Using a single LS coil and distributor, does my math look right?

Hi everyone,

I'm looking for some thoughts on using a single LS coil in a distributor ignition. I have implemented this a handful of times on various engines and have yielded great results with respect to cost, power and drivability. However, after speaking to an ignition manufacturer in Australia I was quickly told this is absolutely a no go...this made me think that even though this has worked, maybe there's power and spark efficiency being left on the table. Similar to the HPA CDI ignition podcast about the drag methanol engines. So I wanted to double check this before buying a scope to test and validate.

After running the numbers I'm a little confused on which equation is right so, here's how I ran the numbers quickly:

Equation 1 -

S Sparks per sec = (RPM X 0.5)/60

T Time between ignition events = 1/S

For a 10,000RPM limit, this evaluates time between ignition events as 12ms

Therefore as long as T is greater than the coil dwell , there should be no dramas?

Equation 2:

S Sparks per sec = (RPM X No of Cyls X 0.5)/60

For a 10,000RPM limit and 8 cylinder engine, Time between ignition events would be 1.5ms, therefore with a LS coil dwell of say 3.5ms, this would result in a loss of spark energy.

I would have expected to experience issues on dyno, such as misfires, poor drivability, rich running etc if equation 2 is right, but from my physical testing so far all the cars have appeared to work fine.

Another consideration is the outright energy output of an LS coil, compared to say a MEC716 or HEC716 which I see commonly in old distributor ignition systems used in conjunction with an ignition module. Does anyone have further specs on LS coils regarding the energy output?

Hopefully this isn't a silly question, if so I apologize in advance!

Thanks!

I think the issue is that those coils have a max duty cycle of 40% (80% for less than 5 seconds with sufficient cooling). Not a problem with firing one cylinder, and big problem using it at high RPM with a distributor.

People have cooked them running highish dwell on sequential rotary applications which is only one spark per 6.7ms at revs.

If it's a turbo or high compression alcohol engine that genuinely revs to around 10,000rpm and you are set on using the distributor I'd be looking at CDI myself. Especially for circuit, maribe, aviation or drift applications.

It also depends on the LS Coil being used, there is a large variance in the dwell times needed for different versions, to same as with the IGN-1A Mercury coils. Some of the LS coils that I have tested require 5~6ms of dwell to get to 80% capacity.

Another important consideration that is missed from your math is spark duration or "burn time". This becomes a big part of the limitation of a distributed inductive ignition system, especially with 6 or more cylinders or high RPM 4 cyl.

For successful combustion the spark needs to continue burning until all the HC around the plug has been burnt, this is usually around 1.5-2mS. If you start dwelling the coil before the burn is complete you will instantly extinguish the spark. The bare minimum spark duration that will support combustion is highly variable depending on cyl pressure and air-fuel mixture etc but is often quoted around 0.8mS.

So, to update your math with an example of how dire that makes things look;

For a 4 stroke engine at 6000RPM your cycle time is 20mS, if we say this is an 8-cylinder engine, then that means we need a spark every 2.5mS. If each spark event has a minimum burn time of 1.0mS, then we only have 1.5mS left for dwell...

Surprisingly though, with mild old-school V8 engines, this is often still enough - the main reason is the VE has usually exited the building long before 6000RPM so the cyl pressure by then is low so not much spark energy is needed. They never spend much time at 6000RPM so the high duty cycle is not usually a problem either. If you are using an ecu then you also may have the ability to run more dwell at the low RPM torque peak, then reduce dwell at higher RPM or factor in spark duration to keep an acceptable burn time, so this gives an advantage compared to old points or other fixed dwell systems.

However, once you get something a bit hotter than mild or need more than about 6000RPM then things get more challenging - this is why MSD/CDI and 16V systems are still so common in the basic V8 drag racing world. Also why you used to see twin distributors on performance V8's and rotaries and all sorts of weird twin coil arrangements on V12's etc back in the "olden days" before direct ignition became easy or affordable.

Thank you everyone for your replies! There's some golden information here. @Adam - The burn time is something I will definitely look into further, if you have any papers or links you might recommend to dive into further that would be great. I will add burn time into my spreadsheet so I can account for that in the future.

Regarding the dwell, I'll build a duty cycle spreadsheet to ensure I don't cook the coil and look at making adjustments as you suggested, and maybe get a data sheet for the current coil as Stephen's suggested to validate the variant of LS coil.

I've been looking at other coils to test (if they're suitable) and do some back to back comparison runs...Racegrade IGN1A or as Stephen mentioned a Mercury or equivalent? Failing that I could look at an old barrel style coil and run an ignition module however, I don't see why a modern smart coil can't be used keeping failure points to a single component rather than multiple...

Burntime is one of the common metrics in a secondary ignition waveform that you look at for misfire diagnostics. If you search "secondary ignition waveform" you will likely find some better explanations than I can give in a forum post.

The genuine "IGN1A" is based on a mercury marine coil, but with a couple of changes to give potentially increased energy over the original mercury part, I believe part of the ignitor was beefed up to give it a little more current headroom, and I vaguely remember Lance saying he changed or moved the rare earth magnet that is part of the core as well. The mercury coil was originally designed to replace a CDI system on a 2 stroke engine, so it was designed specifically to give good energy with short dwell times, which fits quite nicely with your application. The unknown is how well they will cope with the high duty cycle.

Im pretty sure there are many companies selling coils they label or refer to as "IGN1A" that arent actually genuine IGN1A's, so that may be a part of Stephen's comment. That's not to say all the non-genuine ones are bad, just I think that is where the perceived variation is coming from.