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03-28-2008
Ok so we decided to make a new thread about this.
We will be discussing how your IAT ( intake air temperature ) sensor may be hurting your performance due to it's response time/refresh rate.
The following posts are being taken from the previouse post and added to this post to try to get everybody back on track....And sorry if nobody like the way i did this...feel free to fix it or ask me to if needed. thanks
Last edited by TDCRacing; 03-28-2008 at 02:54 PM.
03-28-2008
...Originally Posted by Reptile
Well, and here's another thing to consider. We need better IAT sensors. Not only for accuracy, but do you guys know what the normal refresh rate is? It's anywhere from 7-10 seconds. Voilant (sp?) sells an IAT sensor for the GTO's, and it's extremely expensive, but it's got an almost instant refresh rate, meaning that it's pretty much real time. Think about it. IF you're sitting in the staging lanes and everybody here is going to have heat soak issues. Once you step on that throttle, you're not getting 120* air into the engine, it's considerablly cooler.....Now if it takes 10 seconds to read the correct air, you're almost done, and you could possibly have made up .3 seconds in your quarter time if you could read better. At this point I honestly don't think it matters what type of filter you have if things aren't reading right.
Am I making sense? Maybe not.
Last edited by TDCRacing; 03-28-2008 at 02:58 PM.
03-28-2008
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Last edited by TDCRacing; 03-28-2008 at 02:59 PM.
03-28-2008
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Last edited by TDCRacing; 03-28-2008 at 03:03 PM.
03-28-2008
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03-28-2008
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Last edited by TDCRacing; 03-28-2008 at 03:01 PM.
03-28-2008
I have trouble believing the PCM wouldn't be able to keep up with a fast changing sensor. I think the PCM is unbelievably fast.
I agree that we should have a faster acting IAT but...
It is still reading the air temp before it is compressed and heated by the supercharger (assuming you're boosted). How do we account for that?
Then to make matters even more complicated, say you are running an intercooler. Now wtf is the air temp the pcm should use to make the correct calculations?
I'm not a tuning whiz as far as knowing what tables we have but I'm working on it. However I'm wondering if we should zero out any air temp adjustments in our track tune except for very hot temps and scan for KR.
What do we care how fast our IAT reacts if the calculations it is used in are bogus becasue they are just guessing what is really going into the cylinder?
03-28-2008
The IAT is simply an analog thermal resistor. The IAT sampling rate is SOLELY a function of the input on the PCM. If you want to increase the sensitivity of the IAT, you will need an IAT pellet with a smaller thermal mass that can react quicker with the changing air temps. All said, I don't believe that the incoming air changes THAT fast...
03-28-2008
basically it is just a baseline isn't it? As far as the pcm keeping up, it is programed for a certain time to take a reading so without actually changing that parameter (if even possible) it wouldn't request new readings
03-28-2008
There are tables in the PCM that calculate AFR and Power Enrichment due to intake air temperature.
The calculations between the IAT and the MAF along with the O2 sensor all three coincide with adjusting for the best fuel mixture. This is partially why a VE tune can be so productive, because you're basically helping the computer learn the density of the air, and then the MAF and IAT help it adjust on the fly.
Believe it or not, the PCM has a pretty decent idea on how much air is injested in the engine...but it takes 3 things to help it along.....the MAF is instant, the O2 readings are instant.....Wouldn't you think it would make sense for the IAT to be instant as well?
03-28-2008
Yes, and if you ever look, the PCM does not start pulling timing until IATs reach 122* with a Mg/cylinder pressure of 480 and up. THAT by itself is only -1.01* of timing. At 140* it goes to 1.49* of timing. I forsee only 2 scenarios for this.
1: Your in the staging lanes WAY too long with your engine running. Turn it off.
2: Its 95*+ outside. Dont expect to cut a PB anyway even if you do have all of your timing avaliable to you.
On a nice spring/fall day, this should not matter.
03-29-2008
why do ya'll think the thermisters in the MAF are so small and delicate?
the IAT in the LQ4 maf reacted way faster than the OE unit on the whippled Riv
03-28-2008
This thread makes me glad I use a double walled 4" FWI and don't see more than a few * above ambient in traffic.
03-28-2008
the problem with the IAT is it's Thermal Time Constant. essentially how fast it reads. most GM IAT's are at least 15 seconds TTC. here's a bit of background on the Thermal Time Constant--
Thermal Time Constant (T.C.):
When a thermistor is being used to monitor the temperature of it’s environment then the accuracy of measurement of the resistance of the thermistor is critical.
While the power dissipated in the thermistor is an important factor in this measurement as discussed in the previous section, the thermal characteristics of the system and the thermistor are important also. This is especially relevant in systems where the temperature is changing with time. The dynamic thermal response of the thermistor must be considered in these situations. To quantify this dynamic response, the concept of a Thermal Time Constant (T.C.) is used in the thermistor industry and it is defined as follows:
The Thermal Time Constant for a thermistor is the time required for a thermistor to change its body temperature by 63.2% of a specific temperature span when the measurements are made under zero-power conditions in thermally stable environments.
This concept is illustrated in the example below:
Example: A thermistor is placed in an oil bath at 25°C and allowed to reach equilibrium temperature. The thermistor is then rapidly moved to an oil bath at 75°C.
The T.C. is the time required for the thermistor to reach 56.6°C (63.2% of the temperature span).
The dominant factors that affect the T.C. of a thermistor are:The mass and the thermal mass of the thermistor itself
The definition of Thermal Time Constant arises from the exponential nature of the rate of transfer of heat between the thermistor and the medium that it is monitoring. It is similar in principle to the definition of time constants in describing the responses of systems where physical effects have an exponential response with respect to time.
Last edited by Toasty; 03-28-2008 at 10:35 PM.
03-28-2008
Mr. Toasty, that is quite the block of information there and Im sure will help things along quite well.
03-28-2008
finding a thermistor with a lower TTC in a moving airstream might be difficult. most i see are measured in still air, not moving like in an air intake. and the ones i do see with specs for a moving airstream are still in the 15 second range, which we already have now.
03-30-2008
I'm suprised most of you still have IATs.
I say screw em get rid of them and use a happy knob (connect to your original IAT harness) and be able to adjust timing anytime anywhere. Any vendor out there can put in the proper tables, or if you have a tuner you can set it up yourself. Its a very nice mod, sure beat lugging a laptop around everywhere.
03-31-2008
IS the IAT really that slow? When scanning from a dead stop, my IAT's start dropping immediately and are ambient with in seconds.
03-31-2008
And I think that is the point, seconds. Precious seconds. Seconds while you are trying to make your way to the big end of the track. Normal everyday street driving, probably not a major issue, though the more accurate and responsive the sensors are, the better all the way around the engine will operate in general.
Since timing is tied to IAT's I would think any improvement in responsiveness is a good thing in just about any situation.
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