Why does my 13B blow, twice, when I shifted into high gear (4th or 5th) and acceperated in low rpm?

 

i think you're going to have to give us a better explanation.

 

what blow off valve do you have?

 

The highest loads on the motor are in the tallest gears and low RPM. It's likely those parts of your map where not tuned or not checked under real world conditions. A Dyno can try to replicate load but it may not accurately replicate it under all conditions.

 

high throttle at tall gears (4 or 5th) can cause detonation if not properly tuned