MVP'S ZO6

has anyone ever integrated some sort of resistor into the IAT sensor in order to make the VCM think the IAT's are lower than they really are?

I heard of tuners doing this. Is there any benefits to it on a race car?

LTstewy8

Using the software to change the calibration is much easier and precise, if you have access to it. In HP Tuners: Engine Diagnostics->Airflow-> "IAT Sensor Curve" Table. I used it to limit the effects of a heatsoaking IAT sensor until I could get that fixed- well not fixed, but better.

Neil B

If you're going the trick the sensor, why not just change the tune (spark, etc.) at higher IATs and leave the sensor alone?

GoatKart

Its been done, but usually its done on speed density cars to help stabilize fueling when people don't want to ideal with the bias tables and their impact on fueling...

As far as FI cars, i haven't seen it there, unless again, they are strictly drag racing cars...

For autocross where your IAT can vary dramatically, i wouldn't think it would be a great idea. I would want to keep it as part of the feedback loop for timing and fueling.. Its much more likely that the IAT could get out of control and extreme...

breecher_7

How is that any different than setting the IAT table to pull timing at a higher temp?
I don't get it....

DOUG @ ECS

Because IAT's do more then just timing, so when you do not know the other corrections to make in the tune file, you fool it. Or you're the fool, or something like that.

breecher_7

Well I know I am a fool.... That is not news here....

MVP'S ZO6

So what if you tricked the sensor into thinking the "IAT" was 50 degrees but in reality they were 250 degrees or higher. That could be bad right?

DOUG @ ECS

So many variables to that question, but yes it could be bad, or it could do exactly what you wanted it to do.

GoatKart

I think the point Doug was getting at was... IAT isn't just a timing retard single vector sensor. Its used in basically all air mass calculations for determining fueling... So basically, the car can no longer correctly calculate the air density of the intake charge to fuel it proportionally..


If your only racing 1/4 mile at a time, this isn't a huge problem. People with Alpha N fueling, or SD tunes, fool the sensor sometimes, and lock things down.... However, if your driving it on the road, or racing on a track where this is a real variable. Its a poor idea.

DOUG @ ECS

Yup..

MVP'S ZO6

so where is the benefit fueling wise in a drag car to making the sensor report a false reading to the VCM Trying to understand! I mean if the sensor is reporting say a 50 or 60 degree IAT and THR actual IAT is 250 isn't this dangerous?

Isn't that exactly why IAT alone is not just used for cylinder air mass calcs to determine fueling.

Let's assume a Speed density car for all intents and purposes.

Neil B

I'd say the benefit is consistency when you're hot-lapping. The ECM is adding fuel and removing spark as IATs and ECTs go up so the car will slow down unless you tune around that. But if you've got a ragged-edge tune at 100 degrees IAT, I wouldn't want that tune if the engine was actually seeing 250 degrees IAT.

LTstewy8

I don't see a benefit other than as a a band-aid. In that particular scenario, you would be running richer by tricking the IAT, but not pulling any of the timing that you otherwise would pull at 250 degrees. I've heard of people adding a resistor to untuned cars to trick it into adding a few degrees extra timing, but that's just ridiculous shade tree tuning.

When I used it as a temporary band-aid to fix lean fueling on startup (while I worked on the cylinder charge bias table), I limited IATs to 115 in the IAT Sensor Curve table and interpolated the table to the default 100 degree setting. I then went into the IAT Spark Retard table to pull timing according to the new IAT sensor calibration. It worked as a bandaid on hot starts, but as soon as I fixed my lean startup, I put everything back the way it was. I should also mention that I never saw over 100 degrees while cruising or WOT, anyways, so it was kind of a moot point.

MVP'S ZO6

the band aid situation was exactly what I was thinking.

LTstewy8

It worked well. I wish I knew 100% exactly how the IAT sensor affects the PCM. For example, if I knew I could get away with limiting the sensor calibration to 115 (for fueling purposes) and properly scaling the IAT Spark Correction Table without throwing off anything else in the PCM, I would be tempted to go back to doing that.

Does anybody know how else the IAT sensor affects the PCM?

GoatKart

Consistency in fueling run to run is exactly why... It does a pure drag car zero good, to have a heat soaked iat sensor, that takes the first 1/8th mile to recover and deliver full timing and proper fueling... By the time the car is back to where it should be, the run is over, and they lost..

Simply, by using a variable resistance they can dial the IAT to anything they want... That's just a single reason, but an example...


Either way, for speed density, the fueling is a blended calculation between the intake air temps and engine coolant temps...

Basically, it assumes a certain amount of heat is picked up as the intake air charge passes through all the piping, throttle body, intake, and heads as it travels through the system. This is directly related to how fast the air flows.. The faster the air flows, the less time it has to pick up heat soak, and the more likely the IAT is an accurate reflection of the true charge temp...


Conversely, the slower the air flows, the more heat it will pick up, and the more it will be skewed towards the temperature of the engine.

What ever the PCM determines as the true temperature, is used to determine a charge density, and that charge density along with MAP to determine fueling requirements...



This is all wrapped up in the Cylinder Charge Temperature modeling, be it complex or simple. The charge bias table and the filter coefficient...


All of this in turn rolls itself up into the calculated torque, air mass, etc. Basically, its a large snowball..


This manifests itself in a VERY real way in daily driven speed density cars.... Specifically, when they get heat soak in the intake air temps, the car will swing its fueling trims significantly... There is thread after thread after thread about this single issue.... EFI Live compensates with a table which is AFR vs IAT added, to add or remove fueling as needed with intake temps... HPTuners doesn't have any real elegant way to do similar, but the tables are there none the less... With work, they can be slowly brought into line, however it is no easy chore at all..

LTstewy8

Right, it's more of a problem with OLSD. I could live with a small fueling error in CLSD for a few seconds on startup. Also, are you saying that EFI Live has a table for AFR vs IAT that affects the car in open loop all of the time? Because HP Tuners has the same AFR vs IAT adder table, but it only adds fueling when in PE.

So now all I've got is that the IAT sensor affects (1)fueling calcs, (2) IAT Spark Advance Correction table, and (3) Power Enrich Fuel Adder vs IAT table. Anything else I'm missing?

MVP'S ZO6

Why not just have an adequate intercooling system among other things?

LTstewy8

Because intercooling is not usually the problem. I think when he was referring to heatsoak, he meant the sensor, which picks up engine bay heat while sitting and then tends to read higher than the ACTUAL air temp. That is what throws off fueling. Lots of info out there about this particular problem. You could run no intercooler and have accurate fueling calcs, but as soon as the IAT sensor becomes heatsoaked and inaccurate, fueling gets thrown off.