I dissagree about it being rule of thumb, It is fact, theres a difference. Also, no one suggested intercoolers are there to prevent knock. They are there to maximise the available oxygen at any given pressure for our oxygen consuming machine. Remember that in this thread we are talking about the difference between 20deg C and 100+... not nominal 20 - 40s. As the intake air gets hotter we have to lean off to maintain acceptable AFR. This is a fact, and one that the speed density and Mass systems both take into account and utilise accordingly and correctly, which takes me right back to my first point.....