Not good. You wrote the 7.4 x 8 ?? If that prop is truely the 8, and not the 8.0..? then that 8 should turn close or over 24K...easily. If your "8", is actually an 8.0... then it would depend on what version 8.0 you have. I know that sounds confusing, but APC hasn't made the prop stamped with 7.4 x 8 in many years. They all read 7.4 x 8.0 ....

If a Q40 engine only turns 21.4, you shouldn't fly it. It will just burn itself up. Don't fly unless that tach reads 23K, on the pinch up.

RB