coil specification

[azazzera]

I have been sucked into the coil posts again and although I have not had a bit of trouble running with a mmi coil that is about 3-4 years old I cant find what the specs. for there coils are or should be. I understand that the primary winding should be 3.25 ohms to 4? to avoid overheat and failure. Never found specs on what the secondary should be for healthy coil. I am wanting to do some pm before any trouble. Maybe put info up on FAQ or Tech tips pages.

Thanks Adam Z.

[ndutton]

I understand that the primary winding should be 3.25 ohms to 4? to avoid overheat and failure.

That's an oversimplification Adam. The coil issue emerged with the introduction of electronic ignition and its increased dwell. The critical parameter you need to control is ignition system amperage. Pertronix's advisement of a 4 amp maximum has worked well for us although on this forum we recommend a safety margin 15% below their maximum.

To avoid making the math too ominous we have applied Ohm's Law (V=IR) to the voltage-current-resistance balance to control amperage. An Excel calculator can be found in the first post of this thread. The entire thread is worth a read, there are others too. You'll find, for example, depending on alternator output a 3.25Ω coil may need an additional resistor to check the system amperage and not any resistor will do. It can be overdone affecting starting and performance.

Everyone's coil input voltage is different, that's why optimum system resistance is not a fixed number. Factors affecting coil input voltage are alternator output, wiring harness design (wire gauge + length), wire harness condition, terminal and connection condition and any combination of all of them.

[CajunSpike]

The coil sold by MMI is a Intermotor/Standard motor products UC15. Other part number on box is V18045.
Easily found on the net. I bought one from them, thats how I know.

[azazzera]

each coil should have manufactures specifications. Correct? If the coil spec. for primary is say 3.1 ohms and I get 4 and that is out of spec. it may still work with the rule of 2011. If I don't have the coil spec. than I would not know if coil is out of range and possibly going bad. Just looking for MMI coil manufactures specs so I know if the coil is still in or out of spec. Then I can take readings and do math to see if my system is within range.

[ndutton]

We have seen actual measurements vs. published specifications can vary as much as +/- 10% on brand new, out of the box coils.

[azazzera]

Dose any one know what or were I could find the specifications for the mmi coil?
Should we not know the published numbers for a starting point. 10% +\_ is understood but we should know the published numbers to know if it’s even further out than that.

[azazzera]

I don’t know if it’s just me but the rule of 2011 link comes back to file not found.

[ndutton]

Some links suffered server change malaise. The Rule of 2011 is simple, keep your electronic ignition circuit below 4 amps and go sailing with confidence. To achieve that use our ballast resistor calculator found in this post (the second time it has been referenced in this thread BTW), input your measured coil resistance and your measured coil input voltage at 1000+ RPM, the calculator will return a supplemental ballast resistor value if one is indicated. Simple as that.

Back to coil specifications, I recommend making your own for future comparison. When I buy a coil I measure the primary resistance before installation and record it in an engine log. The resistance can be just as well written on a blank address label and affixed to the coil itself. When doing future preventative maintenance a measurement can be taken and compared. You'll know if your coil has internal changes (changes = not good).

Buying from Moyer will get you the right coil but it should still be measured and the Rule of 2011 calculation performed. If you prefer buying a coil from the local auto parts boutique, there are three basic canister coil classes: 3Ω, 1½Ω and 3/4Ω. You want the 3Ω. Oil filled is preferred over epoxy filled and is recommended to be installed with the wire posts on top (as opposed to horizontally).

I have a Moyer coil on my boat engine (4.3Ω), coil input voltage is 14.0V. Plugging those numbers into the calculator returns a negative resistor value and the note "No resistor required". Calculated ignition circuit current is 3.26 amps.

My spare engine has a Flamethrower® coil, spec is 3.0Ω, actual measured resistance is 3.3Ω, input voltage is 14.0V. The calculator returns "Recommended resistor 0.8 ohms". I added a NAPA Echlin ICR 5500 ballast resistor ahead of the coil, 0.885Ω. Calculated ignition circuit current is 3.35 amps. Please notice that relying on the coil specification would not provide a good baseline for future comparison, that's in part why I advocate measuring for yourself.

[sastanley]

Neil has provided great real world examples in the above post. All of that old discussion is still extremely valid, and I finally did the same as Neil..bought an MMI coil for the boat (if you find an equivalent somewhere else and are happy, good for you), but the MMI coil is good to go for 98% (Shawn's statistic) of engine applications on this forum.

If you really want to get geeky or have an out of the norm situation, the threads that are linked are great reading, and we do not mind (well I don't mind) discussing them again. I went thru a world of hurt fiddling around, but the basics of this thread and what Neil provided are the short and sweet of it that should serve you well on coils.

edit - I'll throw this out again for future readers. The introduction of electronic ignition makes you forget that the timing advance weights under the EI plate still require periodic servicing. When we all did new points/condersers every season we lubed/cleaned those weights up too..with EI, you forget. I have had odd stumbles while running (just a few weeks ago) where the engine stumbled like a coil break down, and I remembered I had not cleaned/oiled the weights in a while!! - just some more stuff to think about .

[ndutton]

Thanks Shawn. . . . . .

Geeky???

[sastanley]

geekie? I dunno, Neil. My sphelling is not that good. My Moyer coil I bought 3 years ago just ran for 4+ hours and burned thru 1/4 tank of fuel last weekend at 175°F FWC without a single hiccup.

Just trying to pass the love along to other A4 owners.

[azazzera]

thank you all. I do deeply appreciate this forum for all I have learned. and how it keeps to the mission of education and how nice it is NOT to have peoples personal social political thoughts expressed, ever. Just sailing/engine talk. Constantly amazed at that.

Ok, out at Greenport today, just saw the Viking ship Draken Harald Harfagre. Very impressive. Leaving today at 5pm. I encourage any one that can to come out and see to do so. Makes the mind drift off to different times.