A few corrections. CDI doesn't necessarily erode spark plugs at a higher rate. Mine erodes them at a lower rate than the Kettering system when used with a conventional inductive coil (which is what this CDI is meant to use). Secondly, a CDI will impress upon the coil whatever voltage the capacitor is charged to. That voltage is multiplied by the turns ratio and is also impressed on the coil secondary even if there is a spark. It may be of extremely short duration prior to spark breakdown, but it does exist and will have an impact on insulation life. CDI voltage must be controlled at the source for that reason. Yes, inductive systems are self regulating but will have a voltage overshoot which is always higher if the ignition system is weak. Finally, very few CDIs have higher energy than inductive systems. Usually less energy crosses the spark plug gap. What they do have is much higher power. Fred
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Originally Posted by M&W Ignitions
My company has been designing/manufacturing very high end ignition systems both of the inductive and cdi variety for many decades so we can speak for both sides unlike most critics. We dont generally become involved with online forums due to time constraints however occasionally there is a need to jump in and correct misinformation being spread.
any CDI system... including an M&W 1000mJ unit
The M&W 1000 Max2 was developed for one specific application - top fuel motors using very high percentage of nitromethane.
CDI is old school technology
Well its not as old as inductive ignition, so what does 'old school' have to do with anything? Modern cdi systems like ours function different to early designs such as the MSD 6AL. Using improved circuitry energy transfer effiiency has been raised such an extent you can no longer compare ignition performance simply by using the manufacturers quoted energy numbers.
Most engines don't need more then 30KV, and a CDI outputs more then double that for no good reason.
This is one of the most misquoted misunderstood functions of any ignition system - they do not make 'X' amount of voltage! It is the coil which generates the voltage and the voltage will reach what ever is required for the spark plug gap to become conductive and no more. The peak voltage required to bridge the spark gap is effected by many things such as cylinder pressure, temperature, spark plug gap size, spark plug construction also fuel type, quantity and quality of atomisation/distribution. Under ideal conditions it may be less than 10kV.
Any inductive setup will fire for at minimum 1.0mS
Show me the scope traces of that occurring alongside the cylinder pressue traces showing its benefit.
In order for reliable combustion you need the spark to stay lit for several degrees of crankshaft rotation.
The ability to ignite fuel is a function of ET (energy multiplied by time), if you dont have the energy you must have longer duration (time) and if you have high energy you dont require the duration. Some of the most efficient engine ignition systems also have the shortest arc duration ie laser ignition (picosecond duration) and systems using peaking capacitors such as Pulstar spark plugs. The only time we have found extra duration may be beneficial is with very lean burn (non performace) applications.
FYI - A peaking capacitor is a very high voltage capacitor located in the high voltage circuit just prior to the arc gap. When the ignition coil is fired the spark gap is initially non conductive and takes a finite amount of time to become so. During this pre conduction period the coil energy starts to be wasted as it cant go anywhere (simplified) but with a peaking capacitor coil energy is stored breifly until the plug is conductive. The effects arc current are extremely dramatic but unfortunately its difficult to implement a robust solution.
no CDI will pass any modern day emissions testing for that reason.
Absolutely they will we have taken many vehicles across the emissions testing laboratories. Inductive ignition sometimes has difficulty passing too especially the cold start cycle and must be multi sparked.
This is why inductive is the ignition of choice in ALL modern day high performance engines
Not its not. The reason CDI is no longer used on street cars is due to the 100,000km spark plug service period requirements for modern cars. Due to its higer energy cdi erodes spark plugs at a much greater rate.
Literally the only people who believes CDI is superior, are individuals who have never seen side by side tests in action
We we have seen and manufactured both and there is a reason we no longer manufacture inductive systems. In one of his books Keith Duckworth (Cosworth) mentioned the benefits in power, torque and combustion efficiency they found while developing the DFV however the reason F1 dont use cdi today is simply to save weight.
Another big bit of misunderstanding is that you can 'back to back' compare ignition systems. To correctly compare ignition, especially cdi vs inductive, you must develop optimised timing and fuel curves for both or the results will be invalid. In some applications it has been found necessary to reduce total advance by up to 10 degrees due to improvements in flame kernel initiation with cdi.
CDI also works very well on older carburetted motors due to its insensitivity to fuel mixture and ability to fire through wet spark plugs. We have been involved in the conversion of many old European performance cars and the improvement in drivability is dramatic.
In general the biggest opponents of cdi ignition seem to be are those without the ability to design/build it and those incapable of correctly installing/understanding it.
To finalise inductive ignition and cdi ignition both have their applications just dont spread BS saying one is outright better than the other.
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