Chrysler 300C & SRT8 Forums banner
1 - 20 of 21 Posts

·
Registered
Joined
·
697 Posts
Discussion Starter · #1 ·
I just finished a mixture control mod. for the Hemi, but am not absolutely sure it will work. It should work any time the fuel injection is in the open loop mode, but I am primarily interested in open loop at WOT. At WOT the fuel injection is in the open loop mode and the oxygen sensors in the exhaust are not monitoring the mixture. In the closed loop mode they do, and force maintain the mixture at 14.7:1, which is chemically perfect for combustion and near optimum for emissions. Between 12-13:1 is optimum for performance. My mod. is a linear potentiometer that is adjustable from 0-50,000 ohms(which is the approximate range of the IAT) and is hooked up in series with "signal lead" of the intake air sensor (IAT). When in the "0" position (and in normal driving) it reads the sensor at face value, which is true intake air temp. Crank it all the way on (or somewhere in between) just before you're going to do a WOT blast, and the PCM computer thinks the intake air temp. is around 10 degrees F, and should then richen the mixture at WOT.....hopefully to the 12-13:1 air-fuel ratio. Right after the WOT run, the pot. must be turned back to "0" or the PCM will start tracking the readings and begin its adaptive corrections. My "Holley Bible" says that there is about a 4% difference in mixture requirements for every 11 degrees F change. I think it will work better in hotter weather than when cool, because of the larger temperature difference. My preliminary figures indicate that it will have to be set 43 plus or minus 7 degrees of true ambient to get the correct mixture. I have taken resistance readings of the IAT at 32, 40, 60, 80 and 120 degrees, so I will have some idea what to set the resistance at. Any comments or input on whether members think it will work are appreciated.
 

·
Registered
Joined
·
697 Posts
Discussion Starter · #3 · (Edited)
JMatt,
Since we had discussed this I have added a switch to mine to go rich or lean. My adjustments with the 50K pot. are obviously not as "fine tuned" as yours Which way are you going....richer or leaner?? How much different from ambient are you going to get your best results. (I know it will vary from day to day) I've still not done enough runs with mine, but have also gained about two tenths last fall. As I recall, I was going leaner...set about 20 degrees hotter than ambient. Let me know, as I am headed to the races in a couple weeks.
Also, did you try it without using the pot??
 

·
Banned
Joined
·
1,300 Posts
I have headers and Magnaflow cat-back. This made my system lean out. I'm adjusting the temperature down, in order to richen the mixture. With a 10K pot, I lower the temps from 20-30 degrees depending on the initial reading. Also, I relocated my IAT to the air box from the intake, in order to get consistently lower readings (i.e. closer to real world and not affected by engine temp) before engaging the pot.

I only ran one time without using the pot, and the time was slower, but still in the 12.9's. I didn't do enough testing with and without.
 

·
Registered
Joined
·
697 Posts
Discussion Starter · #5 ·
How much difference did moving the IAT sensor to the airbox make in readings? I would think that might help a little also. Good idea.
 

·
Banned
Joined
·
1,300 Posts
The big difference is the sensor "recovers" to reading real world intake temperature much quicker than when its sitting practivally on the intake manifold soaking up heat. This makes a big difference in the staging lanes. With outside temps around 50, I would read 75 in the staging lane. The other SRT-8 would read 110.
 

·
Premium Member
Joined
·
4,071 Posts
JMatt said:
The big difference is the sensor "recovers" to reading real world intake temperature much quicker than when its sitting practivally on the intake manifold soaking up heat. This makes a big difference in the staging lanes. With outside temps around 50, I would read 75 in the staging lane. The other SRT-8 would read 110.

JMatt you're the first person to give the REAL reason for IAT relocation! Most people think it will increase HP across the board. The coupling around my IAT in my 'Vette was made of thick steel . :eek: I immediately replaced it with a plastic one.
 

·
Registered
Joined
·
697 Posts
Discussion Starter · #8 · (Edited)
Where on the airbox did you mount it and did you drill a bigger hole and use a rubber grommet?? Is it still in the airflow? I think I feel another mod. coming on....real soon. Any pictures?
 

·
Registered
Joined
·
701 Posts
magnuman said:
... In the closed loop mode they do, and force maintain the mixture at 14.7:1, which is chemically perfect for combustion and near optimum for emissions. ...
I thought that the mixture in normal driving (non-WOT) was supposed to be too rich, in stock configuration.
 

·
Premium Member
Joined
·
4,071 Posts
artichoke said:
I thought that the mixture in normal driving (non-WOT) was supposed to be too rich, in stock configuration.

It's the other way around. In closed loop 14.7:1 is the best anyone should see no matter what car, but the wide band output I've seen on WOT passes on dynos show 11-11.5:1 AFR's which is PIG rig for an N/A motor.
 

·
Registered
Joined
·
36 Posts
JMatt said:
It works. See my track results in the SRT-8 forum. Use a 10K pot though. 50K is too much.
I did this a while back on a neon rt. But it was a set valued resistor. Is it necessary to use a pot ..?
 

