When i went to order some for My S1, the guy looked in the book and said its a special plug for the S1 engine, He recommended the Champion C61ST. They cost me 35 quid for 4. They are a funny looking plug compared to others. I can't say what they perfrom like as i don't use the car, but it never fails to start with em, it don't missfire or anything, as for when under boost i couldn't tell you as again i don't use it. But the guy i bought them off reckoned they were the best plug to use, had to special order them from champion.