IAT 4.7 Ohm resistor trick

[dochidalgo]

These couple of weeks the weather temps have been between 100-102ºF.
Today at 8 am central time the IAT showed 98ºF!!!! in my scanner...at start engine.
I was thinking about to connect the IAT resistor, but does that affect anything in my engine, besides the more fuel and snappier throtle response?
Thanks a lot for your imput
dochidalgo

[Joe 1320]

These couple of weeks the weather temps have been between 100-102ºF.
Today at 8 am central time the IAT showed 98ºF!!!! in my scanner...at start engine.
I was thinking about to connect the IAT resistor, but does that affect anything in my engine, besides the more fuel and snappier throtle response?
Thanks a lot for your imput
dochidalgo

Doc, on your particular year it doesn't alter fuel. All it does is alter the rate of timing advance. By adding the 4.7K ohm resistor, in effect the computer thinks the intake air temperature is 57º. It then allows the miximum rate of timing advance, it doesn't affect total timing nor does it alter the fuel mixture. This is why the throttle response is better..... the IAT sensor doesn't become heat soaked and therefor the computer doesn't retard the timing advance. Technically, you aren't adding anything, you are negating the timing loss from a heat soaked sensor. The only time I take out the resistor and hook up the snesor is if the ambient temperature gets below 57°. then I let the sensor do it's thing.

Before anyone jumps in and says that the resistor mod doesn't do anything, carefully read what the above description says..... it simply negates the effect of a heat soaked sensor. Part throttle performance is enhanced, throttle response is sharper and it does nothing for wide open throttle performance or horsepower. However, your part throttle will be sharper.

[fastTa]

The IAT data is used for many things, and timing advance is one of them....but so is total timing....so are fuel calculations. In fact there are a bunch of PCM tables and/or calculations that are affected by IAT input.

This is why it is important to not attempt to trick the PCM into thinking that the air temperature coming into the intake is a consistent temperature.

Even though IAT input is used in timing advance calculations, it is a secondary value in the equation.

The most important and primary table used in controlling timing advance is the Load vs. Advance. RPM, MAP, and TPS translate into load then the advance is calculated. The only time I have seen the IAT referenced when the PCm is determining advance, is when something is not normal or the reported IAT temps are extreme.

The IAT also works in conjunction with the MAF to not only determine airflow, but air density as well. Since the IAT is a thermistor, it can be referenced by means of a lookup chart to make an educated guess as to what the incoming air density is since temperature has a direct effect on air density. Lots of people focus on airflow(and it is important), but what is really important is the volume of air entering the motor. This is estimated by the MAF and IAT working as a team.

Therfore, things like IAT Scale vs. Air Flow, knock retard attack and decay, and fueling corrections are all affected by IAT input.

It is very important that the PCM sees real time data from the IAT. When the PCM does not see real time data from the IAT, all kinds of tables can be adversely affected however slight it may be.

It is better to just relocate the IAT sensor to a place where it is not as prone to heat soak. Then the PCM will still have real time IAT data, but the readings will be accurate and not be falsely high due to heat soak.

[Rob B (shoebox)]

I tried/tested the IAT trick years ago. It made no difference that I could tell by the SOTP.

[Joe 1320]

I'm curious as to where you get the data that supposrts the IAT being tied in with fuel. It is the case on speed density, but I haven't seen that on MAF cars. You can plug in a variable resistor and go throught the entire range and not see a blip in any of the fuel trims on a MAF system. I'm sure that some manufacturers have their systems operating differently, but I want to see those tables.


Fred?

[fastTa]

I'm curious as to where you get the data that supposrts the IAT being tied in with fuel. It is the case on speed density, but I haven't seen that on MAF cars. You can plug in a variable resistor and go throught the entire range and not see a blip in any of the fuel trims on a MAF system. I'm sure that some manufacturers have their systems operating differently, but I want to see those tables.


Fred?

Get yourself a program such Tunercat or LT1 Edit and start doing some tweaking and you will understand what I explained above.

All late model GM vehicles use what is called an open element IAT sensor. We have done numerous experiments with the IAT sensor and it's effect on the different tables within such software.