·
Registered
Joined
·
697 Posts
Discussion Starter · #12 · (Edited)
LowLimitz,
Yes, it is necessary to use a variable resistor if you want to have the ability to adjust the temperature range. A fixed resistor will just give you one setting only. If the Neon RT has a similar PCM as our LX's, what happens is the PCM eventually takes the resistance value of the fixed resistor into it's computations and makes corrections (adaptive memory) to the mixture to maintain a 14.7/1 air/fuel ratio. This eventually negates the effect of the resistor. What JMatt and I are doing is ONLY turning it on just before a WOT run, and off immediately after the run. This way the computer cannot start compensating for its presence. At WOT the fuel injection is in the "open loop" mode, and mixture control does not use inputs from the 02 sensor(s). Additionally, at normal throttle openings (cruise), the PCM has the fuel injection running in the "closed loop" mode and mixture is contolled by input from the 02 sensor(s). JMatt, or others, can probably add more to this.
 

·
Registered
Joined
·
203 Posts
Sounds like an opportunity to put in a switch on the gas pedal that engages at WOT. Then you don't have to turn it on and off. Just floor it.
 

·
Registered
Joined
·
14 Posts
that or you can flash your computer with superchips, which will eliminate the adaptive memory. After that you can set your little resitor and always have teh same a/f when running wot.
 

·
Registered
Joined
·
482 Posts
Running the Kooks on the 6.1L took the air/fuel into the mid 12:1 range, which I want for a normally aspirated motor. I think the real issue turns out to be that DCX backes off the timing in WOT as well, so as it leans into what I consider a good range, the PCM appears to be pulling timing in the upper RPM range. I cannot verify this, but I wonder if JMatt can confirm this. I am also curious as to where the lower T-stat along with a fan control mod will move the air/fuel ratio. DCX stated that would also richen the mixture.

Either way, my SRT8 is 11.3 to 11.5 :1 across the RPM range, stock with Magnaflow, and that is WAY TOO RICH for a street engine. I can see carbon build up requiring the cleaner being a very common practice for these vehicles in the near future. I recommend driving them hard to help blow some of that crap out :D :D

Todd
 

·
Registered
Joined
·
697 Posts
Discussion Starter · #17 ·
Boy, you're right 11.3:1-11.5:1 is really "fat". The best for the most power should be 12.8:1-13.2:1 air/fuel mixture. Sounds like you need to try what JMatt and I did. I don't think JMatt put the switch in his pot. mod. like I did though. Mine, with the switch, allows me to trick the IAT sensor either to the rich (low temp.) side or the lean (high temp.) side. Like JMatt, I picked up about two tenths of a second using this mod. It's only for WOT runs though...set it just before the run, turn it off right after the run.
 

·
Registered
Joined
·
203 Posts
I'm looking at doing this mod, but instead of a switch, I've worked out a way of doing it automatically for WOT. I'll build it and confirm it works on a dyno, before giving detail, so no one chases ghosts. Or I could just do what everyone else does and claim it's good for 20-30 HP with no real proof. NOT!

Right out of the service manual, you can read...

"When the PCM senses a wide-open-throttle condition through the Throttle Position Sensor (TPS) it de-energizes the A/C compressor clutch relay. This disables the air conditioning system and disables EGR (if equipped).
The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel, based on MAP and RPM."

This matches up with what I've seen on four dyno runs on my SRT8. Since the IAT sensor is a NTC thermistor (Negative Temperature Coefficient), the resistance drops with an increase in temperature. So to fake it out to lean out the mixture, you need to have a parallel resistance to drop the total resistance value. A series resistance will richen the mixture, by faking it in to thinking it's colder than normal.

One more thing I want to do before building the circuit, is to look at the upstream O2 sensors to see if they are usable to assist in this at WOT. It may be possible to build a circuit that monitors the O2 sensors and throttle position and select based on those, what modification is done to the IAT sensor reading. Without intervention, or manual adjustment.

I can't help but wonder why it won't work, because if it did, why wouldn't Chrysler have done it?
 

·
Registered
Joined
·
102 Posts
jaak said:
I'm looking at doing this mod, but instead of a switch, I've worked out a way of doing it automatically for WOT. I'll build it and confirm it works on a dyno, before giving detail, so no one chases ghosts. Or I could just do what everyone else does and claim it's good for 20-30 HP with no real proof. NOT!

Right out of the service manual, you can read...

"When the PCM senses a wide-open-throttle condition through the Throttle Position Sensor (TPS) it de-energizes the A/C compressor clutch relay. This disables the air conditioning system and disables EGR (if equipped).
The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel, based on MAP and RPM."

This matches up with what I've seen on four dyno runs on my SRT8. Since the IAT sensor is a NTC thermistor (Negative Temperature Coefficient), the resistance drops with an increase in temperature. So to fake it out to lean out the mixture, you need to have a parallel resistance to drop the total resistance value. A series resistance will richen the mixture, by faking it in to thinking it's colder than normal.

One more thing I want to do before building the circuit, is to look at the upstream O2 sensors to see if they are usable to assist in this at WOT. It may be possible to build a circuit that monitors the O2 sensors and throttle position and select based on those, what modification is done to the IAT sensor reading. Without intervention, or manual adjustment.

I can't help but wonder why it won't work, because if it did, why wouldn't Chrysler have done it?
So how should you wire this in? I have been wanting to try this but I am unsure as to how to do it.
 
1 - 20 of 21 Posts
Top