Ignition/coil Science... Someone school me...

[nstueve]

OK so after reading many lengthy threads about coils and ign modules I still don't understand why a person couldn't interchange more in between brands... Seems like some ignition systems are mixed and matched, and I want to understand how, when and why I can/should mix and match parts better between saws I have sitting around.

As I see it, you need spark at the top of the rotation which is something that could be supplied by any flywheel and ign combo (some times originally paired together and sometimes not???). So what keeps guys from interchanging say... Husky and Stihl ignition systems onto older saws that have high failure rates in the coils? Here are some more pointed questions for our saw geniuses...

1.) when can you interchange a coil and flywheel of different make and origin? ie: what needs to be the same to make a a coil and flywheel work together?

2.) If a flywheels mass/weight is different than OEM spec one; does that throw off the rest of the saw mechanics? Or does the flywheel just have to match the coil?

3.) Is it the inter-changeability of flywheels & starters, and having to engineer a custom starter that poses the biggest problem to swapping back and fourth?

4.) when is it best or easiest to swap in different flywheels or coils? limited vs unlimited for tuning, older saws with high coil failure rates... etc etc... Please ramble about whatever comes to mind... I'll have the chance to learn more!

Sorry if these are all dumb questions but a guys gotta learn... And after reading 50+ pages of the treads I still can't find the answers I want. TIA

 

[exSW]

A long time ago(the 80's)I ran a 5.0 Mustang with Jacobs Ignition components.Dr Jacobs also wrote a book called the "DR's guide to Optimizing your ignition".If you could find a copy on Ebay it would be worth your time.Made my understanding of spark and everything related a lot clearer.Dr Christopher Jacobs is the guy who invented the MSD 6 ignition(and then licenced the technology to MSD)

 

[zogger]

Proly more to do with how everything bolts up and fits and how it engages with the starter system. The actual moving electrons stuff is way more similar than not I would think, just the mechanics are different, ie, why won't this ferd part work on this chebby? Well, it might, but need to change bolt holes and do some cuttin and weldin, etc. Depends on how much you want to work on stuff I guess.

 

[exSW]

Proly more to do with how everything bolts up and fits and how it engages with the starter system. The actual moving electrons stuff is way more similar than not I would think, just the mechanics are different, ie, why won't this ferd part work on this chebby? Well, it might, but need to change bolt holes and do some cuttin and weldin, etc. Depends on how much you want to work on stuff I guess.

That's exactly what that book talks about.The big threes ignitions of that era were nearly identical except for packageing.The therory behind spark,timing,combustion were all covered in depth and in an understanable manner.

 

[spike60]

Hey, they aren't dumb questions. We're all here to share.

Most common obstacle would be coils that simply wouldn't fit in the case. Flywheels might fit occasionally. But they are weighted and balanced for the clutch and stuff on the other end. Too far out of spec and the engine will be running out of balance.

There are very few options for updating ignitions on older saws. In many cases, we're lucky to find original stuff let alone something that works better. Only one that comes to mind is there are aftermarket coils that allow you to convert an older Homelite XL-12 from points to solid state. IF you can also locate the different coil mounting plate.

For the most part, we just have to live with what's out there.

 

[workshop]

Some of the older stuff you could interchange coils and such, but that was due to the fact that they used the same parts. Newer stuff, I would say, is much more limited. Yes some things could be made to work. The question in my mind is why? I think you would do better to learn and understand basic electrical theory and and how it applies to the ignition system. Most any bookstore or library would have lots of books on the subject.

 

[heimannm]

There are some relatively critical timing issues that must work out properly. The spark does not in fact occur at TDC but rather some ways before TDC. Most older Macs with points ignition were 26 degrees BTDC. The location of the magnets on the flywheel, the location of the coil in relation to the magnets in the flywheel, even the spacing of the magnets (distance between the poles) will have an impact on when or if the spark will occur.

If that were not enough, most modern electronic ignition coils/modules have advance curves, rev limiters, even low RPM cutoff (won't spark at low RPM to prevent it from kicking the engine backwards when starting). A coil designed for a large displacement low RPM saw would not drop in to a small displacement high RPM saw and work properly.

Mark

 

[nstueve]

Maybe I need a makita in signature

Yes you need one... Everyone needs a smurf!

As to the origional topic. I was just disheartened this week looking at all the saws that I have that are DOA because of ign problems. Mark, I do realize actual spark timing is BTDC... But reading more about the poles and magnet spacing helps understanding more.

Which leads to the question why does stihl have the same coil for 024-066?

 

[moody]

Cheaper pproduction and similar design wwould be my guess.