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Old 05-04-2016, 02:57 AM
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exokinetic exokinetic is offline
Motorized Bicycle Elite Member
Join Date: Mar 2016
Location: Lake Forest, CA
Posts: 108
Default Re: Would using this really work?

That looks exactly like a Rocket/HD Lightning CDI (pill bottle CDI). It's copied by many people and sold under many names.

Most people with a "stock" engine see a performance drop with the "performance" CDI's. Or, conversely, anyone with a modified engine, that has not gone very high on the compression.

From my experience with the HD Lightning CDI, it is designed to start retarding the timing as the engine reaches high RPM's.

If you build an engine with very high compression, it will start too loose power at higher RPM's with a stock CDI, as the ignition is too advanced. By retarding the ignition at higher RPM's this CDI allows the use of very high compression (in order to boost low end torque) while preserving top end run out ability.

With this CDI, I run my compression high enough to hit 240psi on a compression test, and the engine (when jetted properly) will still run out to 10,500 RPM's.

But a stock engine actually REQUIRES the extra ignition timing at higher RPM's, because its compression is (relatively) so low.

Here is a thread where Venice Motor Bikes did some investigative analysis of the different "performance" CDI's on the market:

This basically shows how much timing advance each CDI has @3000 RPM's. It seems the CDI's that have less advance @3000 RPM's are beginning the advance curve later, so that they achieve the max 25-30 degrees advance at a higher rpm. So, in essence, the CDI's are not retarding the advance curve at higher RPM's (ideally around 7,500 RPM's to coincide with peak HP), they are just starting the advance curve later, allowing the engine to hit a higher RPM before the total 25-30 degrees of advance becomes too much, and it acts like a rev limiter.

This shows that the "performance" CDI's have less initial advance, witch is not good for a low compression stock engine. It does help high compression engines reach a slightly higher RPM due to the later advance curve.

What would ultimately be best is a CDI that started with a high initial advance, near the total 25-30 degrees, and then begin to retard timing very gradually starting at around 3000 RPM's until the power peak rpm of the engine (7,500 RPM's for my race engine builds), then start pulling timing out more aggressively to allow the engine to maintain efficiency (and thus power) much further past its peak power RPM.

I have found one that meets this exact criteria, and it looks like it is a direct fit for our ignition setup, kill switch and everything.

It is a Yamaha PW80 CDI:

This does not require a battery or a timing pickup like most dirt bike/moped CDI's do. This makes it perfectly compatible with our set-up.

I will be testing these on my race bikes in the near future and will post the results.

Hope that helps!

Don't hesitate to ask me anything!
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