This E-tube has a slightly larger main bore and less air bleed holes providing a better fuel signal at the jet. Improves performance across the RPM range, but especially on bottom end and mid range. You will want a main jet between .035" and .037" and low speed jet around .024" depending on how much you can bore out the carb venturi and which cam you use. This tube is legal in both Superbox and BP. It is not permitted in Box Stock classes.