Bristol Dx6a and DXH6a differences
 

Does anyone know the key differences between these two distributors.
I know the DX6a is for a 400, but Bristol used the DXH6a on the later 100 series engines.
It’s the advance curve similarities I’m actually interested in as I’m working on a 100 d series engine that for some reason has the DX6A fitted.
Regards,
Graham

Graham,
The only difference is that the DXH6a has horizontal outlets in the cap for the spark plug leads , compared to vertical in the DX6a .
Both are very basic distributors . My experience over 50 plus years that even a new Lucas distributor is only good to about 3500- 4000 rpm , much over that they become erratic , probably due to points bounce .
I would not consider the original ignition advance curve ,as today's fuels, higher compressions of most Bristol engines today, often different valve timing etc etc , the large total advance of the 50's is not needed today.
I do not and will not run a Lucas distributor in either of my Bristols , especially if it was a D2 engine

What D2 engine are you working on ??
I have new horizontal outlet distributor caps if you need one .

Geoff
Sydney/Australia

The key difference is the cap. The "H" denotes horizontal ( side entry ) exit holes for the HT leads. DXH6A was used for Bristol-engined AC Ace and Acecas. The cap part number is 407279. The DXH6A distributor part number for the AC cars is 40551 , which will be on the distributor body, as should be "WG-16". The part numbers determine the advance - in this case it is ECM curve 641 using spring set 424216. This is different from the other 2 litre distributors, but I don't know the actual curve, sorry. The Distributor Doctor should be able to supply the springs if you need new ones.

Thanks for the clarification of the model differences.
Oddly my Dx6A has a horizontal exit cap which seems to work fine.
I guess the cap fit both types.
The conclusion for me Is that the Lucas original advance curves can be improved on by using a programmable advance curve and testing on open road / rolling road to get the best out of the engine given modern fuels, higher compression and vernier valve timing settings.
Regards
Graham