Like I said, in order for the PCM to accurately control the a/f ratio it has to know how much air is flowing in and an approximation how dense that air is. This is done by measuring the temperture of the air being inducted into the motor by means of the IAT sensor....meaning it has a significant effect on fuel calculations.

The IAT resistor mod has never shown to give us any performance increase on any GM vehicle that we have experimented with.

[Joe 1320]

Get yourself a program such Tunercat or LT1 Edit and start doing some tweaking and you will understand what I explained above.

All late model GM vehicles use what is called an open element IAT sensor. We have done numerous experiments with the IAT sensor and it's effect on the different tables within such software.

Like I said, in order for the PCM to accurately control the a/f ratio it has to know how much air is flowing in and an approximation how dense that air is. This is done by measuring the temperture of the air being inducted into the motor by means of the IAT sensor....meaning it has a significant effect on fuel calculations.

The IAT resistor mod has never shown to give us any performance increase on any GM vehicle that we have experimented with.

That's where I disagree. The air flowing through the MAF with it's heated element and the TPS singnal is what the computer uses to calculate the quantity and quality of the air for the purpose of fueling. Hence, dinconnecting the MAF moves it into speed density mode. Disconnecting the IAT sensor entirely doesn't do that.

[fastTa]

That's where I disagree. The air flowing through the MAF with it's heated element and the TPS singnal is what the computer uses to calculate the quantity and quality of the air for the purpose of fueling. Hence, dinconnecting the MAF moves it into speed density mode. Disconnecting the IAT sensor entirely doesn't do that.

Explain to me how the PCM obtains a measured quantity of air without knowing the air's density based upon it's temperature?

Our LT1's do not have a cold wire feature like some MAF sensors do to report ambient inlet air temps. The LT1 relies on the IAT sensor for that correction. The LT1 MAF sensor has no way of knowing by itself the quantity or volume of air. It only measures flow. Flow by iteslf doesn't equate to volume. You must also know density. This is where the IAT comes in. They work as a team.

What do you think is used as the correction factor in the Volumetric Efficiency tables? IAT data.

[Joe 1320]

Explain to me how the PCM obtains a measured quantity of air without knowing the air's density based upon it's temperature?

Our LT1's do not have a cold wire feature like some MAF sensors do to report ambient inlet air temps. The LT1 relies on the IAT sensor for that correction. The LT1 MAF sensor has no way of knowing by itself the quantity or volume of air. It only measures flow. Flow by iteslf doesn't equate to volume. You must also know density. This is where the IAT comes in. They work as a team.

What do you think is used as the correction factor in the Volumetric Efficiency tables? IAT data.

Then please explain how a MAF works.

[Joe 1320]

I pulled this from one of Fred's old posts.

This is one of those mods that gets mixed reviews.... and its probably one of the few areas where Joe and I don't agree. First, the 4.7Kohms came from the 3rd Gen MPFI speed-density engines. They benefitted from running a little richer, and this helped.

The 4.7Kohms was carried over to the 4th Gen/SFI LT1 MAF enines, and has become "gospel" and I don't really know why. The guys that developed LT1_Edit used a stock PCM in a simulator. They provided various sensor inputs, and tried to measure the effects on PCM outputs. They were trying to identify relationships controlled by the PCM in order to determine what tables and programming fields controlled those relationships. Although they varied the IAT input widely, they could not find any sigificant output variations. I have also seen a post many years ago, by a group.... maybe it was "Georgia F-Bodys", who did a group dyno day, and every one of the people who tried the "resistor trick" lost HP.

I have played around with the IAT, and I do feel that there is timing pulled when the temperature goes over maybe 120-130degF. There is an elusive 3-deg of timing advance in the PCM, in addition to the tabular/programmed values, the people could not find. I found that simply by relocating the IAT sensor away from the heat soak of the engine, the rough idle, lethargic performance that I had when the IAT sensor in the stock location heat soaked to 140degF in heavy traffic, would disappear.

I suspect there are some timing impacts tied to the IAT, but I believe that the relationships are defined only for very high inlet air temperatures. I know that with my aftermarket MoTeC engine management system, we have timing offsets for high inlet air temps and high coolant temps. Its mandatory, because those are the factors that lead to detonation.

