How TO: Build a 'Nino' Adjustable RPM Module
 

The "Nino" Adjustable RPM Module (ARM) was created to replace the expensive and inflexible RPM plug modules that MSD sells for their RPM Activated Window Switch. In order to come close to having the flexibility the "Nino" ARM offers, you would have to buy the MSD Adjustable Low RPM Module (1000 to 3000 RPM) plus all eight MSD Plug Module packs (3000 to 10,800 RPM), which would set you back about $185! Total material cost for the "Nino" ARM is around $5. This device was created specifically for the MSD RPM Activated Window Switch, but the underlying idea can be used to make modules for other types of RPM switches.

Special Notes

All parts were purchased from Radio Shack. The part number for the crimp terminals corresponds to a package that contains six terminals. Radio Shack offers a package with four terminals for a couple cents less, but it might save you a trip to buy the six-terminal package in case you accidentally break one. If you find a slightly longer crimp terminal of the same diameter, buy it, as these where a little short (but they will work just fine.)

Some of you may wonder why I used a 4.7K-ohm potentiometer and not just two 10k-ohm potentiometers. Well, I figured I would never want to activate my nitrous anywhere after 5000 RPM (!). Plus, by choosing the 4.7k-ohm potentiometer I can adjust the "ON" setting more accurately because each incremental turn of the potentiometer is a smaller change in resistance when compared to the 10k-ohm potentiometer.

TOOLS SECTION

1.Ruler and pencil
2.Drill, including a 3/32" and ¼" drill bit
3.Hot glue gun
4.Soldering iron and wire
5.Multimeter
6.Small flat-head screwdriver

PARTS SECTION

1.Mini Project Enclosure (part #270-288)
2.Crimp Terminals (part #274-226)
3.4.7k-ohm Micro Potentiometer (part #271-281)
4.10k-ohm Micro Potentiometer (part #271-282)
5.Jumper wire, 22 gauge solid core

Assembling The ARM

Begin by marking the holes on the enclosure according to the Figure 1. Notice the dimensions for the holes are referenced from the middle of the rectangular trench. Drill the four holes and make sure that the potentiometers will turn freely when inserted. You may have to make the holes a little bigger than the drill bit to get it perfect, but don’t make them too big or everything will fit loosely.



Insert the four crimp terminals until the barbs lock the terminal into place. Glue the terminals to the inside of the enclosure, making sure that they are aligned straight and that you leave a clean spot for soldering. Cut four small pieces of wire and solder one end of each wire to each terminal. Before you solder the potentiometers, turn both of them all the way counter-clockwise with the screwdriver. Notice that there is a large and small groove on the heads of the potentiometers. Insert the 4.7k-ohm potentiometer into the left hole in the enclosure and position the potentiometer so the large groove points to 7 o’clock. Hold the potentiometer in this position and glue it into place. Repeat this procedure with the 10k-ohm potentiometer.

Now, notice that there are three legs on the potentiometers, one "middle" leg and two "side" legs. Beginning with the 4.7k-ohm potentiometer, connect one lead from the multimeter to the "middle" leg and the other lead to either of the "side" legs. If the multimeter reads zero or very low resistance then you have found the legs that are going to be soldered to the terminals. If you get a resistance near 4.7k-ohms then keep one lead on the "middle" leg, and change the other lead to the other "side" leg. Now you should get a very low resistance. Do the same for the 10k-ohm potentiometer.

You are almost ready to solder the potentiometers, but first you need to orient both halves of the enclosure correctly. When the enclosure is assembled you want to have the 4.7k-ohm potentiometer on the left AND the crimp terminals on the bottom (see the illustration). Now that you have everything oriented correctly, open the enclosure and solder the corresponding terminals to the CORRECT legs of the potentiometers. Assemble the enclosure and we are ready to test.

Testing The ARM

Using the multimeter, check the resistance between the 4.7k-ohm terminals. If you haven’t rotated the potentiometer, it should still read a very low resistance. Now turn the potentiometer all the way clockwise and the resistance should read very close to 4.7k-ohms. Check the 10k-ohm terminal resistance. If there is no change in resistance after turning a potentiometer, then a wire must have become loose so check its connections.

Determining RPM with a multimeter

I picked up this valuable piece of information, along with the idea of using potentiometers, on the net (thanks KK). Basically, the formula goes like this:

Desired RPM = (Resistance in ohms) + 1000

So, say you want to activate the nitrous at 3500 RPM and cut it off at 6500 RPM. Using the screwdriver you would turn the 4.7k-ohm potentiometer until the multimeter reads 2.5k-ohms (or 2500 ohms). Similarly, you would turn the 10k-ohm potentiometer until the multimeter reads 5.5k-ohms (or 5500 ohms).

Labeling the "tach gauges" on the ARM

There are several options here:



1. You can set the potentiometers to the desired RPM and not label anything. In this case, you may want to carry a multimeter in case you want to "tune" the nitrous at the track or on the street.

2. Using the multimeter you can mark off key RPM points (2000, 3000, 4000, etc.) around the potentiometers (so they’ll look like tachometers, see illustration), enabling you to make rough adjustments without a multimeter. In this case, remember that most factory tachometers are off by a couple hundred RPM so, what you "dial in" on the ARM and what you see on the car’s tach may be different. If you use an aftermarket tach, this method would be a good choice since the tach and the ARM will likely match exactly.

3. You can use you car’s tachometer to mark off key RPM points on the enclosure. The following instructions may differ depending on how you have your nitrous kit installed, but I think you’ll get the idea. Connect the "Nino" ARM to the MSD Window Switch, turn the car on, arm the nitrous system, but KEEP THE BOTTLE CLOSED! Rev the engine to, say, 3000 RPM and hold it steady. Turn the 4.7k-ohm potentiometer until you hear the solenoids (or the relay) click. Label the position of the potentiometer as 3000 RPM. Do this for all the RPM points you want to label. This method will match the RPM points between the car’s tachometer and the ARM.

Installing the "Nino" A.R.M.

Once you have set up the desired RPM points, you can insert the ARM into the MSD RPM Activated Window Switch. If you hooked up everything correctly, the 4.7k-ohm potentiometer will be logically positioned on the same side as the Window Switch’s "ON" label. You will now have complete control of your nitrous system’s activation and shut-off RPM. If the nitrous is coming on a little too early resulting in E.T.-robbing wheelspin, simply experiment with a higher "ON" RPM until you find the optimum setting for the existing track or street conditions. You should probably "set and forget" the shut-off RPM at a couple hundred RPM before redline. Now when you flip the switch, your nitrous system is truly ARM’ed. Happy hunting.

Credit to Corral, not me.