Re: Megajolt worth considering?
Posted: June 25th, 2012, 6:55 pm
Here's a pic of the Megajolt components destined for my Spider comprising Ford coil pack and EDIS module (defaults to fixed 10deg BTDC if the Megajolt fails), trigger wheel and trigger sensor, and the actual Megajolt ECU. This was all bought some time ago, so the system may have developed a bit since then.
Regarding the difference between the MSD and an Megajolt, the fundamental difference as far as I can tell (never having used the former, but just going on the spec.), and ignoring the multiple spark trigger strategy, is that the MSD is a capacitive rather than inductive discharge unit. Rather than relying on the collapse of the primary coil circuit, either by mechanical contact points opening or electronically switched by a transistor, to induce a high voltage in the coil secondary, a capacitive system uses an oscillator and transformer to step up the 12v car supply to around 400-500V and charge a capacitor (the size of which determines the ultimate spark energy). When triggered (by contact points or electronically as before) the charged capacitor is discharged across the coil primary thus producing a pulse of several thousand volts at the secondary, and the reason they can potentially 'fry' your tacho if you leave it connected to the primary. The inductive systems tends to give a longer duration but lower energy spark, whilst CD systems produce a shorter duration but higher energy spark. I imagine the multiple spark strategy of the MSD system is to address the limitation of the short duration spark of this type of system. I've built my own versions of both types, mostly with optical triggering from the distributor, and have always felt the CD systems give the crispest starting and response, mostly because they are less affected by a low supply voltage when cranking as you are still putting several hundred volts through the primary. CD systems have actually been around for years; my '70 911 has, an albeit primitive, one as standard. The multi spark system isn't even that new (just the application of microprocessor control in recent years); there used to be an advert in the back of Practical Electronics in the '80's for a kit that fired a 'continuous' spark for the entire period that the points were open. Please note that none of the above is intended to question the performance of, or fully explain the exact details of any of the systems mentioned in this thread (which I am sure have all manner of clever design details and refinements their respective engineers have implemented ), but just to add some additional information.
Regarding the difference between the MSD and an Megajolt, the fundamental difference as far as I can tell (never having used the former, but just going on the spec.), and ignoring the multiple spark trigger strategy, is that the MSD is a capacitive rather than inductive discharge unit. Rather than relying on the collapse of the primary coil circuit, either by mechanical contact points opening or electronically switched by a transistor, to induce a high voltage in the coil secondary, a capacitive system uses an oscillator and transformer to step up the 12v car supply to around 400-500V and charge a capacitor (the size of which determines the ultimate spark energy). When triggered (by contact points or electronically as before) the charged capacitor is discharged across the coil primary thus producing a pulse of several thousand volts at the secondary, and the reason they can potentially 'fry' your tacho if you leave it connected to the primary. The inductive systems tends to give a longer duration but lower energy spark, whilst CD systems produce a shorter duration but higher energy spark. I imagine the multiple spark strategy of the MSD system is to address the limitation of the short duration spark of this type of system. I've built my own versions of both types, mostly with optical triggering from the distributor, and have always felt the CD systems give the crispest starting and response, mostly because they are less affected by a low supply voltage when cranking as you are still putting several hundred volts through the primary. CD systems have actually been around for years; my '70 911 has, an albeit primitive, one as standard. The multi spark system isn't even that new (just the application of microprocessor control in recent years); there used to be an advert in the back of Practical Electronics in the '80's for a kit that fired a 'continuous' spark for the entire period that the points were open. Please note that none of the above is intended to question the performance of, or fully explain the exact details of any of the systems mentioned in this thread (which I am sure have all manner of clever design details and refinements their respective engineers have implemented ), but just to add some additional information.