I’m not concerned about other plug brands, this is a survey based on heat range and what plugs to run with a R75/5 and Siebenrock kit making the engine close to 1000cc.
Plugs are inexpensive, engines & especially those modified tend to react uniquely (especially given overall range of factors) Suggest run in, then chop them for a view during suitable long hard run.
What's the estimated compression ratio of the new set up ? I have always started with one heat range colder plug on a performance build than specified for a stock motor.
^^^^^^this. plug too hot = detonation = bad plug too cold = BFD so you want to run hwe coldest plug you can. a fresh top end will burn cleanly, hot plug not needed.
I don't use Bosch plugs, I use NGK so I can't contribute to the survey. But I have two airheads . The standard NGK plug is a BP6ES. In my mildly tuned road bike ( 9.5 compression, big carbs) I use BP7ES. In my full-on race bike with 10.5 compression, race cam and big carbs that gets thrashed to the red line every gear change I use NGK B8ES. So to sum up: stock engine use 6, mild tune use 7 , full race use 8.
