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Originally Posted by ShadowWorks
I think some cars use EGR to try and lower temperature and reduce knock, what would you say is to hot on a typical car engine?
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It varies. I've never had the IAT in the 3.8L above 140*F but even at that temp mileage was already heading back down. The IAT is more than a foot away from the intake runners so the air is even hotter by the time it goes in. I've shot the headers on a Chevy 350 with a laser temp gun and they were at 200*F and there was no audible knock. The max is a matter of engine design but many forum posts indicate that 100*F is the optimum IAT for many vehicles.
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Originally Posted by dosco
OK. So your car is using temperature and pressure to somehow determine the mass of the air entering the engine.
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My many 4 cylinder cars had very few sensors which were sufficient to run the engine. The 88 Daytona Turbo with multi port FI had IAT, IAT-Manifold, CTS, Knock, MAP, Oxygen, TPS, and VSS. That's all it took to run that engine as well as any modern engine. With that few of sensors there is no cross checking so I had to check each one to ensure it was perfect. Of those sensors the only ones that can be used to calculate air flow are MAP and IAT and every car that was MAFless have had MAP and IAT. 4 years later the 92 Lumina Euro V6 3.1L had the exact same sensor set and ran very well.
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Next question...how is mass determined? Is there a standard atmosphere table in the computer, including a way to calculate density using temperature offsets? Or is there one base density number from which the mass is calculated?
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The 99 Dodge Dakota 4.9L V6 does not have a MAF sensor. Some scan tools will report a MAF reading and others will not knowing that it doesn't have one. However MAF is calculated, it is and with no more than the sensors listed though the Dakota doesn't have the neck twisting torque that the MAF engines do. A MAF provides the necessary data much faster than the calculation from the MAP+IAT.
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That wasn't what I was getting at. If your car has a sensor that can directly measure air density and temperature, in combination with the lambda sensor it should be able to very precisely control the fuel-air ratio.
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You're describing a 1985 Chevette with an oxygen sensor and a variable carb. I usually removed those and put the old carbs on because the system was so crude that it had poor driver response. You could push the pedal a long ways and not get anything more because your change was being compensated for by the feedback loop.
A lambda sensor is too slow so the only thing it regulates well is steady state. That's what the Chevette system was doing. Since rich is always acceptable the system can richen during state changes and lean out when the state steadies again. That runs fine but the emissions and efficiency are crap. All the rest of the sensors are needed to regulate when the state is changing before the slow but reliable lambda sensor can get the information out.
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It doesn't make sense to me that the intake air temperature would be the primary variable controlling the fuel metering. However you have made an interesting observation, and your vaporization hypothesis could be correct.
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MAF equipped cars always use the MAF as the primary sensor for air flow since it is the fastest and most accurate. I disconnected my MAF and the car wouldn't start on the first try. It did start on the second try but it was having trouble staying running, probably in limp home mode already. A MAP is totally unnecessary on a MAF car and many do not have one but it is probably there for finer tuning and cross checking as required by the OBD-II standard. IAT isn't being used for metering on a MAF car but it along with the CTS is being used to richen to make FI run well in the dead of winter.
Here vaporization is only affecting mileage, not what the sensors see.
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Originally Posted by R.I.D.E.
The computer reads
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Humidity
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I've not seen a humidity sensor on any car. The effect is so little and lasts for so long that the oxygen sensor can handle the difference through long term fuel trims.
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Throttle postion determines desired load applied.
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TPS is not used for mixture control other than at idle or WOT. Its only purpose is to provide instant driver response. When the driver stomps the pedal to the floor the computer learns it from the TPS first. Other sensors ramp up more slowly. I disconnected the TPS from the Daytona and all it did was to hesitate on acceleration, surge on slow down, and refuse to idle. The car in steady state ran no different. The TPS makes the car pleasant to drive.
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Originally Posted by dosco
Unless of course his ECU is using a less than optimal method to compute density.
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That's what the 4 bangers do to save money. The turbo was plenty peppy so apparently using MAP+IAT to calculate MAF works pretty well.
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The only other thing that makes sense is when I looked at a standard atmosphere calculator, where I compared the air density at 1000 ft versus a 100 degree F temp at sea level. Turns out that the air density at 1000 ft is much higher than sea level at 100 deg F.
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My ears have a different opinion when going up the elevator to the air conditioned 26th floor.