably doesn't care about the breakpoints anyways, it's more for the car than for you) and you really only need a rough separation between idle, light cruise, moderate acceleration and speeding ticket.

    Funny enough, that's basically the partitioning we have here. It's also why it's still the stock numbers on my tune. Some tuners may adjust these numbers slightly, but don't worry if they don't; you're fine without it.

    Congratulations, you've just learned a thing you probably didn't know!

    Laminar Flow
    Here we go. This is the good stuff; smooth, fluid, graceful, and most importantly, predictable. Laminar flow is when a fluid like air (yes air is a fluid FITE ME BISH) flows in a highly organized manner. If you've ever seen a fountain of water that looked like glass rods but it was actually flowing water, that was laminar flow. For a more accurate description and cool ass demonstrations, I defer to Destin:

    You can find videos and pics of how to make a laminar flow thingie to connect to a hose out of soda straws and PVC just about anywhere.

    So, why does THIS matter? Well, the MAF sensor can only measure airflow from one part of the intake, and the closer to the center of the intake you go, the higher the flow rate. This is why the MAF is shaped the way it is; to get as close to the middle of the intake as possible; any air touching the inside of the intake is going to be dragging against the intake wall, slowed down. Any air touching THAT air will be slowed as well, but slightly less, and so on and so forth.

    Airflow Straighteners
    Thus, any intake you buy will most likely have some kind of "airflow straightener" built in. These take many forms, from wire mesh to honeycomb to just two vertical pieces of metal.

    Some tuners will argue that you don't need anything but the intake, and while this is *largely* true, they do, in fact, make drivability better and fuel trims more stable. Some anecdotal examples of this include people with no straightener at all having their cars die from an idle either with their hoods open on a windy day or when the radiator fan kicks on. Yes, this is a real thing that happens.

    I personally use a honeycomb insert on my intake and they work wonderfully; they can be found at Treadstone Performance's website among other places, but if you get one elsewhere make sure you get one with decent thickness and smaller cell size; the goal here is to have enough cells small enough that airflow is forced to even out at idle but not so small that it can't breathe at higher airflows. Anywho, here's what mine looks like, taken earlier this year:

    gvbsI9t - Imgur.jpg

    6KyHBau - Imgur.jpg

    You may notice that the edges are quite damaged; this is perfectly fine. When you install these, you don't generally need any kind of glue or welding or anything; just crush the sides that stick out too much until you can just barely press it in. We want the airflow that hits the MAF to be clean and turbulence free; the outside edge near the tube wall doesn't much matter.

    Further Notes and TLDR
    I've been at this for several hours now and I'm pretty beat, so I'll make this section quick.
    • Fuel trims fluctuate, like all the time. Don't sweat this. If your trims swing +- 10%, maybe drop an email to your tuner and/or look for leaks or change your air filter or something.
    • Factory spec on trims is +-8% LTFT, plus up to another +-19% on STFT before you throw a code (someone correct me if I'm wrong here). That is a WIDE range to work with.
    • If your trims look jank but your tune isn't done yet, don't panic and post all over the place that the world is ending. Your tuner probably knows and is likely going to adjust stuff; ask, but try not to pester.
    • Honeycomb air straighteners are THE SHIT. Seriously. If you are on the market for an intake, get one with no straightener or a high cell count honeycomb one if you can; it makes things so much nicer/faster to tune.
    • If you do get honeycomb, get something at least half an inch thick with a higher cell count. If you want to know why, look at designs for garden hose laminar flow thingies.

    Additional tips and tricks
    • If there's an issue with trims going wonky, throwing codes and the car runs like crap but you don't have light or tools or are in an "I just need to make it home" situation, shut the car off, pop the hood and disconnect the MAF sensor plug. You can limp it home like this, but don't do this for long and avoid hard accelerations, if it even lets you.
    • If your trims have started to drift out of normal range, you might be able to rotate the air filter and see if that is the cause (uneven flow). This can show you if your air filter is too dirty.
    • High trims on the rich side (negative numbers) can suggest you have an EVAP leak/issue. There's a hose next to the high pressure fuel pump that has a filter on it and goes from the firewall to the intake (NOT the brake booster). If you clamp that or disconnect and plug the end going to the intake manifold with your thumb and the STFTS swing heavily the other direction, that might be your problem...Keep in mind some flow through there at certain times is normal, though.
    • Fuel trims can be reset by either reflashing your current tune or disconnecting the negative battery terminal for 30 seconds.
    • While changing the intake requires a tune, changing anything other than the intake should *not* require any changes to your MAFCAL, unless other drastic changes have taken place to your engine (in which case, you'd need a full retune anyways).

    Welp, I'm bushed. Too much thonk and not enough lube.
    That said, I leave you, my little sciencelings, with this moment of zen:


    Vs the guy she told you not to worry about:
    QxpkoE6 - Imgur.jpg

    Like this thread ya little shits or I'm gonna give you pinkeye in the worst way possible.
    Last edited: Dec 12, 2023
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