Performance comparison between jag and stock CDI

How interested would people be in a hard number, objectified comparison between the stock CDI and the Jaguar CDI? I see people constantly knocking it, claiming any gain is a placebo effect, and some people praise it. I'll try to get as much data as I can, my jag has 4 jumpers for a total of 9 settings, I just recently got ahold of some jumpers that I hope fit. If there's interest in this by tomorrow after work (around 17 hours from the time of this post), I'll do some testing. Only objectified numbers will be posted (and if people want, subjective vibration ratings) Please post any additional things I should try to look for when comparing before I go about the tests. I hate misinformation so I want this to be the end-all argument about the CDI. Of course your motors will differ from mine (as all HT's will) but being run on the same bike, through the same tests, should provide a good enough baseline as to which is better, and who's right. I will update this first post and bump when the results are posted.

If you want "spark strength" I do have access to a digital volt meter, but I have no idea where to test (or even what setting to use... I've never used one). In theory I could do the last 4 tests all at once.

So Stock CDI vs Jaguar CDI (all 9 settings)

tests:
Hill climb time and speed (20kph run at it (to remove human clutch feathering bias), base to crest, 1/4 mile)
Top Speed (taken over 5 blocks ~1/2 mile, both ways averaged)
Heat (WOT run, taken immediately after top speed test)
0-60kph time (No pedaling)
Vibrations (I might be able to find something for my phone, but it may not pick up the bike vibes)

warping rims is usually from too much power twisting and loosening the spokes. Sadly my plans for testing today have to be put off, wind storm coming in from the south and knocked my bike over in the bike rack, bending the front tire (and the wind gusts are near constant and enough to take my speed down from 40k to 20k). I'll get around to it soon, dont worry, I want to know if I should buy the CDI again for my next build.

Unfortunately this wont tell you which setting to use, only me. The point is to see if theres a setting that provides more speed than the stock cdi. Your settings will differ due to timing and parts differences

looks like the weather's finally cleared up in my area, the storm I got apparently flooded alot of Alberta (some of Calgary had to be evacuated, crazy) and a lake just north of me is going to be flooded as some rivers north of that had to open flood gates. I'll see if I can get some testing in today

Seems I've lost some of my top end, I think I didn't put the right jet back in (and for some reason my REAL #62 dellorto jet came apart [destroyed] when I tried to remove it, almost had to drill it out) when I was trying to get the bike running again. top speed is now 55kph before it fourstrokes and wont go past that point (even downhill) so I'm going to try and get the bike running up to 60kph again before I test. Acceleration tests flat-out are a good way to measure the low end as are hill climbs and I cant find an app for my phone that will measure a 0-30kph time. I promise I will get around to this very soon, but only once I can get the bike running to my specs.

Dave31 said:

I dont get it? my motor idles at 1500-2000 how can they only max out at 5500? One of these days I've got to set my camera on my tach. The highest I have twisted my motor 9800.


http://www.youtube.com/watch?v=DGZaXllrtSQ

Click to expand...

Mine revs to 7300 no problem, I'd say 6500-7000 is the upper limit for a stock 66cc, the 48cc can rev up to like 7500-8000. But yes, with added power (ESPECIALLY added compression), the heat gets pretty insane and advancing ignition adds heat. While it is good for snappy performance, it's not very good for the two strokes. 4-strokes can do it because the flow turbulence they experience is pretty constant. 2 strokes experience more turbulence from higher RPM's. I cant quite remember the physics behind it but igniting them earlier and earlier adds more heat, and makes it harder for the piston to push against (you're aiming for a complete burn around 20* ATDC for max power), but can only go so far as well, plus the extra force the piston deals with means your rod bearings are taking a much worse beating. retarding the ignition will reduce power at high RPM's but will net you MORE because it doesnt have to fight the gases as much, and heat isn't as big of an issue. So yes, for max power, the 4 stroke (stock) CDI is best, but at consistent high RPM's will damage the motor (this is why the racers use it, you want max power all the time and the motor is expendable). For every day users like me, you dont want that, you want reliability, cool running, motors. You dont need a ton of power at the top end, and if you do, modifying the motor itself (expansion chamber, port timing, etc.) will net you better long-term results than a CDI that beats it to death.

But if you feel the jag CDI is too pricey, you can build your own and buy an ignition coil for about $30-$40. For me the $80 is iffy, but considering I run WOT all day every day for the last year and the motor barely gets hot enough to boil water, I'm quite happy with it. It's the shipping that kills me because I'm in canada (so it comes to a total cost of around $125). Worth it? Maybe, I have no idea how long the motor would have lasted with the stock CDI, it's never been on my new bike, but if it's saved me $150 on getting a whole new motor, then by all means, the price is justified

Huffydavidson said:

did I miss something where are the test results on the Jags vs the stock CDI ?

