I have just finished fitting a Megajolt system to my Mark 2. I thought the group might like to see some pictures and also share in the results I obtained.
Although this system has been fitted to many E Types, the Mark 2 does not seem to have been so fortunate. One of the difficulties is the location of the timing wheel which I wanted to fit behind the fan belt in what would be a sensible spot. However the space is pretty crowded and one has to deal with the jockey pulley and its possible interference with the timing wheel (ask me how I know!). In any event I ended up with a wheel diameter of 6.2".
I used the Ray Livingston method of mounting the coil pack and set the EDIS-6 module close by on the inner fender. That way I could simply slice the cables from each together. The EDIS module was set on two small vibration mounts so that it would not be subject to stresses when mounted on the curved fender wall.
The pick up bracket was machined from aluminum block in two pieces.
I think the attached pictures tell the rest of the story, particularly how I fitted the custom timing wheel to ensure it ran as true as possible. I set the gap to the pickup at 0.070" and this gave plenty of signal (about 1Vrms at 500RPM).
As well as the pictures attached, I have added a description of the ignition map I used and its rationale, plus the recordings of a test run.
The main motivation for doing this was that I had run out of things to do on my car and it adds a nice level of complexity, my being a retired engineer.
In principle it provides a greater level of timing accuracy than can be achieved with a distributor, since there can be issues related to gear back lash. The load on the gear, being the drag torque of the distributor, is quite low and this can lead to irregularity of timing.
Essentially the Megajolt system allows complete flexibility to set vacuum and centrifugal curves as well as several useful add-on functions such as rev limiting and temperature control if desired. I was planning to play with these and still may do so but I doubt I can improve on the factory settings for my engine which is in a standard configuration.
I have used one of the early 123 ignitions that had several fixed ignition curves, none of which matched those of Jaguar on my E Type. I see that later versions allow a customised curve and that is better. I do not know whether you can program the vacuum advance on these later 123’s.
From a price aspect, I bought all new parts and they came to around $600, not including the cost of the timing wheel and pickup bracket that I made at home. So a little more expensive than a 123 but not egregiously so.
The car does seem to run very smoothly and I had to close down the idling screws to achieve 650 RPM so that is some indication.
In conclusion it was a fun project and I had the time to do it properly. BTW I keep the old system in my trunk just in case …
I just converted my Mk2 with a 4.2 and 123/TUNE to EDIS and the improvement is noticeable. The 123/TUNE is very adjustable but not as much as the EDIS. The cost for me was about the same, maybe 5-600$ total, the most time consuming bit was researching everything, fabricating the parts, and getting everything tuned.
I do have one question about spark plug gaps. The coils are designed to fire plugs that are gapped to about 0.050", needed for modern lean burn engines.
Obviously they will fire the normal gap in a Mark 2, but I do wonder if there is any advantage to opening the gap. In my case I have compromised and set the gaps at 0.040"
Bruce
I doubt you’ll notice much from the gaps. Timing is key. I run quite advanced maps compared to the original. The second step is to fit WBO2 sensors and a datalogger to see what the carbs are actually up to (and then start planning the EFI conversion).
I have dual O2’s in my car and it does make a big difference in dialed it in. I actually thought I heard some lite pinging around 1500-2000 so I backed that range off slightly, but I’m going to increase some other areas and see if there’s any benefit. I’ve leaned out my carbs a bit with the EDIS and found the idle is very steady and good mix, while keeping the under load mix richer.
I wonder whether the spark plug gaps affected their life.
Could you share with us the maps you use and also the engine(s) they are used on?
I do have one channel of WBO2 and have data logged the output when I changed my intake air filter from pancake to “oil-bath”. I used it to select the best SU needles.
My readout gives a 0-5V signal as well as showing AFR.
It would be neat to run the Megajolt datalogging and record O2 at the same time but I don’t see how this can be done . Ideally it would be great to feed a 0-5V signal into the MJ and get readings out. Alternatively set up a multichannel data logger (which I have) and record all the MJ channels +AFR.
I run three sensors, one for each carb, connected to a datalogger. You need a sensor for each carb to be able to see how synchonized they are (not). My engine is a 4.2, modified head and high lift cams.
Could you explain the sensors you use? I built a sensor using a linear pot connected to the piston. It sat above the carb. The challenge was to maintain the action of the dashpot I never really debugged it and there was too much friction
Here’s a picture of the design. I also had a tapping to measure the suction chamber pressure.
I did actually consider something like that, but the complexity and the need to drive around without the bonnet on made me think otherwise. In reality though, with the wideband O2 sensors you’re getting the same information in a much easier way, given that you start with the carbs at the same initial needle height.
This is where things go a bit sour with carbs though. If you have a smooth-running engine you can do a reasonable job of synchronizing the carbs by looking at the trends of the mixtures. Don’t expect any wondrously smooth curves though, at least if my carbs are anything to go by. And it gets worse once you start driving.
Having adjusted my tri-carbs over a few years, I was pretty confident in my skills. The car ran strong and the tailpipe mixture was in spec. Once hooked up though the truth was revealed. I guess I should be a little chuffed by the centre carb being pretty well tuned, but the front one was super lean and the rear was really rich. The plugs looked ok though. Once on the road though (and this is where the data-logger is important), those three carbs are all over the place, rising and falling quite independently of each other. A general trend, yes, but thinking the mixtures rise as hoped, no way.
Hence, maybe, with brand new carbs, a lot of testing and tweaking to ensure they actually are totally equal, it might be possible to feed the cylinders with something approaching an equal mixture, within a certain tolerance. In reality though, it’s like with an old distributor, thinking it’s anything like the factory engineers planned it 50 years or more later, we’re kidding our self. Hence, electronic ignition and fuel ignition.
The fact that the XK-engine is incredibly tolerant to crummy tuning hides a lot of this though.
This is very true, but I have found that the O2’s helped get CLOSE. My engine ran well (I thought) but when I installed the O2’s I realized it was off the charts rich. I tried to lean it out and found that the needles were not correct, good mix at idle went way lean under load. Went to richer needles, much closer. Then I found the front carb was more even while the rear would get much leaner under load. Replaced the rear needle with an even richer one (I know this sounds crazy but I adjusted the floats and everything else and could not solve it). Now both carbs are generally within ideal range at idle and under load. Every day is slightly different but now I mainly use the sensors to make sure either a float isn’t stuck or something is wildy out of tune, or to make sure I don’t lean out under heavy acceleration.
I have 2 carbs so two sensors is a perfect system. For three I don’t know, I guess it’s just less exact since you’re trying to approximate a balance.
My car has headers, collecting each pair, then into one, so I welded a sensor bung for each pair of cylinders. For two carbs the original cast iron series 3 XJ6 manifold is great, having fittings already for O2 sensors, as I recall.
First road test and ignition mapping.pdf (1.0 MB)
