IAT mod..problem

[Adam H.]

Hi all,
I tried the radio shack mode for the IAT sensor. My car threw a ses code. So, I took off the sensor and reset the computer by unplugging the battery.
Hooked it back into the lid. Now it is back to normal. Did I use the wrong resistor?

It is a 4.7 1/4 sensor. Is that right for a LS1?

Thanks,
Adam H. 2002 T/A Firehawk #536 M6
Magnaflow exhaust
SLP stb, 85mm Maf, Y-pipe
Texas-Speed 85mm Lid
PPC bellow
BMR lca's, tunnel brace
Sphon adj. Panhard Rod
Hotchkis sfc's
Edelbrock torque arm
Holley Powershot Filter
Msd wires w/NGK Tr55's gap at 0.55

[Jeff_in_Atl]

I have seen this topic on here several times but from what I have read, I'm not sure this mod is a good idea. Read this thread http://www.f-body.com/forum/showthre...ghlight=inlet.

The temperature sensor tells the computer the air temp so the computer can optimize the fuel amount being mixed with the air. I'm sure there will be some responses to this but here is my opinion on this. If you take out the sensor and replace it with a resister it will tell the computer the air temperture is a fixed amount. I'm not sure what the amount is but I think I have read it to be 57 degrees. Well if it is 80 degrees outside, the mod should trick the computer into adding more fuel to the mix. This might net a slight gain depending on the fuel curve programmed into the computer. With an LT1, I believe it is already set to pretty rich. OK now if it's 57 outside, there will be no change in performance. If it's 20 degrees outside, the mod is going to make the engine run lean. That's not good at all.

I think the propper way to change the fuel curve is to reprogam the computer.

[tabahr]

From what I understand the IAT does not control mixture. It controls the *rate* of timing advance. (Cooler=more advance faster) It does not control total advance, so WOT performance is not affected.

I've never heard of it on a LS1 car, but I dont see why it wouldn't work. Maybe the LS1 would require a different resistance?

I'd use a meter and measure its resistance at about 55* then go buy a resistor that value.

With the additional timing, you will need to run 92-93 octane.

[Joe 1320]

Originally posted by Jeff 95 Z28

The temperature sensor tells the computer the air temp so the computer can optimize the fuel amount being mixed with the air.

No..... the computer uses the temperature for the rate of timing advance, not fuel mixture. The MAF and 02 sensors are in the loop for fuel trim.

As far as the resistor, a 4.7K ohm should be the correct one, but I can't verify the effects in an LS1. Now as far as the code, if you disconnect the sensor with the engine running, you will set a code. If you switched the resistor for the sensor and then fired it up, there should have been no code.

[TraceZ]

Originally posted by tabahr
From what I understand the IAT does not control mixture. It controls the *rate* of timing advance. (Cooler=more advance faster) It does not control total advance, so WOT performance is not affected.

I've never heard of it on a LS1 car, but I dont see why it wouldn't work. Maybe the LS1 would require a different resistance?

I'd use a meter and measure its resistance at about 55* then go buy a resistor that value.

With the additional timing, you will need to run 92-93 octane.

oops... wrong login. Cookie should be right on this PC too now.

[Injuneer]

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.

[Jeff_in_Atl]

My mistake. I forgot the MAF does temperture on these cars. That being said, substitute the word timing for fuel amount in what I said. Wouldn't it still hold true?

"The temperature sensor tells the computer the air temp so the computer can optimize the fuel amount being mixed with the air. I'm sure there will be some responses to this but here is my opinion on this. If you take out the sensor and replace it with a resister it will tell the computer the air temperture is a fixed amount. I'm not sure what the amount is but I think I have read it to be 57 degrees. Well if it is 80 degrees outside, the mod should trick the computer into adding more fuel to the mix. This might net a slight gain depending on the fuel curve programmed into the computer. With an LT1, I believe it is already set to pretty rich. OK now if it's 57 outside, there will be no change in performance. If it's 20 degrees outside, the mod is going to make the engine run lean. That's not good at all."

My point is I could see adding a resister inline to the stock sensor to fool the computer but the sensor is there to compensate for the air temperture. Substituting a fixed value resistor for a sensor is going to make it better at one temperture and worse at another.

[Joe 1320]

Originally posted by Jeff 95 Z28
Substituting a fixed value resistor for a sensor is going to make it better at one temperture and worse at another.

You are correct. It will make cold air performance worse, warm air performance better. Actually this mod was only intended for the summer anyway where heat soak and hot ambient temps zap the throttle response. It is recommended that the sensor do it's job the rest of the time. Well, for here in Florida the rest of the time would be most of the time as it rarely dips below 57º anyway.

[fastTa]

Originally posted by Jeff 95 Z28
My mistake. I forgot the MAF does temperture on these cars. That being said, substitute the word timing for fuel amount in what I said. Wouldn't it still hold true?

"The temperature sensor tells the computer the air temp so the computer can optimize the fuel amount being mixed with the air. I'm sure there will be some responses to this but here is my opinion on this. If you take out the sensor and replace it with a resister it will tell the computer the air temperture is a fixed amount. I'm not sure what the amount is but I think I have read it to be 57 degrees. Well if it is 80 degrees outside, the mod should trick the computer into adding more fuel to the mix. This might net a slight gain depending on the fuel curve programmed into the computer. With an LT1, I believe it is already set to pretty rich. OK now if it's 57 outside, there will be no change in performance. If it's 20 degrees outside, the mod is going to make the engine run lean. That's not good at all."

My point is I could see adding a resister inline to the stock sensor to fool the computer but the sensor is there to compensate for the air temperture. Substituting a fixed value resistor for a sensor is going to make it better at one temperture and worse at another.

Jeff you are correct in assuming that the IAT sensor data does influence the fuel system. The IAT sensor is used by the PCM for several things:

Injector pulse width, ignition timing, rate of canister purge, and lastly idle speed.

I would have to agree with Joe and say that fooling the PCM in regards to engine management is usually ineffective. The LT1/LS1 PCM is not dumb and has many alterior methods of making calculations to adjust fuel and spark based on electrical resistance measurements from various sensors.

[Joe 1320]

This is the kind of mod that is a trial and error as to whether it will work on your car or not. I really wish folks would get the idea that this mod is meant to sharpen the throttle response for the first 1/3 of the pedal travel, create gains at part throttle and not meant to increase power at WOT.