Sorry, i was trying to use the KER information to explain my problem, I've seen that the VAB values dont always equal the values in flight. If i scale a thermal or plasma nozzle up from its base size, it still produces the same thrust, same isp. I have tried it with a large surplus of thermal and charged particles respectively, and it is not a lack of resources or cooling causing a change. Nansuchao's signature has a link to an interstellar extended guide, and under the heading MAGNETIC NOZZLE, it shows 2 pictures of magnetic nozzles, one at base size, the other scaled larger with much more thrust. Mine won't scale like that, the larger version has the same thrust (well, slightly lower when accounting for the extra weight of the scaled engine). All of my KSPI engines I have unlocked have the same issue.