There is a reason they call it RANGE OF MOTION.
A little schooling kid, the rep range you are working in is the strongest range for biceps. Compared to the starting posistion (arms fully extended) which is the weakest. The only way to growth (hypertropy) is to work thru the full range!
You can curl that much all day long in that range.
Lighten the weight, you have years to try and impress us and 8" arms won't get it done....