Now it appears we are exptrapolating a temperature based mod that was "invented" for the 3rd Gens, and their SBC 1, speed-density systems, and applying it to the SBC 3 LS1 family of engines. Does anyone have the slightest idea how the LS1 PCM programming compares to the LT1 or even the old MPFI SBC 1 programming? I know I don't.

I would agree that it is not likely that the LS1 PCM would use the IAT sensor for fuel management. That function is taken care of internal to the MAF sensor, where it measures the inlet air temp, and heats the wires to a fixed number of degrees above the inlet air temp, then calculates mass air flow based on the specific heat of air, and the power consumption required to heat the wires.

I'm not a big fan of "fooling the PCM". Think about what happens if you get really high inlet air temps, and the PCM does not pull the required timing. You get knock, and the PCM has to pull timing using the knock response. That's the last thing you want, because it typically pulls much more timing than it needs to, then puts it back in very slowly until it gets back to the condition that produced the knock in the first place... you lose more timing, and you lose it for a longer period. A lot to think about.

that goes with what I was pointing out, changing the IAT resistor values did nothing to the fuel mixture. I tried! Nada...... nothing....... however, it did pick up some additional timing at low throttle inputs.

[fastTa]

Then please explain how a MAF works.

The MAF sensor on our LT1's is a hot wire positive temperature coefficient(PTC) type resistor.

When the MAF is powered up, a constant voltage known to the PCM is applied to the heated elements. These film-coated elements are obviously positioned within the air stream of the inside of the MAF housing. The elements are heated by the electrical current that is flowing across it supplied by the hot feed. As air begins to flow over the heating elements, the heating elements are cooled thus requiring the PCM to apply a specific variation of voltage to maintain the constant temperature.

The more the air flowing across the heating elements, the relatively cooler they get and the less resistance that exists between the elements.

This measure of potential difference is called Ohm's Law

V= I*R

V is the potential difference
I is the current
R is the constant

This measure of PD is then converted into a frequency and sent to the PCM. The PCM looks up the respective air flow value from the MAF table. This table tells the PCM what each frequency reading corresponds to with respect to flow rate in GPS.

Get the IAT over 130 deg F and you can bet a wideband will show lean. I know because we have proved it on multiple cars.

Just the same if you were to place a potentiometer strategically within the MAF circuitry to where you could skew the measured resistance across the elements, you would see the outcome of the a/f respectfully altered.

The PCM depends on the IAT, BARO, and MAP to alter any tables related to altering a/f…..especially the 3 main VE tables

I've spent literally hundreds of hours getting VE tables, timing tables, and so forth dialed in and let me tell you, after a while some of this junk actually sinks in.

[Joe 1320]

The PCM depends on the IAT, BARO, and MAP to alter any tables related to altering a/f…..especially the 3 main VE tables

I've spent literally hundreds of hours getting VE tables, timing tables, and so forth dialed in and let me tell you, after a while some of this junk actually sinks in.

Great so far, now explain how the disconnection of the IAT or the inclusion of a resistor to alter what the computer sees has not shown to be any impact on fuel mixture. I can hook up a resistor box to the IAC harness, toggle though every range and nothing appears on a scanner except timing increases and decreases at part throttle.

I hear what you've been studying, I am questioning either an interpretation or a misconception somewhere. If the IAT were indeed tied to fuel trim, then one would be able to alter it via that same IAT reading and that just doesn't appear to be case with these cars. As it was pointed out, the 3rd generation cars responded this way, obviously alot has changed in 20 years. Theory is great, but there needs to be experimentation to test the theory and documented results to prove the hypothesis. Without that, we're merely revving at the lights without ever putting it in gear. I do believe however that when readings in either get to an extreme range, there is a high probablity that the input from both would be used as a backup. Hence, 130 degrees would be considered an extreme temp. The whole idea of why heat soak of the sensor is bad, why people come up with relocation kits, etc. Even the WS6 with the IAT located far from the engine but still above the radiator is not immune. It's actually a good thing to keep it out of the extreme. I can't think of any place in this continent (maybe Death Valley on a real scorcher) where the ambient air temperature is over 130. There are times I guess where the heat reflecting off the pavement makes the lower air that hot, But at cruising speed I seriously doubt it would be that much of a factor. One thing I wish to really point out. Better to be a little rich, than a little too lean.



