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I recently cared enough to look beyond what I read here about setting up coil dwell tables. I'm a pretty average dude who sucks at math and I could be dead wrong.
I did this based on cop config. Am I full of ****?
So the formula is
Dwell/cycle @rpm
In my case my coil is rated for 9ms continuously at 40% duty
Cycl is time between ignition events
Last edited by mikey D; 07-19-19 at 08:28 PM.
Reason: making a readable table
I'm not factoring in voltage correction.
The coil doesn't care about map or tps when it comes to it's own capabilities correct? My numbers are just literally what dwell can be achieved at a given rpm on a rotary and stay at the rated duty cycle.
I could add the spark duration in.
-x to each dwell which brings up the fact that you have to exceed the rated duty cycle at high rpm.
Why would you run the coil over it's rated duty cycle for an extended period? Just trying to wrap my head around a new piece of info.
Originally Posted by RGHTBrainDesign
Yes, you're wrong. Where is your spark duration in that formula?
You should also be setting up dwell as MAP or TPS vs. RPM vs. Voltage.
at 9000 RPM you actually have 80% Duty Cycle to use 3ms of coil dwell...
I'm not factoring in voltage correction.
The coil doesn't care about map or tps when it comes to it's own capabilities correct? My numbers are just literally what dwell can be achieved at a given rpm on a rotary and stay at the rated duty cycle.
I could add the spark duration in.
-x to each dwell which brings up the fact that you have to exceed the rated duty cycle at high rpm.
Why would you run the coil over it's rated duty cycle for an extended period? Just trying to wrap my head around a new piece of info.
You are simply not understanding how the charging/discharging events are happening and that's why your formula is wrong.
You wouldn't want the same coil dwell if your battery was at 8v vs. 14v.
You also wouldn't want the same dwell if your engine is at cruise around 60kPa vs. WOT at 250kPa. Unnecessary wear on spark plugs and coils.
MAP or TPS can be swapped as your "load" in this scenario.
You NEED to factor in Spark Duration since you're counting everything as combustion time. Do you think that simply comes out of nowhere as "free time"?
Essentially what you said was that discharge time was not necessary. It's absolutely necessary and that's why CDI is the answer for any high horsepower car over 9000 RPM.
Your misunderstanding me completely. I never said duration was not important, I said I need to add it.
In fact I don't disagree with anything you just said.
However if you assume 14v and use dwell/(cycle - duration) @ rpm
Is it accurate to say that formula could be used to figure out what the duty cycle a coil would be ay a given point?
So this calculated table is for an end-all answer to an old debate of the IGN-1A Coil being a good choice for a Rotary Engine. Here are some notes as to what I did.
Q: Why did you subtract 2.9ms?
A: Spark Duration is 2.9ms +/- 10% on these coils. So unless you want to ignite the next homogenous charge, you better subtract that value as to have ZERO chance of doing so.
Q: What's up with the 25%, 40%, 60%, and 80% Duty Cycles?
A: I like to alter coil dwell by both RPM and MAP. Under low boost I would think 40-60% DC is fine depending on RPM. Low load and cruise I'd want the coils to be cold since they don't have to break through so much air and fuel. Under HIGH loads, I'm aiming between 60-80% DC.
Personally, if you're over 8K RPM on a high boost application, CDI is hands down, the way to go.
If you're locking your coil dwell at 3ms, you shouldn't rev over 9K RPM.
On track setups where you have LONG durations of WOT, you're absolutely going to fry these suckers in a hot engine bay. Pay attention to placement and provide cooling passages.
Keep sharing... We have a niche community full of a LOT of wisdom. Don't be swayed by bullshit or marketing. Do the math, enjoy the design process, and never stop pushing the envelope.
Last edited by RGHTBrainDesign; 07-20-19 at 03:08 PM.
I dont really agree that the duration should be included but otherwise it seems like you agree that the equation is correct. thanks
I did come across some info from some well known rotary tuners that my thoughts more or less lined up with. I had a brain fart on the duty cycle thing, which is funny because I found an old post where you had the same exact brain fart, I dont feel so bad now.
I was aware of your chart, but I wanted to go though the process myself and make sure I understood what was going on so I (and maybe some others) would be able to use the formula and apply it to whatever it was they were using for coils or even a different engine entirely.
Anyways I applied things to a load/rpm axis table, nothing melted or got hot.
Im not sure there was a noticeable change going from a flat 3ms dwell time to something a little more aggressive.. I plan on spending a day on the dyno in October. Ill share the results.
Your calculation is correct for duty cycle. I agree that spark duration exists and should be taken into consideration, but the term 'duty cycle' has a definition among the math and physics communities and your calculation matches that definition. I've heard two trains of thought regarding coil dwell. One is to use as the least amount you can get away with, to keep the coils as cool as possible as long as there aren't noticeable misfires. The other philosophy is to find the dwell time that gets the max energy from the coil so you're never 'leaving spark energy on the table'.
For what I've seen, most OEM ECUs are doing a constant coil charge time (for instance a 2.5ms pulse for all RPM) and sometimes decreasing at high RPM. They are usually leaving some spark energy on the table, at least 0.5ms - 1.0ms below the charge time that would result in maximum energy.
07-19-19, 04:32 PM
