Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties double fire on u flatty

double fire on u flatty

Post Fri Apr 14, 2006 1:37 pm

Posts: 24
Location: itzstedt germany
:?: hello alltogether
i want to build an double fire ingnition on my old lady (74 big twin)or i closed the oem hole an make only one in the middel of piston area.
are there any experians(sorry i know its bad english).thanks for your help

Post Fri Apr 14, 2006 8:36 pm

Posts: 801
Location: Planet Earth

Don't think that would be a good idea. The bulk of the intake charge is not over the piston it is over in the area of the stock spark plug location. On the compression stroke, the air fuel mixture is squeesed over to the area above the valves. There is minimul charge left in the area on top of the piston. If I remember correctly this was designed to stop detonation and provide a more controlled burn of the mixture. If you were to put the spark plug over the piston you would be trying to ignite the mixture in the wrong direction. Probably all hell would break loose in the motor when it fired. If I am not correct I am sure others will correct me. Or might have more input on it.


Post Sat Apr 15, 2006 7:38 am

AFAIK in the quench area is useless as JIm describes.
My choice for a second plug would be at the farthest end of the "transfer" area above the bore, just before the quench area.
If both plug positions are new, I would try there, and the second plug at the exhaust valve (rather than the intake).
You can use smaller plug bodies (14mm, 12mm, 10mm) for more choices of location, but as they get smaller their fouling resistance goes down because the path to ground on the porcelain is shorter.
Remember that a second plug reduces spark advance requirements - you can't run as much, I would start with 5 degrees less as a test.
The 2 plugs will almost certainly not run at the same temperature, the plug near the exhaust will be hotter, you may need 2 different ranges (B6 and B7 etc.).

Post Fri Apr 21, 2006 12:53 pm

Posts: 24
Location: itzstedt germany
ok...i will leave much action and to little helpfull
thanks for answering
i watch this board for other tunning tricks
so long howie :!:

Return to Big Twin Flatties