Fitting 123 ignition

[Geoff Dowdle]

David,
I'm certainly not an ignition expert but have had experience with Bristol engines and distributors over the past 40 plus years.
All Lucas advance curves are quoted in Distributor degrees and Distributor revs .
These figures have to be doubled to correlate with engine RPM & Deg advance .
There could have been a misunderstanding between Andy and the 123 literature as to whether Distributor or engine revs & degs were quoted.

Ask yourself "what would the manufacturers of 123 distributors know about what advance curves Bristol 6 cylinder engines require in their current mostly modified condition, without even considering the various types of Bristol engines produced".

I suggest you don't think too much about the curve until the rolling road tells you what advance curve is required for your engine.
Yes the camshaft , compression , fuel octane and other factors can change the advance curve required.

A vacuum advance is probably not necessary with any good quality distributor.

Geoff

[David C]

Update on 123 ignition.

I had been using one of the stock curves recommended by others who had fitted 123 to the Bristol engine. It worked quite well but had a couple of flat spots. I tracked down an advance curve for distributors on 100A and B engines. This had max and min values and 3 straight portions forming a rough curve. I drew a smoother curve between max and min values and read values from it to put into the app. The result was very good and a rolling road session suggested they were very close to ideal.

the values I used were as follows:

RPM Degrees crankshaft
500 0.0
700 4.0
1200 14.0
1600 20.0
2400 25.0
3000 28.0
4000 29.0
5000 30.0
8000 30.0

Static timing 10 degrees.

The last value is academic since the rev limiter is set at 5000 but the app required a value.

I had thought that individual engines would need tailored curves but it seems these figures should be good for 403 and 405 engines and may well work for 400 and 401 as well.

I had considered using the vacuum advance feature but rolling road results suggested that this would offer little or no benefit.