Factory Ignition Curves : Is there any point ?.

[111Robin]

Over the years as I have been looking for the correct distributor (Mk2 998 Cooper) I have always been told there is little point in trying to replicate the original factory advance curve due to the effect of modern fuels on the ignition timing requirements. So is there any point having one rebuilt to factory spec' any more ?. If not it just becomes a need for having the correct number stamped on the body for the sake of originality. This is pretty much why I stopped looking as my existing distributor (serial number shows it is from an 850 automatic, I have measured the advance through the rev range and it's not that far from the correct figures) gives a perfectly good performance for my needs. I dare say there would be a few bhp to be had if it was tweaked but probably not that noticeable. Any thoughts on this ?. One of my winter jobs was going to be to send the distributor to the Doctor to reinstate the factory curve but is there really any point in this ?.

[bwaminispeed]

Unless the curve is plotted on a Dynamometer, with the fuel you normally use, you can only guess, you may guess right, you may guess wrong, the factory curve may be right, it may be wrong.......

The whole thing is pretty much a crap shoot.......With some time and patience, you may get reasonably close doing road tests, but, you have to be consistent with your procedures, and, only make one change at a time....

Overall, if you are happy with the way it runs, I would just go with that.....

[Spider]

While no longer optimal for the new fuels, they are close enough to give engine performance that you'd be hard noticing the difference between what they give and what would be optimal. It becomes more important as compression ratio goes up.

The other, and possibly more importance aspect of this, is that with the wrong curve, you can do serious engine damage



That's an exhaust valve taken from an 1100 engine that was fitted with a 998 distributor (our Aust 1100 engines had a higher compression ratio than the UK counterparts), and when the curves are plotted for each, it's clear why this happened



Right in the cruising zone, the ignition timing was quite retarded.

Crap shoot? No.

[111Robin]

Understood, however having checked the advance through to maximum there is not a lot of difference compared with the Cooper 998 spec' so I don't have any concerns about this sort of eventuality. I might still do it just for completeness, it's an itch I've needed to scratch on this car for 30 years.

[whitewithblackroof]

The Cooper curve has more advance, mine was recurved as was an 850 curve and I did have retarded at cruise which caused coughing from the carb
It it now much happier, there is a service document which states 26 degrees at 2300rpm as a check

[111Robin]

I have it set at 32 degrees all in and no hesitation through the rev range, pulls like a train which is why I'm doubtful changing it will make a difference.

[bwaminispeed]

If you do go for a factory re-curve, send a different distributor......

I would keep the one that works well to fall back on, in case things don't go well....And, quite often they don't.....

[111Robin]

If you do go for a factory re-curve, send a different distributor......

I would keep the one that works well to fall back on, in case things don't go well....And, quite often they don't.....

Makes sense, I have one that needs a full rebuild anyway so can send that in the meantime.