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Discussion Starter · #1 ·
Ok, lets say your fuel mixture is set so as to provide max power at 75F. Then lets say the temp drops 25F. To make more power do you lean it out because you can without running hot, or do you richen it because there is now more O2 in the chamber?
 

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You richen it to maintain 14.7 :1 ratio. I think thats what the o2 sensors do. it is the same in normally aspirated piston airplane engines as you gain altitude you lean engine because there is less o2. thats why you get better milage up high.
 

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The toothdoc is right, except at wide-open-throttle (WOT). The 02 sensors are one of the main inputs to the PCM anytime the fuel injection is in the "closed loop" mode, which is most of the time. Anything you try to do to "adjust" the mixture when in closed loop will be negated by the O2 sensor's detection of the improper mixture. The PCM will then adjust accordingly to maintain 14.7:1. At WOT the fuel injection is in the "open loop" mode and does not receive input from the 02 sensors. Instead it operates off of pre-programmed air/fuel ratio tables, using inputs received from the IAT sensor, ambient air temp., engine coolant temp., engine speed, throttle position, knock sensors, and MAP sensor. I cannot find anywhere that says what air/fuel ratio it tries to maintain. I do know that at the drags when I dial in my homemade WOT mixture control to the lean side by 20 degrees my E.T. went down two tenths of a second, so I am assuming the PCM is keeping it around 14.7 to 1
 

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magnuman said:
The toothdoc is right, except at wide-open-throttle (WOT). The 02 sensors are one of the main inputs to the PCM anytime the fuel injection is in the "closed loop" mode, which is most of the time. Anything you try to do to "adjust" the mixture when in closed loop will be negated by the O2 sensor's detection of the improper mixture. The PCM will then adjust accordingly to maintain 14.7:1. At WOT the fuel injection is in the "open loop" mode and does not receive input from the 02 sensors. Instead it operates off of pre-programmed air/fuel ratio tables, using inputs received from the IAT sensor, ambient air temp., engine coolant temp., engine speed, throttle position, knock sensors, and MAP sensor. I cannot find anywhere that says what air/fuel ratio it tries to maintain. I do know that at the drags when I dial in my homemade WOT mixture control to the lean side by 20 degrees my E.T. went down two tenths of a second, so I am assuming the PCM is keeping it around 14.7 to 1

I'm not trying to start something because I could be wrong on this; and I heard this second hand. but here goes:

This is correct that MOST engine management systems go into open loop at WOT (GM, Ford, older Chryslers) but I have been told that the new hemi's engine management does not go into open loop, even at WOT; this is one of the reasons that companies are having a hard time with superchargers on these engines; they can't get enough fuel under boost and are burning up pistons due to lean condition.

Perhaps this only applies to the 5.7 and not the 6.1; perhaps it's total B.S.; I was given this info by my brother who has a 5.7 hemi Ram 1500 and was thinking about supercharging it; this was what he found out in his research and from reading various car magazines.
 

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Discussion Starter · #5 ·
2 things: 1 I'm not referring to the 300, but to just an engine in general (in particular one with mechanical injection, but what I'm trying to get at is the basic physics of it).

Another question: magnumman, when you said 20 degrees to the lean side (I'm guessing you mean you tricked the IAT sensor), did you trick the car into thinking it was warmer or cooler than it really was?
 

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Typically, the IAT sensor has a much more direct input on the timing table than it does the fuel tables. If the temperature drops 20 degrees, it will continue to advance the timing according to preset tables programmed into the PCM. Of course, if it hears 'knock', that timing will be pulled a bit and slowly re-introduced to see if the knocking is gone.

I do know that in the 6.1L Hemi that very high IAT temps pull timing big time. I cannot verify that it adds fuel, but it is my belief that it added fuel as well. As I drove I-8 through Arizona this summer, to and from Magnaflow in Cali, I had speed control set and a very close 'range' of average mpg figures going. As soon as I reached the 115 degree ambient temp read, my average mpg according to the display started dropping. I dropped over 2 mpg through the very hot Arizona western desert. I don't think pulling timing alone would drop it that bad, but it could depending on how much timing is pulled. I was running 91 octane and just cruising(no acceleration runs). MPG dropped from 20.6 to 18.2(according to my logs I took).

