IMRC delete tune, link missing

[MouTine]

Yes did try search with no luck.

Could someone please post here a direct link to the topic where some expert explained very well throuly what hapens in tune when IMRCs work. There was some writeing about timeing adjustments and what should do when deleting IMRCs and common misstakes that not-so-pro tuners had done when tuneing off IMRCs.

Thank you!

 

[Shake&Bake]

op:

 

[whtcobra1998]

when you turn off the imrc's in the ecu the imrc ign timing map is also turned off. tune like a normal car.

 

[luke87gt]

I will be getting my car tuned in the next few weeks and am also interested to know what an experienced tuner does to address IMRC deletes.

when you turn off the imrc's in the ecu the imrc ign timing map is also turned off. tune like a normal car.

I would assume the same. If you delete the IMRCs, the IMRC Spark Advance goes into a "safe" map mode (PCM assume the IMRCs won't open) and you won't get the benefit of the "normal" map mode spark advance.

I will be data-logging the stock tune, base tune, and any iteration tunes (I'm doing mail order) so I can see exactly what the commanded A/F is, spark advance, and actual A/F ratio over the RPM range at WOT. Once the A/F is where I need it to be and I am satisfied with the spark advance, I will dyno.

Interestingly (not sure if this is '96 specific), I have had my IMRC deletes installed with the short runner for about 800 miles now, and I have not gotten a CEL :??:

 

[Shake&Bake]

I will be getting my car tuned in the next few weeks and am also interested to know what an experienced tuner does to address IMRC deletes.



I would assume the same. If you delete the IMRCs, the IMRC Spark Advance goes into a "safe" map mode (PCM assume the IMRCs won't open) and you won't give you the benefit of the "normal" map mode spark advance.

I will be data-logging the stock tune, base tune, and any iteration tunes (I'm doing mail order) so I can see exactly what the commanded A/F is, spark advance, and actual A/F ratio over the RPM range at WOT. Once the A/F is where I need it to be and I am satisfied with the spark advance, I will dyno.

Interestingly (not sure if this is '96 specific), I have had my IMRC deletes installed with the short runner for about 800 miles now, and I have not gotten a CEL :??:

I'm surprised you haven't gotten the cel yet.

 

[MouTine]

I think I found the linky... http://www.svtperformance.com/forums/sn95-cobras-24/615042-imrc-delete-without-re-tune-come.html And it was Mr Mwolson's post. Heres direct quote...

IMRCs are powered by 12 volts and ground. There is a PCM output that drives to IMRC control input. That is how the PCM commands the IMRCs to open or close. When the PCM drives the signal high, the IMRCs close. When the PCM drives the signal low (ground), the IMRCs open.

The PCM needs to know if the IMRCs are both open or closed when they are supposed to be open or closed. There is an IMRC sensor input to the PCM that can read voltage. That input has a resistor on it that is hooked to 5 volt power to pull the input up to close to 5 volts when nothing is connected to the input. There are two switches and two resistors in parallel inside the IMRC controller. One side of each resistor is connected to the PCM IMRC sensor input. The other side of each resistor is connected to a switch that goes to ground on the other side of the switch. Each switch is mechanically connected to the controller end of each IMRC cable, so that it closes when the IMRC opens.

If both IMRC cables are in the "IMRC open" position, then both resistors are switched in parallel to ground, pulling the IMRC sensor PCM input down to less than 1.6 volts. This is how the PCM knows that both IMRCs actually opened. If only one opens, only one resistor will be trying to pull the IMRC sensor input down, and that is not enough to get it below 1.6 volts. This is how the PCM knows if one or more IMRC is stuck closed. If the PCM detects they are stuck, it sets a DTC.

If both IMRC cables are in the "IMRC closed" postion, then both resistors should be out of the circuit since the switches should be open. This lets the IMRC sensor PCM input go up to over 3 volts. This is how the PCM knows that the IMRCs are actually closed. If one or both are stuck open, the resistor(s) will not allow the IMRC sensor PCM input to go over 3 volts. This is how the PCM knows if one or both IMRCs are stuck open when they should be closed. If the PCM detects they are stuck, it sets a DTC.

If you remove the IMRC controller, the IMRC sensor PCM input will always see a voltage of greater than 3 volts, so it will think the IMRCs are closed, regardless of what it commands.

I do not yet know the actual values of those resistors because I sold my IMRC controller long ago and have not been able to measure one yet.

Assuming the IMRCs are working properly, the stock tune in the PCM (from my 97 LLX4 tune) opens the IMRCs above 3250 RPM and closes them below that RPM. When the IMRCs are open, the PCM adds spark using 3 tables: Spark_Adder_to_Borderline_for_IMRCs, Spark_Adder_to_Max_Allowed_Table_for_IMRCs and Spark_Adder_to_MBT_Table_for_IMRCs.

The Spark_Adder_to_Borderline_for_IMRCs table adds from 0 to 8 degrees of spark advance to the borderline knock table when the IMRCs are open, depending on Load and RPM. The Spark_Adder_to_Max_Allowed_Table_for_IMRCs and Spark_Adder_to_MBT_Table_for_IMRCs tables add from 7 to 15.75 degrees of spark advance to the Max_Allowed_Spark table and the MBT_Spark table, depending on Load and RPM. So you can see that when the IMRCs open, the PCM can add a lot of spark advance and hence a lot of power.

When you delete IMRCs using SCT, then those IMRC spark adder tables are deleted. As a result, you tuner needs to compensate by adding the IMRC spark advance for the IMRCs being open back into the Borderline Knock, Max Allowed and MBT spark tables.

Some tuners just add the values from the IMRC adder tables back into the main tables, but I don't care for that approach for a KB blower equipped car. I prefer to make my spark tables look like the spark tables from 03/04 Cobras, since we have similar boost onset characteristics, and there is no long a jump in air when the IMRCs open, so why should we have a jump in spark under those conditions.

I digress. I do not know if you lose the IMRC spark adder tables if you delete the IMRCs in the tune using other tuners such as Diablosport. I do know you can delete the IMRCs without losing the spark adder tables if you are using TwEECer.

The other thing I do not know is if the PCM does not add in the IMRC spark adders if the PCM thinks that the IMRCs are stuck closed. If that happens, you will lose power by deleting the IMRCs and not deleting them in the tune or doing something with springs or a circuit to fool the PCM into thinking the IMRCs are still there. You will also get the CEL.