HOW TO: Make a Timing Retard Box for a Nitrous Oxide system...
This mod is for those of you who want to retard the spark timing in your car while spraying nitrous but don’t want to give up that horsepower while on the street the other 99% of the time you aren’t spraying. Yeah you could buy a more expensive timing retard box that costs in the $275 range or you could do this mod for about $30. Your choice. I got the idea for this mod while tuning in my IAT Spark Tables for methanol injection spark advance. But like everything else, when I looked around online to see who else had thought of it of course somebody had already beat me to it so I guess I wasn’t the first with this bright idea.
Use this mod at your own risk. I will not be responsible for any wiring or tuning debacles you may put yourself in if you misuse the information in this post.
To understand how this mod works you need to first understand how the IATS (Intake Air Temperature Sensor) works and how it interacts with the ECU (Engine Control Module). Basically an IATS is just a thermally modified resistor (aka thermistor). At different temperatures that it encounters the sensor outputs a different electrical resistance (measured in ohms). The ECU outputs 5 volts on one wire leading to the IATS and measures the return voltage on the other wire coming from the IATS. The difference in voltage is in direct proportion to the amount of resistance the IATS causes but the ECU still needs to know how to interpret this. That is where the IATS Calibration Table comes into play that is in the ECU tune.
Here is an IATS Calibration Table from a 2009 Pontiac G8 GT. The tables for the GTO’s look similar.
IATS Calibration Table.jpg
As you can see, different intake air temps are represented by differing resistance values. For the purpose of this modification we want to focus on the lowest temperature cell, which is -40*F in this case. Now we all know that short of a VERY few exceptions there is no one who will be driving their G8, GTO, or Corvette in -40*F weather so this cell is virtually useless for normal driving use. But we can use it for the purpose of retarding spark timing while spraying nitrous. Notice how the resistance value is 100,866 ohms. We will use that later in this writeup.
Now let’s move onto what the ECU does with this temperature data with regards to spark timing.
Below is your typical IAT Spark Adder Table. This one is from a 2009 Pontiac G8 GT but the ones in a GTO tune will look similar.
Nitrous IAT Spark Retard in HPT IAT Spark Adder Table.jpg
As you may have noticed, I have already modified the tune so that -4 degrees of spark is taken out at all cylinder loads in the -40*F cells in the IATS Table. Pretty simple, but there is one more item to change in the tune before you are finished.
Enter the IATS Spark Adder Multiplier Table. As always, this one is a G8 GT table but your GTO table will look similar, with a few small temperature cell differences.
Nitrous IAT Spark Retard in HPT IAT Spark Multiplier Table.jpg
This table will multiply whatever figures are present in the corresponding temperature cell of the IAT Spark Adder Table. In this case we know that we are requesting -4 degrees in the IAT Spark Adder Table across all cylinder loads. So at whatever RPM vs IAT cell I request a multiplier of 1.00 I will retard -4 degrees of spark advance out of the tune below what is already in the Base Hi/Lo Octane Spark Tables. If my multiplier is 0.50 then I will only be pulling -2 degrees at that RPM. Most people start spraying at around 3,000RPM’s so that is what this example is setup to represent. I like to feather spark changes in and out gradually to keep things smooth so that is why I am already requesting that 50% of the IAT Spark Retard be taken out in the 2,500RPM cell.
Now what we have accomplished is setting up the tune so that it only pulls spark advance when it senses -40 degree IAT’s and only from 2,500RPM’s on up. Yes the ECU will also interpret the cooler aircharge as a more dense one and will want to add a bit more fuel to compensate. But you will want to use a richer WOT AFR anyways, so that won’t really be a problem because you will be jetting your wet nitrous system to enrichen WOT AFR’s anyways.
Now onto making the Timing Retard Box.
First off is a list of parts that will be needed:
• Radio Shack plastic “Medium Small Project Box” x1
• Relay, 12VDC, 5 pin automotive relay x2 (<----Make sure these are 5 pin with pin #87a)
• Resistor 0.5 watt, 100k Ohm x1
• Resistor 0.5 watt, 5k Ohm x1
• Electrical Spade connectors, ¼” x 18gauge x10
• Relay Harness pigtails x2 (this is optional and would replace the 10 ¼” spade connectors above)
• Wire, 18gauge various colors x10’
• Solder
• Heat Shrink Tubing Small and Large sizes
• Grommet, Rubber roughly 3/16”-3/8” x1 (Size of grommet depends on size of wire you go with)
• Zip Ties, Small x5
Total Price: $30
Tools Required:
• Basic Wiring Tools
• Digital Multimeter (<----Not 100% needed but helpful for testing resistance, voltage, and connectivity)
• Heat Gun or Hair Dryer
• Soldering Iron or Gun
• Drill and Drill bit
• Philips Screwdriver
Below is an electrical diagram of how everything will be wired.
