Let's talk about GY6 CDIs: Rev limits, timing... what is true?

[Travis @ Buggy Depot]

CDI misinformation in the GY6 market is a big problem.
I want to talk about a few myths pushed by sellers. Some of these are stretches, others are just outright wrong.

First... no! There is NO stock rev limiter on stock GY6 CDIs.
I've tested several stock and performance 150cc CDIs and found that they will produce stable spark to a maximum of 30,000RPM. At that point spark begins to attenuate, and weakens abruptly between 29,000 and 30,000rpm right before becoming erratic. This is due to the physics involved and electrical characteristics of the internal components inside. Not due to any intentional rev limiter function.

The 6,000RPM limit claim is completely false!
Let's call it what it is. The 6k rev limit is a lie. Complete fiction. The 6k limit was made up by eBayers a number of years ago to push sales of "9,000rpm limit" CDI's. Unfortunately this misinformation spread through the forums.

Speaking of 9k limit: Another fiction. If you're reading this and thinking about getting a "9k limited" CDI on eBay, you'd be taking a big step backwards and paying them to rob yourself of performance. There is no stock rev limiter on any stock CDI I've tested. And there most likely isn't a limiter on the eBay CDIs either, just snaky advertising that sells a false story.

CDI sellers claiming to advance beyond 25° BTDC are falsely advertising:
More garbage marketing. Any outfit claiming that their $20 CDI is able to advance timing beyond the stock 25° BTDC is selling a lie. It's not physically possible for the CDI to advance timing on the GY6 beyond 25 degrees BTDC without an internal microprocessor calculating a delayed offset. This would mean to produce a 10° advanced firing event on the GY6 (35° BTDC), you would "retard" timing 350° from the last trigger signal received. This produces a spark event that is truly advanced ahead of the trigger signal on the following firing cycle.

Adjustable "advance" CDIs are actually only capable of retarding timing:
None of the adjustable CDIs that I'm aware of on the market have the complex circuitry and programming described above, and are only able to retard timing, not advance.

Are CDI upgrades worth it then?
With all of that said, I believe that some CDI upgrades on the market are worth the money. Some aftermarket CDIs produce a significantly stronger spark that you can visually see and hear the difference. The most noticeable difference are easier and more reliable starts. But in general, performance benefits of CDIs aren't fully tested.

Just be careful of outfits making sensational claims. The stronger more reliable spark is the greatest benefit that I've been able to test for so far. As far as timing though, at best performance GY6 CDI's lessen the 20° timing retard between 1500 - 3200rpm that is found on some stock CDIs.

One way to truly advance a GY6 CDI:
I've used a 20mhz AVR microprocessor to advance timing by feeding a modified trigger signal to a standard CDI. Take a look at the scope screenshots below. Yellow is filtered trigger signal from the stator, blue is the modified trigger signal delayed 350 degrees to produce an "advance" of 10 degrees before the trigger position on the following cycle.



I was able to get this down to ±1° accuracy on the bench.

[PGH]

From what I can tell, the retarded ignition timing at slower RPM aids in starting the engine. There are some CDI’s that use different timing curves based on motor speed, but I never had a “seat of the pants” difference between them.

[Travis @ Buggy Depot]

From what I can tell, the retarded ignition timing at slower RPM aids in starting the engine.

This was intended to help against kickback when the engine initially starts to turn over. Before the rotating cranking assembly has enough momentum to overcome a firing event set too early before TDC. But... this seems to sacrifice stable idle, and the ability to hold a low idle.

[PGH]

You can change ignition timing through physical means too. I’ve seen people shift the pickup coil on the mount to gain some advance - It will give you few degrees of adjustment within that limited space. The other method is to use an offset crank key that will shift the flywheel a set number of degrees forward or back - There is no level of adjustment other than using a different offset key, but it does give you repeatable results.

[Travis @ Buggy Depot]

True. I used the word "physically" in relation to physics and causality, as there can't be an effect (spark) before the cause (trigger). I was referring to it not being physically possible to advance timing within the CDI itself (without a delayed offset as I described to produce a pseudo-advance on the next cycle).

A microprocessor or other similar logic circuitry would be required for the offset. The problem I'm highlighting is that there are vendors claiming to accomplish this with nothing more than their CDI's internal SCR/triac. That's not gonna happen, their claims are snake oil.