the plugs stick out further and so are nearer the centre of the combustion chamber, this does three things;

1. puts the spark nearer the centre of the spherical combustion chamber so the flame front has less distance to travel to complete the combustion .. so combustion is over sooner ... you get more power as you have more mechanical advantage .. as the later it finishes the further away from 90 deg ATDC you are .. so less time on a smaller/decreasing arm .. kind of difficult to explain .. see Corky Bell's book Maximum Boost - nice graph... basically the current spark is on the edge of the ball of fuel/air whereas these plugs put the spark nearer the centre

2. This action of moving it nearer the centre does advance it by some 1 or 2 degrees (theory again.. no actual measure done)

3. The plugs may run hotter as they have a harder time cooling down .. this may promote pinking but then so does the 1 or 2 degrees advance .. so who can tell which came first. This heat means that you may want a colder plug .. so go to a 6 instead of a 5.. (or 7 instead of a 6 in my case). So .. there have been good reports on power up by 1-2-3-4 hp but my take on these plugs are .. they cost the same as the stock ones .. some people have reported more power .. so you have nothing to lose and something to gain. .. and number is NGK ZFR6F-11 (this is a Colder plug than ZFR5F-11 which is a stock equivalent to Stock Plugs:'90-93 BKR6E-11 '94-99 BKR5E-11 but grade 5,6 and 7 listed as an option in US 1.8 workshop manual) .. anyway the 5/6/7 bit is not too important according to people who have tried .. I did 'wear' a set of 5 plugs by 30% in 1200 miles with the turbo but should be on 6 or 7s... see bottom of engine section on my 'page if you want more. I had to buy a box of 10 '7' grade plugs to get mine.. so go with 6 or 7 ... (2/00)