The best sparks I was able to achieve was 1250mm (thats 2 times the winding-lenght of this coil) The tank cap was changed to 30nF for this run. Power input is 9KV@200mA from a neontransformer base supply.
The first light of this coil reached 950mm sparklenght to a grounded object.
This is the recordspark of the 15nF configuration The spark is 1000mm long.
In this picture you see a 1200mm long ground hit and some strong steamers.
This is an 8 seconds exposure time picture with 30nF and the 2,5KW PSU.
Here you see that there is no more space in my workroom.