Click to expand...

Unless I can find an app for my phone that will measure 0-30kph time, I'm waiting on an expansion pipe so I can hit 60kph consistently.

However yes, the retarding will weaken the motor a bit at high RPM's, thats why racers dont use it. However the retarding does SIGNIFICANTLY reduce vibrations (I discovered this when I figured out I was playing with the wrong jumpers), my motor's ridiculously smooth now (this whole time I thought the vibrations were normal). And it saves the bearings by reducing the sudden forces and vibrations. In racing, this isn't required, a motor is replacable, so max power is preferred. however those of us with limited income dont have the option of replacing parts every ride, so longevity is important, as well as it keeps the motor cooler (I havent had a chance to check temps since adjusting the jumpers though).

So in a nutshell: any power gains are, as people expected: a placebo effect. The jag CDI will reduce power UNLESS the timing was horrible to begin with. The jag does, however produce a better spark and retard timing for longevity (and maybe a few extra mph if it's rev-limited, retarding will allow a couple hundred extra RPM). It's not just listed on Jag's site but Jenning's book and virtually every 2-stroke book. This is ridiculously important for people running high compression as well, the stronger spark wont 'blow out' and it'll keep the motor a bit cooler.

come to think of it, I have a fairly quick motor as it is, could be used for testing. But I dont have a lightning CDI.

That said, IMHO if you're experiencing a boost in power in your low range from a different CDI (Jag or Lightning), it's not a testament to the CDI, it's a testament to the shoddy quality control of these engines (or you modded it and the spark timing is off now). From what I gather, the spark has no bearing on flame speed. It simply gives you a more reliable spark under compression, and an adjustable timing curve. The adjustable timing lets you find where the optimal point is, but the flame does not care how big and bad your spark is, it is only a point where the flame can start. That said, being centrally located (single plug) is the best choice, thats why straight-fire heads tend to produce more power, the plug is in the center, producing the most even, short distance all-round burn. Twin plugs produce more power simply because there's even less distance to complete a burn, but it doesnt add *much*.

Basically an engine (2 or 4 stroke) is powered by the expansion of gases, the conversion of gasoline and air into CO2 and other exhaust gases. Exhaust gases want to occupy considerably more room than oxygen and gasoline droplets (this is how Dynamite works too, it's not the flame that's deadly, it's the conversion of the materials into CO2 so rapidly that it produces a shockwave). This forces the piston downward. The faster you can make that flame burn through the chamber, the more power you will achieve. This can be done with more turbulence (better mixing), higher pressures (compression), ignition point (shortest distance, depends on if you run 1 or 2 plugs. 1 plug in the center for shortest distance to all sides), Air/fuel ratio (14.7:1 by weight burns the fastest, too much or too little will leave unburned oxygen or fuel), Volumetric efficiency (this includes the carb, manifolds, exhaust, ports/valves, transfers, etc. Basically it's all about how much you can pack into that cylinder, at the proper AFR, without losing any out the exhaust or carb. Remember: the more gas and air you can pack in, the more room those exhaust gases want to occupy, and the harder and faster they'll push on that piston). This is why performance mods will change where your spark timing needs to be.

So why does spark timing affect your power if it doesn't affect your burn rate at all? Because once all your fuel has burned up, you've reached maximum pressure. If you're too advanced spark or even detonating, your peak pressure is going to want to push your piston back down, while it wants to go up (compressing). Because compression hasn't been completed yet, the burn rate is slightly slower, so you have a bit of play room. However you're losing a ton of power (pumping losses) because you've reached peak pressure before the piston has hit TDC, while it is going to push just as hard as if it were fired at the right time, the fact that it was pushing the piston back down before it was supposed to, is where you're losing your power. Retarding too much means the burn hasn't fully completed as the piston is already on its way down. Optimum point is about 40 degrees ATDC for a complete burn. 2 strokes need to retard at high RPM because the high speeds increase turbulence, meaning better mixing, meaning a faster burn speed, meaning the spark needs to be later. But I digress, if you've ever tried pushing someone on a swing, you can feel maximum push as they're coming to a stop at their peak height, or when they're coming toward you. But you'll notice that it becomes increasingly difficult to push them faster and higher when they're going the same direction you're pushing. It's the safest way? Yeah, all that weight moving weight could throw you off easily, or you're going to waste all your power slowing them down coming back. Consider the peak rear height as TDC, obviously you can inject the most power at that point, doing it half way through is possible, but doesn't add much. The burn rate is still a finite, calculated number, just as your pushing strength is.