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Old 05-05-2000, 09:00 AM   #1
rob
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Question Why does MY00 inake = rich?

Hi all. From all the posts I have read, it is pretty well known that any upgrade to the intake of a MY00 will cause the ECU incorrectly run the car rich. But I am having a hard time going through the logic why. Please follow through my thinking and point out where it is flawed.

For a MAP based system, the ECU reads the absolute pressure in the manifold to calculate how many oxygen molecules per time are entering the engine (mass flow). Now, to calculate this, the ECU needs to know a few things; atomospheric pressure, intake temerature, and volumetric efficiency. The values of the first two are taken from sensors, but VE is a characteristic constant (constant enough) of the 2.5SOHC engine. Any changes to VE is where the ECU will start injecting the wrong amount of fuel.

So say we change the intake setup to a high-flow cone filter. Essentially this makes it easier for engine to fill it's cylinders, which translates to an INCREASE in VE, right? Ok, so now for a given MAP value, the engine is now able to pull in more air than stock....wait, is this right? Let me do this with electric circuits...

NO, I'm wrong! Ok, so with a less restrictive intake, there is LESS air flow per pressure drop (V=IR hehe). So for the same MAP value, there will be LESS air, hence the rich condition. Bingo. I'd like to take this oppotunity to thank all of you for the help sorting this out

I will still post for your amusement. You know this should be a real easy fix. If somebody with an OBDII reader could do some datalogging of the pressure drop across the stock airfilter, then all you would have to do to add a real kick ass intake would be to build a voltage divider circuit that would alter the atm pressure sensor to compensate. Super simple.
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Old 05-05-2000, 10:29 AM   #2
8Complex

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Heh... thinking out loud can sometimes be a good thing.

Yeah looks like you're right about that, but now I wonder why Honduh's and other vehicles with MAP's don't run rich when adding an intake. Only seems like it'd make sense, right?

As for the OBDII reader and voltage divider circuit, go ahead, I have on clue what you're talking about but if it is simple and you can get a simple small circuit to fix it, go for it.
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Old 05-05-2000, 11:23 AM   #3
rob
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Ok, here is what I meant.

With the stock intake and OBDII scanner, record the values of atmospheric pressure and the MAP. This will tell you what the pressure drop is for the stock intake system (at least for idle).

Then put in your snazzy intake and do the same measurement. Now, the pressure drop will be less, and with an OBDII reader, you will know exactly how much.

Then you build a little electronic device that alters the signal from the atm pressure sensor to compensate for the difference in the pressure drop.

And bingo-bango, the ECU should be happy again!

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Old 05-05-2000, 11:28 AM   #4
8Complex

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Hmmmmm... too bad that the MAP isn't good enough to deal with it correctly in the first place. IMHO, if a car ran a EGT sensor and compared it to how it was running at all times, that'd be ideal. Keep the EGT's in place and it wouldn't have to worry about pressure drops or anything like that. Maybe just eliminate the MAP/MAF and just use a stock setting until running then have it recalibrate itself to the EGT's and O2 sensor levels. It would make for a car easily turbo-able. Or am I dreaming?
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Old 05-05-2000, 03:11 PM   #5
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Ok Rob, you've touch on what I do for a living, so I'll try and add a few things.

First off, pressure drop through the intake system (from the outside to the manifold) is not constant. It will vary with three things; throttle angle, flow rate, and with how dirty your filter is. I'm sure you already understand the throttle concept, so I won't talk about that. And an increase in pressure drop due to a dirty filter is also a simple concept. The one that seems to be missing from your reasoning is how it varies with the flow rate.

For this part of the discussion let's assume the throttle is in a fixed position and the filter gathers no dirt. Your pressure drop will increase with the flow rate. The curve for pressure drop versus flow rate is not linear. It will start off with a relatively low slope and get steeper and steeper as the flow rate increases(Part of my job is predicting this curve for different systems). For this reason, your idea of a "little electronic device that alters the signal" may be more complicated than you think. Such devices, like the Apex-i AFC, already exist.

How MAP based systems (Speed-Density as they are sometimes called) calculate the mass flow rate will depend on the particular implementation. As the name implies, there are to basic parts to the calculation. The Speed (volumetric flow rate) of the flow, and its Density. The manifold absolute pressure, manifold absolute air temperature, and the perfect gas law are typically used to get the density. The speed part is where I believe different manufacturers take different approaches.

The ideal volume flow rate is the RPM x displacement/2. Of course this is just the ideal, so this is where the volumetric efficiency that you mentioned comes in. Now for the stock intake system, the Manufacturer takes measurements during the cars development to determine the efficiencies. Thus the efficiencies are known and can be programmed into the ECU's memory (look-up table), but they still vary with RPM, throttle angle, and flow rate. Flow rate and throttle angle we have already touched on, that leaves RPM. Now we start getting into talk of intake tuning, which would make this long post much longer, so I'll skip it for now.

OK, this still leaves "Why do MY00s tend to run richer when intake mods are done". Well my guess is that the ECU may only be using RPM and the difference between atmospheric pressure and MAP readings (pressure drop) as input for finding the right volumetric efficiency in the look-up table. Say you just drop in a less restrictive panel filter, your pressure drop will go down through out the operating range. The higher the flow rate, the greater the reduction in the pressure drop. Which is all in good as far as increased flow goes, but I think the higher MAP reading (lower pressure drop) is causing the ECU to extract the wrong volumetric efficiency from the look-up table. In this case, a higher efficiency than actually exists, thus it calculates a higher mass flow rate than exists and you end up running rich. Mind you, this is just a guess on my part.

