Slightly different IAT mod - Page 2 - Fuelly Forums

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Old 11-23-2007, 08:36 AM   #11
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Originally Posted by froggy81500 View Post

Some of the data I'm looking for are this. What is the typical normal operating temperature (engine warmed up) of both an unmodified air intake, and a warm/hot air intake? This is the baseline I need to know to work this from, the starting point. Also, what are the desired "modified" air temperature readings for optimal economy? I've seen 242f out there but issues over that I've also read about.
On my Saturn in stock configuration the IAT sensor was right in the fresh air inlet so it read virtually ambient temperature, maybe 5 degrees over. For best economy I like to see arount 180-200 degrees F. Sure I suffer a large power loss but my mileage peaks in that range. Around 220 F the mileage starts dropping. I know these are higher numbers than most recommend for actaul IAT but I have over 6 months experience with this and have done pretty well.
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Old 11-23-2007, 08:40 AM   #12
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On my Saturn in stock configuration the IAT sensor was right in the fresh air inlet so it read virtually ambient temperature, maybe 5 degrees over. For best economy I like to see arount 180-200 degrees F. Sure I suffer a large power loss but my mileage peaks in that range. Around 220 F the mileage starts dropping. I know these are higher numbers than most recommend for actaul IAT but I have over 6 months experience with this and have done pretty well.
ambient temps, meaning the outside temp. Wintertime, depending on where you are, will considerably drop that. That will make it a little harder to reach 180-200 when its like 10F outside.
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Old 11-24-2007, 01:13 PM   #13
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I few weeks ago I replaced my 4Runners IAT thermistor with a variable resistor.
After adjusting it too read all over the usable range I can say it made absolutely no noticeable difference for me while running in cool air. Still would get about 2mpg lower FE while its 50f compared to 75f outside.
My A/F sensor goes into closed loop in a matter of seconds and that takes over mixture settings so the initial F/A settings coming from the IAT are apparently rather pointless on my truck.

The most recent test I have done is to totally block off the upper grill but left the lower grill open. Also blocked off any holes that allow outside air into the engine bay so the only air getting in from the front is through the radiator. This has helped increase the IAT temps by 20-30f but also really raised the radiator water outlet temp from 90f to 120f when its in the 50's outside. Somewhere here a difference was made in that I can get the normal summer FE when its 50f now after some warm up time. Having the radiator run warmer also increases the transmission temp a bit... So whats really helping the FE? Likely a combination of affects.

Think I need some automated grill louvers to regulate all air intake flow to control the radiator outlet water temp.

Lastly its obvious but your tire pressures drop quite a bit when its cold so thats something to check.
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Old 11-24-2007, 02:52 PM   #14
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Think I need some automated grill louvers to regulate all air intake flow to control the radiator outlet water temp.

Lastly its obvious but your tire pressures drop quite a bit when its cold so thats something to check.
The louver thing was something I thought about a few years back. I was trying to think of something that could actually fit in the front end of a car. coming up with some kind of opening and closing mechanism could be a matter of a manual cable, like a manual choke kit for a carb, or something more automatic, like a bi-metal spring that would open it upon heating up. Never got farther than working the idea in my head though.

Yes, I believe the general rule for every 10F drop in temperature, a tire will lose 1 psi of air. I try to keep on top of that.

Yesterday I was looking at preheat ducting over at Advance Auto, but I haven't had a chance to get a better look under the hood again. Even though I know plumbing hot air right into the filter box would be ideal, I don't want to start hacking things up in this car. What I have been contemplating though was to try and run some ducting from the exhaust manifold around the front of the timing cover and underneath the intake ducting to heat it up. I don't know, however, how I will fasten it to the exhaust manifold though because I haven't been able to see far enough back there. and I also don't know how effective this will be in bringing heat from the firewal side of the motor to the front. I have to keep looking in the front corner to figure out where the air box pulls its air supply from and see what I could do there.
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Old 06-21-2008, 07:05 AM   #15
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cheapybob let me know over at saturnfans he tried this and it has worked out. He used a variable resistor (instead of a fixed resistor) in series to one of the IAT's to tweak the output slightly. I am reviving this thread so hopefully he can jump in and share some data and what not.
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Old 06-21-2008, 08:25 AM   #16
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After my first post and thread, here's the idea I was discussing with cheapybob.

I see a lot of ideas about modding the IAT signal with resistors to obtain a certain reading equivalent to a desired temperature to get a leaner mixture. The problem with using a resistor in parallel is that it doesn't change its value and will affect the IAT negatively in some temperature ranges, like the cold temperatures.

I studied electrical engineering for a few years. Adding resistance to a circuit is easy, just putting resistors in series and they add up. Lowering resistance is a little trickier. I played around with the parallel resistance calculations and came up with this idea.

Why not use 2 IAT's in parallel? Yes that is what you are reading. 2 IAT's that are resistors that will both be changine values at the same time. Effectively, what happens when two resistors in parallel of the same resistance effectively total half of that value. I did some calculating and compared them with the corresponding temperatures for given resistance.

this is what I found out. At 0F, the mid range of the IAT is 24K ohms. two of them would divide that in half to around 12K. That falls in the range for +20F. Effectively you are bumping up your IAT reading by 20 degrees.

Now at the high end. At 140F temperatures, the IAT mid range is 690 ohms. Divide that in half and you have 345 ohms, which actually falls between 160 and 180F, call it 170 ish.

One thing cheapybob alluded to was cold start drivability issues when using a resistor in the IAT. I think this dual IAT mod will correct that and maybe even work better than a resistor.

I also did some tweaking of the idea by placing a resistor in series with one of the IAT's to do some further adjusting of the ending resistances at several different temperature ranges.

Anybody care to chime in and ask questions or share ideas, please do. Also, what I'm really looking for is what temperature reading you guys try to obtain in order to maximize your fuel economy. If I know that, I can further tweak these numbers and give you possibly a better way to do it.
The cold start problem was if you tried to tell it that the air was 246f when it was only 40f. It was dumb to even think that would work because it was stretching things way to far.

Anyway, it was good to see your theory on the resistance works. It will take a while to see if I get better MPG by running it slightly leaner than stock. I think I will, but i don't think i can rely on the scangage to tell the truth on it till its used a full tank of fuel and been recalibrated for actual consumption.
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Old 06-21-2008, 08:29 AM   #17
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I look forward to positive results.
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Old 08-23-2008, 05:10 PM   #18
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IAT air density correction

In MAP based systems the IAT can be used to estimate air density. The Ideal Gas Law states PV = nRT, where P = Pressure, V = Volume, n = moles of gas, R = gas constant, and T is temperature in Kelvin. Kelvin temperature is degrees Centigrade plus 273.15. Here is an example. If the intake temperature is 26.85C for nominal conditions (low for real example) then T = 300K. If the IAT circuit is modified to indicate a higher temperature (10C) than real temperature, the ECU "may" try to reduce the fuel amount by 300/310 or 0.968 for a reduction of approximately 3.2%. Just because the fuel is reduced by a percentage, the engine response may not improve MPG by that amount. There are also other ECU actions that may occur in engine timing, or setting of error codes. The O2 feed back may also adjust fuel back.
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