Todd
 

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Discussion Starter · #7 ·
WhiteDiamond said:
As I drove I-8 through Arizona this summer, to and from Magnaflow in Cali, I had speed control set and a very close 'range' of average mpg figures going. As soon as I reached the 115 degree ambient temp read, my average mpg according to the display started dropping.
Ok, right, here is where I get confused. It got hotter so the car made it richer (or at least logically would have). So when it gets colder out it would lean the mixture right? But when it's colder the combustion chamber will have more O2 in it, so shouldn't it be getting richer to keep the ratio the same?
 

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marlinspike said:
Ok, right, here is where I get confused. It got hotter so the car made it richer (or at least logically would have). So when it gets colder out it would lean the mixture right? But when it's colder the combustion chamber will have more O2 in it, so shouldn't it be getting richer to keep the ratio the same?
adding fuel to the engine can also cool the engine. 2 different things. that may be confusing you?
 

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Discussion Starter · #9 ·
Ok, I knew richer means cooler but yes that was what was confusing me. So the only end trying to be achieved by modern cars when they automatically richen the mixture is too keep things cool and not pinging, right? I.E. in hot weather, if you are neither pinging nor running warm, there is no need to richen the mixture on a car that has 0 computers right?
 

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From the LX service manual.

MODES OF OPERATION
As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to the output devices. For example, the PCM must calculate different injector pulse width and ignition timing for idle than it does for wide open throttle (WOT).
The PCM will operate in two different modes: Open Loop and Closed Loop.
During Open Loop modes, the PCM receives input signals and responds only according to preset PCM programming.
Input from the oxygen (O2S) sensors is not monitored during Open Loop modes.
During Closed Loop modes, the PCM will monitor the oxygen (O2S) sensors input. This input indicates to the PCM whether or not the calculated injector pulse width results in the ideal air-fuel ratio. This ratio is 14.7 parts air-to-1 part fuel. By monitoring the exhaust oxygen content through the O2S sensor, the PCM can fine tune the injector pulse width. This is done to achieve optimum fuel economy combined with low emission engine performance.
The fuel injection system has the following modes of operation:
² Ignition switch ON
² Engine start-up (crank)
² Engine warm-up
² Idle
² Cruise
² Acceleration
² Deceleration
² Wide open throttle (WOT)
² Ignition switch OFF
The ignition switch On, engine start-up (crank), engine warm-up, acceleration, deceleration and wide open throttle modes are Open Loop modes. The idle and cruise modes, (with the engine at operating temperature) are Closed Loop modes.
As far as the A/F target, look at the dyno chart link in my sig.
The a/F is plotted at teh bottom of the Dyno chart.
 

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Thank you, maneval69, for the clarification, confirmation on open loop operation. Just as I thought!!

Marlinspike, yes, I am tricking the IAT sensor, but only for WOT operation. I turn it on just before a run, and off right after. I built a little box with a potentiometer and a rich/lean switch. I use my dash mounted (velcro) Scan Gauge to dial in the desired IA temp. On that run I set it to 20 degrees hotter than ambient. Since the hotter (lean) side bypasses the IAT sensor completely, it maintains that temp. for the full run, unlike the rich side, which is in series with the IAT sensor. The rich side combines the dialed in resistance (temp.) with the true IAT resistance (temp.), and the result is a somewhat lower (richer) than dialed in temp. by the end of the run. More testing of this device is definitely in order, to see how much it helps. On that run I also used my 25 cent cooling fan override mod. too, which may have also helped. I used that the whole evening though, so it may have only been a slight contibutor. Engine coolant temp. at the start of that run was 190 degrees. I also did one run, setting the mixture mod. 20 degrees rich, and the E.T. was the second slowest of the night, whereas the lean run was the quickest.
 
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