Coil Dwell Time

[Red 90]

Can someone tell me how to calculate the coil dwell time. 

At 14V on my miata the dwell vs rpm is at 100.  The voltage to % is at 50% and the coil dwell factor is at 35.

Does this mean my dwell time is 100x0.5x35=1750microseconds or 1.7ms

[Red 90]

I just logged my dwell time and it's showing 3.5ms.  How does this relate to the graphs.  It would seem like it's the dwell vs rpm table value X the Coil dwell factor.  But if that's the case, what does the Voltage to % table do.

I wish AEM would have better documentation.  Seems like such a simple thing, but can't find any information anywhere.

[SB]

(Disclaimer: to avoid damaging ignition components and/or your EMS, AEM suggests that users utilize the Coil Dwell Wizard. Be sure to change Coil Dwell settings in very small increments, and verify changes with an oscilloscope.)


Red 90, your formula is almost perfect. However, the Coil Dwell Factor is just a unitless number, so a programmer somewhere decided to multiply it by 2 in the calculation. I spent some time working on Coil Dwell settings today, and verified these values with an oscilloscope.

Since there are a few factors being multiplied, there are a few different ways to achieve the same solution. Here's how I've been doing it, using a 1993 RX-7 as an example.


Here is some data that I gathered from the stock ECU:
RPM  --  Dwell (ms) --  Duty Cycle (%):
2000 -- 3.0 -- 20
3500 -- 2.9 -- 33
5500 -- 2.7 -- 50
7000 -- 2.6 -- 60
7500 -- 2.4 -- 62

Notice the dwell time is decreasing, which will decrease the amount of spark energy (which isn't desirable in most cases). This is to avoid overheating the coils from having an excessively-high duty cycle.


The Dwell Max sets the max duty cycle allowed, regardless of other settings.
Set this to (Max Duty Cycle Desired * Number of Crank Teeth Per Ignition Event)
For instance, the RX-7 uses a 12-tooth crank sensor, and is firing the coils every 6 crank teeth, so in order to limit the maximum Duty Cycle to 66%, I set Dwell Max to 4  (4 = 0.66 * 6)  Setting this to 3 would yield a 50% duty cycle.

The Dwell Min sets the minimum duty cycle, regardless of other settings. You could perform a similar set of calculations here, but I prefer to set this to a low value (0.1-0.5) and use other settings to decide the minimum dwell time.

Ignition Charge Time = 0.02 * Coil Dwell Factor * (Dwell vs Batt Volts value) * (Dwell vs RPM value)

For instance, here are my table values at 2000 RPM:
Dwell Factor: 30
Dwell vs Batt Volts: 50
Dwell vs RPM: 100


This results in a 3.0ms coil charge time:
3000 us (3.0 ms)  = 0.02 * 30 * 100 * 50


Hope this helps.

[Red 90]

SB,

Thanks that makes a lot of sense.  I wish AEM would have provided better documentation to lets us know about how these are calculated.  How would anyone know there's a additional factor of 2 mulitplied by it.

[scottie1134]

Why not have a box that you could say, input, 2.5 ms dwell. And that's that. No calculation. Just a simple number to input. Why this would not work on inductive type ignitions, not very many people in the high performance car market are using inductive ignitions. I think most people are running msd or equivalent ignitions. So having a box to input "2.5 ms" would make it a lot simpler. At least in my simple mind.

[SB]

scottie1134 ,

If you want to set the dwell to 2.5ms, use these settings:

Dwell Factor: 25
Dwell vs Batt Volts: 50  (across the entire table)
Dwell vs RPM: 100  (across the entire table)
Dwell min: 0.1-0.5
Dwell Max: depends on number of ignition teeth  (see my post above for more info)




SB

[Red 90]

You can set your coil dwell time constant as SB has put in his post.  However, with inductive discharge systems, it is important to have the voltage compensation on the dwell time.  You'll typically need more dwell time with lower voltage and less with higher voltage.

[scottie1134]

That was my mistake on how I worded the second part of my post. I wasn't asking a question as to "why" this wouldn't work with inductive type ignitions. It was a statement saying, this will not work for inductive type ignitions. Either way, maybe someone will learn something new from this post about inductive and capacitive ignitions. As I do get that question a lot.
And my vote is still for a box to input a # for coil dwell.
Peace,

[Red 90]

Sorry misread your post.  Your right on capacitive discharge type ignitions, it would not be as big of an issue.  It applys mainly for inductive type ignitions.  However, capacitive discharge ignitions are not necessarily better than inductive ignitions.  They are different with different firing characteristics.

[lazzer408]

scottie1134 ,

If you want to set the dwell to 2.5ms, use these settings:

Dwell Factor: 25
Dwell vs Batt Volts: 50  (across the entire table)
Dwell vs RPM: 100  (across the entire table)
Dwell min: 0.1-0.5
Dwell Max: depends on number of ignition teeth  (see my post above for more info)


SB

I think his question is: Why isn't there just a map with time values across it? Why is all this math involved in calculating the time? The 'computer' is supposed to do the computing for us.

[SB]

That does sound like a good idea for AEMTuner: make the coil dwell settings a bit more intuitive.

In the meantime, I hope this explanation helps people use the software that is already in circulation.