I guess if it walks like a duck and sounds like a duck, maybe......... Beyond that, I guess we can just agree to disagree.



Doc, getting back to your question, The only times I decided to change the resistor back to stock was in two conditions. One, when the temps were going to be cooler than 57 degrees F. The other instance is when temps were really hot. The reason is because the extra timing was a concern of mine in regards to part throttle spark knock. As was pointed out in Fred's post, the retard was a little extra insurance. I found as well that when the ambient temps got really hot, there was a tendency to get that part throttle knock depending on fuel quality. You definately can't even think about running lower than 93 octane in those instances. So if it is getting hot and fuel quality is a concern, put 'er back to stock. Good octane though and no worries.

[fastTa]

I hear what you've been studying, I am questioning either an interpretation or a misconception somewhere. If the IAT were indeed tied to fuel trim, then one would be able to alter it via that same IAT reading and that just doesn't appear to be case with these cars. As it was pointed out, the 3rd generation cars responded this way, obviously alot has changed in 20 years. Theory is great, but there needs to be experimentation to test the theory and documented results to prove the hypothesis. Without that, we're merely revving at the lights without ever putting it in gear.

Scanners are great but they don't come close to telling the big picture. They show 10 to maybe 20 frames per second of what is the sensors and PCM output. In reality O2 sensors can switch over 1000 times per second and Cell changes, table correction factors, and algorithm selction can also occur 100's of times per second. The only way to see what is actually going on is to have the car on a high resolution wide band and tuning software that actually allows to see what's going on behind the scenes.

I am not proposing to you some speculation or theory on my part, I am telling you how the IAT and MAF work.

Joe, let me say this as a friend and as respectfully as possible and give you a few pointers. Because quite honestly Joe, it is apparent that you don't have much experience with the actual tuning and/or manipulation of the tables within the PCM. Therefore it is disrepectful when you question work that has been proven by tuners across the nation and myself for along time now.

Don't believe me? Feel free to call any professionally experienced dyno tuner and ask. It's OK to learn Joe. Even I am still learning. We all are.

You haven't answered a single question that I asked you. Instead you have just thrown back questions in my face avoiding having to answer my questions. So I'll ask you one more time.

Explain how the MAF sensor, all by itself, determines the precise volume of air entering the intake manifold without knowing IAT or BARO?

[Joe 1320]

Scanners are great but they don't come close to telling the big picture. They show 10 to maybe 20 frames per second of what is the sensors and PCM output. In reality O2 sensors can switch over 1000 times per second and Cell changes, table correction factors, and algorithm selction can also occur 100's of times per second. The only way to see what is actually going on is to have the car on a high resolution wide band and tuning software that actually allows to see what's going on behind the scenes.

I am not proposing to you some speculation or theory on my part, I am telling you how the IAT and MAF work.

Joe, let me say this as a friend and as respectfully as possible and give you a few pointers. Because quite honestly Joe, it is apparent that you don't have much experience with the actual tuning and/or manipulation of the tables within the PCM. Therefore it is disrepectful when you question work that has been proven by tuners across the nation and myself for along time now.

Don't believe me? Feel free to call any professionally experienced dyno tuner and ask. It's OK to learn Joe. Even I am still learning. We all are.

You haven't answered a single question that I asked you. Instead you have just thrown back questions in my face avoiding having to answer my questions. So I'll ask you one more time.

Explain how the MAF sensor, all by itself, determines the precise volume of air entering the intake manifold without knowing IAT or BARO?

When one relinquishes his monopoly on wisdom, that's when he finds more truths. I asked a few questions, you answered them, but you don't like more questions? Not once did I make this personal, I'm the one that said let's just agree to disagree, now you're talking about disrespecting people? That is out of line and uncalled for. Period. And with that, this thread is locked. This has not turned into a healthy debate.............