I guess i will chime in and explain what Ed started to:
Brake Mean Effective Pressure is the average mean pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured brake power output. The equation for figuring out BMEP is:
BMEP= IMEP-FMEP-PMEP
The simplest explanation without confusing everyone is BMEP is the overall cranking compression. Compresson ratios can be figured out a number of ways, some engine builders use Swept volume (Total Cylinder Displacement) divided by Head Volume (Installed cc), some use the BMEP. The thing about BMEP is that because it totally relies on effective pressure, the reading will be different from engine to engine ie:
Comparing a brand new engine to a 4 gallon identical engine. The new engine will always read higher because the piston to wall clearances are tighter allowing less blow by which will indicate higher cylinder pressure. What Ed is saying, because the seal of the standard plug is at the copper washer and the turbo plug is at the base of the plug, the turbo plug will have a higher BMEP acting as if the engine has a higher compression ratio thus giving the engine more "pop" as some call it resulting in a faster burn giving the engine the feel of more torque. the thing you have to understand about engines is that there is a hell of alot more to them then BMEP. There is Indicated Mean Effective Pressure(Overall Pressure), Friction Mean Effective Pressure(Rotational losses from bearings, rod, piston etc.), Pumping Mean Effective Pressure(Vacuume created during power sroke), Piston Speed(Feet per second), Rotational Mass, Elavation, Barometric Pressure, Humidity etc.
Basically the BMEP can only be used as a guideline and will not be the same from engine to engine as every engine has different load bearing, friction loss and vacuume pressure that will affect usable torque. If you take 1 engine and switch between a standard plug and a turbo plug, provided the installed cc's are identical, the plug heat range is identical and the BMEP is identical, you should not see a difference in performance between the two because to put it simple, if all of the above is identical (in the perfect world) the mixture does not know the difference between a standard plug filiment and a turbo filiment. All you need is a candle to light the fire and all the other forces mentioned above take care of the rest.
This is why you have all sorts of opinions on the subject because most of them have put together different combinations of plugs, on different aged engines, on different brand engines, compression ratios, pipes used etc. and all have had different results.
I hope this explains a little more for you guys
