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MG MGB Technical - Aldon Amethyst

Is anyone using one of these? I've had one on my roadster for two or three years and it certainly seems to work ok. I have a fast road setup and was wondering what curves other users have set up. Also I have mine connected to the vacuum port but currently have the vacuum settings at zero. I have found no reference to vacuum settings for any programmable units. Since rebuilding the car I have promised myself a trip to Peter Burgess for a rolling road session. Hopefully he can help me with optimising the Amethyst as well!
S Church

I put an Amethyst on my Lotus Elan about 3 or 4 years ago and I couldn't get it to programme properly. If I set the idle timing and then revved the engine it retarded the ignition rather than advancing it. A pal offered to trailer the car to Aldon for me so it and I spent a day with Roger there. Despite swapping the unit for two different new units, he couldn't make it work either. They were completely baffled and ended up giving me my money back. Never did discover why it wouldn't work on my perfectly standard Lotus Twin-Cam unit.

I now have a 123-Tune distributor on my BV8 and like you I have no idea how to set up the vacuum curve. I even contacted Mr Burgess about it, but he wasn't willing to make a guess. I've tried various settings for the vacuum and have come to the conclusion that whatever setting I use makes no difference at all to the way the engine runs, so I've stopped worrying about it. The 123-tune is a smashing bit of kit.
Mike Howlett

The point of the Amethyst and others like it is that you can set the curve by experimentation to suit your engine and fuel exactly. A pal did this and ended up with a curve that was significantly different to any of the published curves, but did feel faster. He didn't use vacuum, which is primarily intended to improve cruising economy but also sharpens part-throttle acceleration. But because of that you have the vacuum 'curve' interacting with the centrifugal curve through the throttle range and it takes a lot longer to set up for best results. This is why competition use doesn't have vacuum - cruising economy is hardly a factor.

The benefits of Amethyst and Accuspark Stealth are that they use a conventional trigger or points, so if the unit fails you can still limp home. If the 123 fails you are stuffed. Also the 123 was 50% dearer than Amethyst the last time I looked, and three times the price of the Stealth.

At the end of the day whether any of them are going to give significantly better results than a correctly set-up conventional system is very debatable.
paulh4

I use an Amethyst and have the "significantly different" curve. It's coupled with a pertronix Ignitor. The car is supercharged, and I fitted it as I was getting detonation. Consequently I would use the "boost" (as opposed to vacuum) settings to back off the advance under high boost had it been necessary.

The curve was "created" by many trips up and down a bit of local road. The start point were the settings that I had measured with the standard (D43) distributor from 1000 to 4500 in 500 intervals.

One of the key benefits of the Amethyst is the "offset". This allows the whole curve to be shifted "up" or down". This highlights the rev range at which any detonation occurs, whilst being able to see if it "goes" better. It is all a little bit "seat of the pants", but so is driving. At a session with PB we discovered we could back off the whole curve by 2* with no loss of performance within the 2500-5000 range. So I run with a 2* offset!

In terms of "other curves", you presumably have it so it feels right. You could use the offset facility to see if a degree more or less helps.

Not really sure about fuel economy, but when on holiday with a pal with a V8, we seem to fill up at roughly the same time!



Michael Beswick

i had amethyst with my newly rebuilt +40/Econotune/K&N.

peter got another 10HP out of it. 80 to 90.

G
Graham Moore

This thread was discussed between 11/03/2019 and 12/03/2019

MG MGB Technical index

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