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HT4100 4.1, 4.5, 4.9 Discussion, Fooling the Inlet Air Temp (IAT) sensor? in Cadillac Engine Technical Discussion; Clever way to resurrect an old thread............ What did you use to measure before and after air duct temperatures in ...
  1. #16
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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Clever way to resurrect an old thread............

    What did you use to measure before and after air duct temperatures in order to determine that your modification actually resulted in "greatly" lowered temperatures ??

    A handy gadget is available at www.scangauge.com . It will blow all so-called "CAI" myths out of the water. Just call up IAT as one of the display gauges.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    The best possible CAI for your car came from the factory. The point is to pull air in from outside of the engine bay and as long as the intake can flow equal to or more than what the throttle body flows at WOT then there can be no more improvement. If your TB flows 500CFM and you install a pipe in front of it that flows 1,000,000 CFM, you successfully accomplished NOTHING. A chain is only as strong as its weakest link.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    The OP was asking about mods to the MAT although the thread has drifted to "real" CAI. First of all using a resistor is kind of silly since you can just move the damn thing to somewhere else that has colder air. Second fooling the computer will work right up until the car enters closed loop at which point the O2 will probably cause the computer to lean out the mixture again. So it's pointless to try and trick it unless you have made other mods that are causing the car to run lean during open loop.

    A real CAI will improve performance a bit but you won't be winning NASCAR or anything. Seat of pants dyno may be worth the price, that's up to the owner. A good AEM dry-flow filter in the stock housing is probably the smartest thing you can do here.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    The factory air filter and intake already flow more than the engine can use. Aftermarket intakes are louder which only adds power to the placebo effect. The only time you get any gain is when you upgrade the heads, cam, bore, stroke, etc at which time the factory intake becomes the choke point.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Keep in mind that not only is the factory intake system already breathing "cold" air, but it is also routed over the PCM under the air filter box to cool it. They must have done that for a reason, don't ya think.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    So what about moding the map sensor. Or is that cut out when the engien goes in to closed loop?

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    The MAP is always used by the ecm. The engine wont even run without it. Its output varies constantly with engine vacuum so adding a resistor would just send one value to the ECM. It just wouldnt work.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Yeah, that ^^^

    Adding a resistor either in series or parallel with the MAP output skews the entire output range so that the engine always "thinks" it is running with more or less load than base calibration. You can fool with it, but you run the very real risk of holing a piston from lean mixtures at or near WOT unless you own some very sophisticated fuel curve mapping electronics.

    MAP (Manifold Absolute Pressure) is one measurement of manifold vacuum (engine load) and is calculated as the inverse of atmospheric pressure: Atmospheric pressure at sea level is about 14.7 psi or 30" of mercury (close to a 2:1 ratio), so a MAP of 14.7 = 0" of vacuum (engine OFF or totally unrestricted intake ducting at WOT). With the engine running MAP decreases to a very low value at idle - about 3, so then manifold vacuum: 2 x 3 = 6; 30 - 6 = 24 (inches of vacuum at idle). These values vary as atmospheric pressure changes with weather patterns and mountain driving.

    For a supercharged engine there's another set of values calculated in to account for manifold positive pressure.

    So, basically, screw with MAP and your air/fuel ratios go all over the place, and you don't know which way.........

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    i was thinking some thing like this http://www.angelfire.com/my/fan/MAP_adjuster.html now i know its for a jeep but i thought it might work for any engien

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    That's an interesting idea and it would initially command more fuel but once in closed loop, the oxygen sensors will tell the ecm that the mixture is rich and the ecm will dial back the pulse width to bring the mix back to normal. I would suspect that the constant re-correction of the mixture would cause some lag and driveability problems. It's a redundant, self correcting system. All you would accomplish is pushing the mixture out of the range that the system can correct it thus causing more driveability issues.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Fuel mixture values are stored in tables in the ECM. As the vehicle is operated in each "cell" fuel trim data is calculated and stored.
    The cell is determined by load and sensor data.
    "Tricking" the ECM into thinking that something is not what it really is, well... is just stupid.

    The computer takes the inputs, processes them and monitors the outputs to adjust the table values. Sure changing the MAT or IAT will bump it into another cell but it is watching the O2's and will adjust accordingly. Making it think the air is cooler and more dense will only make the air fuel ratios get screwed up long enough for it to figure it out and adjust it.
    I am not sure how fooling the IAT input will reduce ping, okay maybe rich plays a part there are a lot more factors in ping than just rich/lean.

    Anyway, all of this is an exercize in futility if you ask me.
    Comparing anything modern to tweaking an 87 GN is like saying you did something on the Mayflower that you think might make a nuclear sub go faster. (okay that is an exaggeration but I think you get my point)

    PS, did anyone ever stop to think that powertrain engineers actually might know this stuff?
    (and therefore take it into consideration during design)

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    In addition to the above, the basic principle of fuel injection is that the driver's foot only controls airflow, and the computer adds the appropriate amount of fuel to match based on various sensor readings. The most basic part of the equation is matching fuel to air quantity, as determined by volume and density of the air in the combustion chamber. Volume is fixed, but density is a function of pressure and temperature (straight math) which are read from the MAT and MAP sensors directly. Messing with either the MAP or MAT will cause the air density calculations to change, which will result in the computer applying the wrong amount of fuel.

    If you change the volume of the combustion chamber (such as a stroker, or putting a 4.9 block under the 4.1 or 4.5 intake) then you can use an adjustable MAP to compensate for that--this is pretty common in the Jeep world where the 4.0 stocker is bored out to 4.7 or more. Another option is to use a variable fuel-pressure regulator to increase the injector pulse width, which is the more common approach in this forum.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Increasing the fuel pressure with an AFPR will actually cause the ECM to [i]shorten[/l] the pulse width because the o2's will see the increase in fuel. You can see this by watching the fuel integrator counts. When everything is stock the intergrator will read close to 128 + -. When you add more fuel by hacking something the integrator will show a lower value meaning that the ecm is taking time off of the pulse width to get the mixture back to where it wants it. The system can compensate for alot of adverse conditions. This is why we can put mustang injectors in our cars.

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Maybe you weren't paying attention but I was talking about adj MAP and FPR being two ways to deal with increased volume

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    Re: Fooling the Inlet Air Temp (IAT) sensor?

    Quote Originally Posted by ehall View Post
    to use a variable fuel-pressure regulator to increase the injector pulse width,
    As long as the o2's are in play(closed loop) the system will self correct up to a certain point. After that point you run rich. The volume your talking about is one of many values programmed into the prom. The ecm will always try to correct based on those values. If any value is physically changed from what the computer is programmed to expect then it no longer can maintain the right mixture. If you were to add volume to the combustion chamber then the values in the ecm need to change to reflect that. You cant do a whole lot with a speed density system because the volume of air going into the engine is calculated based on certain known(stock) values. A mass air system (MAF) actually measures this volume in real-time and CAN account for bolt ons.

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