Timing Retard Box Wiring Diagram.jpg
The basic gist of how to wire this is so that when the relays are not powered the signal travels from the ECU, through the IATS, and back to the ECU like it would in stock form. Then when the relays are powered by 12 volts they will switch connection with the pins so that the IATS is temporarily taken out of the circuit and instead the resistors are connected for the ECU to “see” and think it is encountering -40*F IAT’s. Normally I am very adamant about not tricking the ECU with false signals, but in this case it is for the purpose of being on the safe side versus some other hack-job trick to gain power at the cost of safety so it will be ok. The IATS does not have a positive and negative pole so you don’t have to worry much about which wire coming from the ECU that you hook up to which Pin 30 on Relay #1 or #2. For those of you who are also skilled in DC wiring I understand that there is more than one way to wire the relay(s) in this box to allot the IATS signal to be bypassed, but this is the way I prefer to do it. So that is what my diagram is geared towards. NOTE: DO NOT USE CRIMPED BUTT CONNECTORS to connect the wires! You will have a total of 6 wires exiting the box that will route to various places on the car. The first two will be the 12v power and ground wires that activate the relays themselves, in the diagram above that would be the single red and single black wire that split off and each route to pins 85 and 86 on the relays. Then the next two wires that exit will be the wires that will connect to your wires coming from the IATS. In the diagram above those would be the wires coming from pin 87a on each of the relays. The last 2 wires that will exit the box will be the ones that attach to your vehicle electrical harness. According to the diagram above these will be the wires coming from pin 30 on each of the relays.
Activating the Box: Now there are a few options of how you can send 12 volts of power to these relays. You can just keep it simple and run an extra power wire to the box from your nitrous master arm switch or you can run 12 volts from the solenoid activation wire in your nitrous controller. The first choice will mean that the ECU will “see” a -40*F IAT at all times that the master arm switch is flipped, while the second choice will mean that the ECU will only “see” a -40*F IAT when the solenoids are being powered (aka spraying). Before some of you start asking, well why trick the ECU into thinking it is seeing -40*F temps when the nitrous is sure to cool down the IAT’s to that cold of a temp in reality? Well the difference is that there will ALWAYS be a delay time between what the actual intake temps have dropped to on the nitrous versus what the IATS is reading. The IATS just aren’t quick enough to react to temperature changes to keep the engine from knocking for the first few seconds of nitrous activation which is what gives this mod purpose. The ECU only checks IAT’s 2 times a second so in theory if you hook the box up to a solenoid trigger wire then there can be a chance of a slight delay between when the noids are powered versus when the computer “sees” the -40*F temp and pulls timing. Just something to think about.
Locating the Box: While you could seal the box with silicone and keep it under the hood I would suggest that it be located inside the cabin somewhere, perhaps under the dash somewhere. You don’t really need access to the box to activate it so it can be tucked up somewhere hidden. Just make sure that if you mount the box away from the IATS that you make good connections with solder and heat shrink tubing on all sensor wire connections so that you don’t inadvertently add more resistance that isn’t accounted for in the IATS Calibration Table.
Extra options/Multiple Timing Retard Settings: One other item worth mentioning is for those of you who may want to retard different amounts of timing with the flip of a rotary switch for whatever reason. You can use this box for that too but you will need to do a few extra things. First off you will need to go to the next coldest cell in the IATS Spark Adder and Multiplier Tables and input whatever spark retard value in them. Then reference the IATS Calibration Table to see what amount of IATS resistance in ohms that ECU expects to see with that temperature so you know how much total resistance your resistor(s) need to add up to or slightly exceed when wired in series. After this you will add in the rotary switch between Pin 87 on Relay #1 and Pin 87 on Relay #2. The wire coming from Pin 87 on Relay #1 will go to the common pole on the rotary switch. Then the first resistor set for the coldest IAT Spark Retard setting will go on Pole #1 of the rotary switch and connect to Pin 87 on Relay #2. The second resistor set for the second coldest IAT Spark Retard setting will connect to rotary switch Pole #2 and connect to Pin 87 on Relay #2 also, and so on and so forth for further Spark Retard settings. Just remember that you are limited on the amount of IATS Spark Adder temperature cells that you can use before getting into cells that you may actually use while daily driving your car in normal weather.
01-Parts.jpg
02-Resistors.jpg
03-Half Done.jpg
04-All Wired Up.jpg
06-Project Complete 2.jpg
07-Resistance Test.jpg
This is the resistance, measured in the thousands of ohms, that the ECU will see when the box is powered thus making it think it is encountering -40*F intake temps.
I hope this mod helps those of you who want an inexpensive and effective way to retard your spark timing while spraying nitrous. There are some other options for how you can set your timing retard without having access to HP Tuners or other tuning platform so if you want to do something like this but don’t have the ability to modify your tune itself, just ask and alternative options can be provided. They aren’t quite as good as directly modifying the IAT Spark Tables, but it is a viable second option. Overall it is a pretty simple setup, especially if you are familiar with DC wiring and have access to HP Tuners, EFILive, or other GM custom tuning platform.
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