IAT Sensor position - Pre or Post S/C ?

[Lloyd_D]

I have a 1999 Mk 2 1.8 JRSC M45 air to air intercooled fitted with a Powercard Pro (latest HotSide Flash update).

Its running about 8psi intake manifold pressure.

It currently has the Standard IAT mounted in the silicone tube that goes to the cone air filter pre S/C. It seems to be drinking fuel at the moment and I'm rarely going into boost. Now i know the powercard does not add fuel off boost. So my question is should I try to fit the IAT sensor post S/C so it adjusts fuel to suit the warmer air going into the engine ?

What do other people with similar spec cars do ?

At the moment the off boost measured air temp is lower than what is actually going into the engine after its been warmed by the S/C, so I'm guessing its adding more fuel to suit when it doesn't need to..

Lloyd

[FormerDatsun510Man]

In a word, no :). The reason is that the stock ECU uses the IAT to determine the mass airflow. The mass airflow sensor by itself is not enough. The mass airflow sensor is really measuring the airflow rate (volume rate)... not the mass airflow. The mass airflow of course is influenced by the density of the air by the equation mass_airflow = density * airflow. To determine the density of the incoming air the stock ECU needs to now it's temperature. Thus, it needs to know the temperature of the air AT THE SAME PLACE AS THE AIRFLOW RATE IS MEASURED BY THE MAF. If you move the IAT downstream post SC where the temperature is much higher you will thus get erroneous calculations by the stock ECU to determine the mass airflow. You would probably get the correct reading if you also moved the MAF downstream of the SC, however, I don't think this would be a good idea for two reasons. One is the MAF probably would not fare too well at 200-250 deg F air. Secondly, the IAT would be likely at a reading out of range for what the stock ECU is expecting from the IAT (just a hunch there) and may end up going into some strange mode (such as fuel dumping to protect the engine).

So leave the IAT and MAF and the stock locations if you are still using the stock ECU. The reason for your engine running way rich must be one of the stock sensors being faulty, plugged cat, vacuum leak, etc. You need to do a step by step process of elimination to find the fault. First hunch I have is you have a bad primary O2 sensor, which would cause the engine to run rich. If that is the case, then you also probably have a bad cat, which would also need to be replaced (and would be currently causing further mpg losses). First thing first though. Hook up a scan tool (they do it for free at Autozone) and see if there are any codes. Fix any problems related to those first. Next, check the voltage of the primary O2 sensor with the engine running. It should fluctuate around .25-.75 v with the engine idling or at a steady cruise. It should go up to a steady .8-.95v when in boost.

Also check your coilpack. Pull the spark plug wires off and look for corrosion on the terminals. If there is you likely will need a new coil. Check the resistance of the terminals and also your plug wires. Also pull the spark plugs and inspect. Are they caked in black soot? Or are they light brown? Etc.

Hope this helps...

Bill

[Lloyd_D]

Ok I get what your saying.

I understand that the MAF will measure the same air that goes into the S/C that gets pumped out. The car has an efficient air to air intercooler fitted so intercooler oulet temps are fairly low compared to non intercooled.

I will probably leave it as is like you said, just thought it would be worth a discussion.

I know the aftermarket ECU guys fit the IAT sensor post intercooler close to the inlet manifold.

Thanks,

Lloyd

[FormerDatsun510Man]

Let me clarify this :), the IAT and MAF work together for the stock ECU to determine the mass airflow. If you seperate them you are not getting an accurate measurement because the stock ECU needs to determine the airflow rate and the temperature of the air at the same place to basically solve the equation PV=nRT. Having "T" at a different place invalidates the equation, even if it is close in the case of a post Air/Air IC. You really want to know the temperature of the INCOMING air for the stock ECU to calculate the fuel needs correctly.

The reason many standalones put the IAT downstream is because they have an ECU that will calculate it properly that way. You see, they all use Speed Density systems, which is actually an older system than the Mass Airflow method that the Mazda (and most other factory) ECU uses. For a speed density system to determine the mass airflow is a little different. It still is basically solving the equation PV=nRT. However, they measure the pressure in the manifold (P) and thus also need to measure the temperature (T) in the manifold. Note that both methods arrive at the same end result (mass airflow to determine amount of fuel to inject). The mass airflow systems generally are more adaptable for regular driving and emissions and is the reason they are used on all cars today from the factory. The speed density system works also and eliminates the need for a MAF, which can be slightly restrictive, however they aren't as good overall in having accurate fueling in all situations of normal driving. However, they certainly get the job done for performance. There are many factory ECUs now that use both systems. Perhaps there are some standalones that can do this also.

Bill

[Tom @ Fast Forward]

Make sure when you go to start the car that you turn the key to one for a few seconds before starting. This let's the card calibrate for atmospheric pressure. Otherwise, it may be in vacuum but think it is in boost and add fuel.

[Lloyd_D]

Ok people thanks for the advice I will be leaving it in the intake tubing by the air filter. The O2 sensors fine and I can't think of anything else so will just keep filling her up. Wish juice was as cheap as US ;)

Does the accelerator pump feature on the new Powercard Pro software add extra to standard fuel off boost during acceleration ?

Lloyd

[Tom @ Fast Forward]

Only for a second or two when you press hard on the pedal.

[PXC94]

The reason is that the stock ECU uses the IAT to determine the mass airflow.


On the flip side of this, I'm about to start tuning my car with a megasquirt and e-cool, given that I will be using MAP, should I relocate the IAT to the manifold post supercharger?

I had assumed that even though the IAT will not be correct using an sensor mounted pre supercharger, tuning to get the correct numbers out of the wideband O2 would make sure all was OK.

[FormerDatsun510Man]

I am by no means a Megasquirt expert, but I would think the IAT would be best in the manifold since it is using a MAP based system and you would want the temperature to be measured at the same location as the pressure to arrive at the correct air density. The amount of fuel needed for a given a/f ratio is directly proportional to the air density. While you can tune as you suggest with the IAT pre blower, and the IAT post blower should correlate (by a given ratio) for a given rpm/boost cell on the map, it probably would be less variable measuring the IAT post blower.