Quote:
Originally Posted by Racer_X
Well, the nature of karman vortex air flow meters is to measure based on the velocity of the air going through the meter. That is why the stock meter also has an intake air temperature sensor. The velocity of air flow through the calibrated meter is calculated to be a certain CFM value based on a normalized temperature, and the mass of the air is calculated by adjusting that normalized value for temperature with the actual IAT. THe ecu does this by injector duty cycle percentage. That is why it is really hard to keep the same tune on a really cold night vs. a really hot day if you have bigger injectors. The ecu is adding or subtracting fuel to try to calibrate for the air density change based on a downstream adjustment (downstream of the normal logic flow). Kind of like an afterthought. The EMU does create a karman vortex frequency based on MAP and RPM, and IAT is simply an option. This is exactly how the OEM system does it (although it is more accurate). You can get very close by saying air flow through x cross sectional area will be predictable given the engine RPM and manifold pressure, assuming intake IAT is constant. The "density" may change with temp, but the velocity will be the same.
Now, if you hold the temp the ecu sees as constant (with a resistor) then the ecu no longer is able to make the downstream injector percentage adjustment to compensate for air density changes, and you "have" to make that change with the EMU if you want the tune to stay the same at differnet inlet temperatures. This can be done much more accurately adjusting from scratch with the IAT fuel adjustment in the emu as opposed to doing the adjustment on top of the ecu's adjustment. Also, vehicle speed and intercooler flow surface area has a good bit to do with the density adjustment as well. This is because the charge temp is different at 100 mph at 15 psi at 6k RPM with 65 degree ambient temp than it is at 15 mph. That's why the EMU has vehicle speed based fuel adjustments as well. OF course, if you put the IAT in the manifold, you wouldn't need vehicle speed adjustments, but you would also need a way to produce an entire base fuel map at a static temperature.............................................not going to happen without an engine dyno.
To get a really basic tune, just use the map sensor. Trying to adjust the fuel trim using the IAT after the turbo brings in way to many variables to be worthwile. How do you know exactly "how much" to adjust when you have 100 degree temp swings in a single pull???????? Sure you can guess, but your guess is going to affect your entire map in a bad way and then you are trying to adjust one value with 2 variables. Doesn't work well.
If you put the IAT in the turbo inlet pipe, or near the filter, you can get a much better calibration, but it still won't be perfect. After you adjust for vehicle speed, then you will have a much better reading. That is assuming that the ecu isn't doing that as well..........that I am not sure of.
Irregardless of who is running what on whatever or why, this is how it works.
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Your post is correct and that is why OEM's choose a mas based system because they can measure air mass correctly. Air temperature and therefore density is a direct relationship and can be inputed into an air temp correction table.
It is more vital to measure air temperature going into the cylinders versus going into the turbocharger inlet (when you are trying to switch to a MAP based system and are ignoring the MAS). Compressor efficiency, intercooler efficiency are vital keys to the air volume/engine efficiency.
If your vehicle has a 100 degree temperature swing during a pull, you should re-evaulate your turbocharger choice. You are either:
1)Off the compressor map. This could be from flow demand of the engine, or boost pressure, but you are probably not on the island.
2)Intercooler efficiency. Simply put, your intercooler should keep up. Knowing that your IAT's rise 100 degrees during the pull is vital for the safety of the vehicle. As they rise 100 degrees, I guarantee they DO NOT drop 100 degrees when you shift gears and pull the next gear. Eventually you will end up with a circumstance where your IAT's are higher than you were calculating based off your inlet air temp to your mas.
OEM's know and understand this, but as they aren't pushing the factory turbochargers or intercoolers beyond that point, it isn't an issue.
You need to remember that most oem's have a +/- 3% tolerance for target versus actual AFR. Some tune tighter, but those are rare.
Velocity of the meter, therefore flow, is a theoridical maximum based on the orfice size during nominal settings (usually SAE), anything outside that boundary is indeed trimmed out, like you stated.
I still perfer and have tuned way to many vehicles to count to have the IAT as close to the valve inlet as possible. A MAS based system is awesome as it measure density easily. Once you know air mas, you just have to know engine VE, which is something that rarely changes unless you are dealing with a variable cam setup like MIVEC, VANOS, or VTEC, and just raw fuel flow.
The rest is math and understanding what a vehicle wants.
Trust your sensors, but use them wisely
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1993 Green 3000GT VR4 - Sold - Best ET 11.7 - Best Trap 122 - 475hp / 477tq (old 368sx setup) - *Some items listed not current, see for sale thread - Pauter rods/Ross pistons/High rev kit--springs and titanium retainers/Ported heads/Dual pump, dual feed fuel system with hotwire/Dejon intake/Scanmaster/550cc injectors/MSD 8.5 wires/HKS DLI/H&R springs/GAB adjustable struts/PST CFDS/EMU/SBC-iD/2 Greddy 60mm EGT with peak hold/Greddy 60mm Boost with peak hold/2 SPA dual digital gauges (oil pressure/temp, water temp, fuel pressure)/AEM WBO2/18x9 Bronze Pro N1s/265/35 Yoko AVS Sports/PPE FMIC with PPE HPC-coated pipes/Flexalite fans/Spec 3+ clutch/RPS flywheel/Tial BOV/DR 3.5" exhaust/Non-resistor copper plugs/Brembo F40 calipers and rotors/short shifter/poly motor mounts/rear control arms/Krank vents
1995 Red Dodge Stealth RT/TT - Sold - RPS Stage II Clutch
