Robert56

iTrader: (2)

I broke one of my msd pills and got to thinking, maybe I can make pills. I got the trusty ohm meter out and started measuring, here's the results:

6000rpm = 1616 ohms resistance
6200rpm = 1650 ohms
6400rpm = 1680 ohms
6600rpm = 1714 ohms
6800rpm = 1745 ohms

So, it seems that they are just resistors? I could just put a resistor in that calculates to my desired RPM? Am I missing something or would this work? I don't know if there is a variable voltage to contend with or if a standard formula would work? I am going to do some backwards math to see if I can figure that out or not, but help is certainly needed.

Edit: Yes, I know, I have no life.

BMN

They are just resistors. Unfortunatly they do not chart linearly, fortunetly, you can find the Ohm's to RPM chart in the web. Use to have it, but is long lost.

Robert56

iTrader: (2)

Thanks BMN, I figured someone would know. Now is the fun part, searching the web.

BMN

Even funner is taking them apart. Lift the little sticker up that has the RPM value on it. Then take a peaky at the $.02 resistor hiding in there.

01FbTaWs6

iTrader: (12)

please post the chart here when you find it, would really appreciate it, thanks alot

NOSjohn

iTrader: (9)

Good information Robert, I'd like to know this as well.

BMN

http://www.corral.net/tech/powerplant/arm/

shows a homemade box. I don't think his math looks rite. Not sure what cyl mode and to lazy to do research on this cool sunday.

Robert56

iTrader: (2)

Yea, I think your right on his math, however, it may be correct for non ls1/ls6 type motors because we run the MSD in 4cyl mode (looks as though his jumper wires are not cut). Also, his math doesn't correspond with res reading on the pills I listed above. That doesn't mean using a pot won't work. Just some details to work out. I am going to measure some low rpm pills and see if what he say's is correct. I've got a couple 2.2k resistors right now and will test them compared to his math using my car and a test light to see what rpm it comes on. We are getting close. I do like the idea of using the variable resistance.

01FbTaWs6

iTrader: (12)

is the math correect? It should work for any setup right, the only difference is he is running a ford, which isnt the 4 cyl setup, so all we would have to do is cut the loop on the msd switch, and this "box" should work regardless right? After all, you cant beat the $5.00 price tag of making it. Let me know asap, I might be heading to the old radio shack and making one of these before I get my setup, while it is fresh in my mind. Thanks

Robert56

iTrader: (2)

Correction, I had my ohm meter at wrong setting, here is correct numbers:

MSD Pills:
6000rpm-4.57k res
6200rpm-4.78k
6400rpm-4.99k
6600rpm-5.21k
6800rpm-5.43k

So the backwards math (suggested math in link above) of established values works out to:

4570 + 1000rpm = 5570rpm (6000 msd) Difference 430rpm
4780 + 1000rpm = 5780rpm (6200) 420rpm
4990 + 1000rpm = 5990rpm (6400) 410rpm
5210 + 1000rpm = 6210rpm (6600) 390rpm
5430 + 1000rpm = 6430rpm (6800) 370rpm

We are closer, but there seems to be a discrepantcy. The math is off by 430rpm to start and looses 10rpm at each level as you go higher. This pattern means something if it can be figured out. Something else is at play here? We'll see.

Robert56

iTrader: (2)

Maybe I have it. Ohms law: It takes 1amp to push 1 volt through 1ohm of resistance. Now if you change the percent of anyone of those values you'll get differant results (simple math). So in conclusion, we need to know the amperage (or rather wattage) that MSD is sending through the resistor to get an accurate formula (a little more tuff math). Thats all a resistor does is change the output from the input. Any electrical Engineers floating around that can help us, if I can't figure it out?

Robert56

iTrader: (2)

I think I have a correct equation figured out (there may be an uncontrolled variable inside the MSD though), however, I am going on a drag run to do some logging and will get back to this later.

Robert56

iTrader: (2)

Ok, for those who are following my tangent, here are some more numbers.

MSD Pills
6000rpm = 4.57k ohms or
12volts / 4.57k ohms = .0026mA (signal leaving resistor)

6200rpm = 4.7k ohms or
12volts / 4.7k ohms = .0025mA

6400rpm = 4.99k ohms or
12volts / 4.99k ohms = .0024mA

6600rpm = 5.21k ohms or
12volts / 5.21k ohms = .0023mA

6800rpm = 5.43k ohms or
12volts / 5.43k ohms = .0022mA


Then using suggested math:

5k ohm resistor + 1000rpm = 6000rpm (or desired rpm), however,
12volts / 5k ohms = .0024mA

5.2 ohm resistor + 1000rpm = 6200rpm, however,
12volts / 5.2k ohms = .0023mA

5.4k ohm resistor + 1000rpm = 6400rpm, however,
12volts / 5.4k ohms =.0022mA

5.6k ohm resistor + 1000rpm = 6600rpm, however,
12volts / 5.6k ohms = .0021mA

5.8k ohm resistor + 1000rpm = 6800rpm, however,
12volts / 5.8k ohms = .0020mA

So, we can see the differences. Does this mean that the pot won't work, no. I think it'll work fine, just the suggested math (link) doesn't work for single resistors and/or compare with known values (read: msd). The math for the MSD pills should work fine for making individual pills and be right on. Maybe someone can spot why things don't jive. I think I am going to make one of the rheostats (variable resistance) when I get time and use a single res for now.

NOSjohn

iTrader: (9)

It's not a tangent, it's a great technical lead into something great and useful.
I will have to contact my electrical engineer friend tomorrow and see what he says.

rons 00z

iTrader: (10)

shouldnt this be 6400 rpm?
5k ohm resistor + 1000rpm = 6000rpm (or desired rpm), however,
12volts / 5k ohms = .0024mA

and i think you want to measure current before the resistor so you know how much your attenuating with your msd pill

BMN

It's not the current that they effect. The resistors change the frequency of the local oscillator inside the circuit. It then uses comparators to see if it above or below the freq for the on and off then uses simple logic to switch the N2O out leads. + goes - and - goes +.

Robert56

iTrader: (2)

That's my point, shouldn't it be 6400rpm, but the math supplied in the link dosen't support the way MSD does it.

I didn't measure current at msd because just using a constant 12v on each system/idea and they should follow with the same results.

Robert56

iTrader: (2)

So, if we want to shut my n2o off at 6700rpm (msd dosen't make a pill) we use this formula.

mA x ohms = volts, so
.00225 * x = 12 then,
12 / .00225 = 53.3k ohm resistor for 6700rpm shut off.

I'm heading to Radio Shack soon.

rons 00z

iTrader: (10)

resistors do not change frequency at all in this case it steps down voltage. the only way a resistor can effect frequency is if it's used with a capacitor then it acts as a filter. not neccisarily changing a freq but removing unwanted ones. if you hook a tach signal up to a o-scope it's going to show you a square wave of 12v p to p hook a resitor up to it and you will see your wave drop in voltage depending on the current.

NOSjohn

iTrader: (9)

How was the trip to Radio Shack and what can you report back in your findings?