reasons a CDI ignition is needed

[Barry Donovan]

I found some examples here from scott, bbm..
any others?

with HEI being modern since the 70s, not much reason is explained.

I found from ads, helps cold starts, and lower rpm range when air is slow, fuel has more chances for mistakes.

also if a stroke is small enough .. induction as a regular standard is so very slow. Motorcycles can't do a car setup.. I have a car with a 2.65 inch stroke. I literally looked at motorcycle ignitions before going with accel 300+.

I have all the mistakes with a liquid boxer, and a 43mm carb. 3k rpm seems to be a leap in stoichial related power..all while never going into 20s mpg (low to high 30s at all times).
this indicates ignition problem. mileage is good, and fire just is not right at lower rpms.


is there any other important reasons? I did find it can take away concepts of engine retarding by oems (always good),

speed up induction covers alot of facts.

[Matt Cramer]

The biggest reason I can think of is if a distributorless ignition isn't in the budget and you're making a ton of cylinder pressure.

A lot of motorcycles with CDI don't have very powerful alternators, and you can drive a CDI straight off a high voltage source at the stator.

Overall, not the biggest CDI fan myself, but it's got its uses.

[JeffMcKC]

The leaner you get the harder it is to fire.

[dieselgeek]

I'm not so sure about the idea that "leaner = harder to fire" based on some testing I did trying out lean mixtures on the dyno recently. But hey, maybe we were doing it wrong.

The whole trick to CDI ignition is that they make a far shorter duration spark than an Inductive ignition. The CDI spark doesn't last very long. At low RPMs, sometimes this short spark is not enough to light the mixture properly - and that's why they came up with "multiple spark discharge" - not because it makes the CDI better than the IDI, but because without it the CDI can cause a crappy idle when mixtures aren't as good at low RPMs.

CDI is good for firing poor mixtures. But if your tune is in check, there's a benefit to having the longer inductive spark especially in turbo and supercharged engines.

I wish I took video of the Ignition Test system that Lance Nist runs at Pantera. It's pretty interesting to see which ignitions get off the bus and when, as you increase pressure in the test chamber.

[Barry Donovan]

found some more reasons, the above are quite correct.

Adding up reasons in a summary for CDI is

• stop run on bleed (so the off is OFF)
• short stroke was too short for the fat slow elongated weak induction
• 3750 rpm in 5th gear is the cars limit…with no computer, this indicates ignition..plenty of power locked up on my build in the guts.
• 9000 rpm dead rev needs a limiter
• a chance for some diodes to really guide this around peacefully
• keep the stainless pipes from echoing sounds of a mig welder (again…ignition is too long on the dwell)
• avoid lightning (recorded a hit while driving)
• save the hood edge from disintegretaing the steel (lit)..along with all other panels replaced.

This could help battery and light bulbs..but the battery goes in 5 year cycles, never errored.

[TheSilverBuick]

•avoid lightning (recorded a hit while driving)

I don't need a CDI box to let me know to avoid lightning

[BigBlockMopar]

Here's a nice write-up I found on ignitions and such;

http://www.gill.co.uk/products/digital_ignition/introduction/spark_ign_index.asp