And don't forget, the above only applies to modes of operation where the ECU soley relies on the estimated mass flow rate number for proper fuel metering. In closed-loop operation, the O2 sensor readings are used to keep the A/F ratio around stoichiometry.

[This message has been edited by Scooter (edited May 08, 2000).]
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Old 05-05-2000, 05:13 PM   #6
larryganz
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I was also told that as we pull colder air into the system from the right inner fender, that the temp sensor will increase fuel delivery to match the predicted increased air density and you will get even richer...

Will a free flowing exhaust lean it out some?
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Old 05-08-2000, 04:50 AM   #7
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larryganz,
In your example, I don't think you'll run richer. Colder air equals denser air, so fuel delivery should increase just to maintain the correct A/F ratio.
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Old 05-08-2000, 08:18 AM   #8
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Hey Scooter, just an idea, but what if the atmospheric air pressure sensor (I have no clue what it is or where it is for that matter) was moved into a location behind the pass. side headlight so it got cooler air? That'd make it (consistently as far as I can tell) read higher outside pressures so it'd look the difference up in the correct spot on the fuel map chart. Is this a correct assumption?
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Old 05-08-2000, 09:01 AM   #9
boxerman
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Quick question? I am new to this EFI stuff. Where is the map pulling the air pressure signals from to compare pressure drop. Is it in the intake before the throttle body vs after the throttle body. Is it outside of the intake (atmosphere) vs after the throttle body. I am assuming that at least one of the sensons is in the manifold after the throttle body hence MAP (manaifold absolute pressure) but where does the ECU get the atmosphere reading from?

Tim
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Old 05-08-2000, 09:12 AM   #10
rao
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The MAP sensor (also includes a temp sensor in one unit) is just after the throttle body on top of the intake manifold on the 2000's.
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Old 05-08-2000, 11:23 AM   #11
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Tim,
I believe the atmospheric sensor is mounted on the front side of the front passenger side strut tower, but I could be wrong. I have an MY99 RS.

Some Speed-Density systems only have the one sensor, the MAP. The atmospheric pressure is read only when the ignition key is first turned, before the engine starts to crank. At which time the manifold pressure equals the atmospheric pressure (pressure drop equals zero when there is no flow). This obviously has it's drawbacks when your trip involves large changes in altitude.

[This message has been edited by Scooter (edited May 08, 2000).]
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Old 05-08-2000, 02:24 PM   #12
rob
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larry: there is an intake temp sensor that will tell the ECU about the cooler air, and it will compensate for that.

8complex: Thanks for the thourough explaination. My description was a little oversimplified. I understand that pressure drop would be dependant on a few things and would probably not be a linear curve over the different load/rpm range.

Here is what I was thinking. Using an OBDII data logger, you could crunch a bunch of data to get a fairly good pressure drop trendline. If you have this trendline for both stock and w/ other airfilter (along with A/F info) you could get a pretty good idea on what it would take to compensate for the new filter.

It wouldn't be too hard to come up with some simple electronics (voltage adder, Voltage control voltage source, etc) that would cut back on at least some of the extra rich portions of the fuel curve.

I understand that the S-AFC would solve this problem, but I think $300 is overkill.

Does that seem reasonable?
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Old 05-08-2000, 02:57 PM   #13
8Complex

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I think it's Apex's display that brings up the price (personally I like the display, but you're right about the price, still shouldn't be that high).

Also, what about moving that outside pressure sensor? Move it into somewhere cooler so it'd get denser air and the pressure drop would be more like stock (or is that backwards, should go to a warmer area?).
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Old 05-08-2000, 03:21 PM   #14
rob
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won't work. The engine bay isn't a sealed container so it will all be the same pressure regarless of temperature differences.
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Old 05-08-2000, 04:52 PM   #15
8Complex

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I beg to differ... I'm positive that the pressure is lower near the hood vents (when I have them open) then near the throttle body (with the large scoop closed). Air is probably drawn up from either side of, or in front of, the engine and out the vents so it is moving there constantly, but by the throttle body it never moves. And what about in the fender? Both cooler and in a different compartment there...
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Old 05-09-2000, 07:36 AM   #16
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Cool

I have my hood scoop open (the post on "intakes")on MY00 and a cone filter w/a simple bell at the throttle body. I did not reset the ECU or any other adjustment (I did add a borla header and cat back exhaust). There is more power available and I am getting 26 mpg on 370 mile trips (10K on the engine, 0w30 oil, 17x7.5" wheels - 215 40, w/average speeds at 80 and brief trips past the 120 mark...oh yeah, the trips were on one tank of gas each). If that is running rich, wait till I get my Tech 2...but I don't think so:-)
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Old 05-09-2000, 07:50 AM   #17
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How about fooling the temp senson into thinking it is gettin a charge that is a couple degrees warmer than it really is rtecieving. The temp sensor would incorrectly indicate a less dense charge and cut fuel accordingly. I would think it is easier to trick a temp sensor than a pressure sensor.

I think I located all the sensors last night (except 02 and RPM) there was one on the pass side strut tower (atmos) one just behind the TB (map/temp) one on top of the TB (throttle pos) of course the rpm senson probably pulls it signal from the flywheel?.

Tim
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