Here is what I found today.
I have a 14" riser and a 18" Apogee (A&H) riser.
I have a set of 58" and 62" ACS CX limbs, both are 40# on the 14 " riser and 36# on the 18" riser.
So, I can shoot several combinations of limb lengths and riser lengths in 40 and 36 lbs.
Here are the chrono speeds I shot today with two different weight arrows.
14" riser, 58" bow, 40lb, 8.1 gr/in = 187 fps
10 gr/in = 171 fps
14" riser, 62" bow, 40lb, 8.1 gr/in = 191 fps
10 gr/in = 175 fps
18" riser, 62" bow, 36lb, 9.0 gr/in = 182 fps
11.1 gr/in = 168 fps
18" riser, 66" bow, 36lb, 9 gr/in = 187 fps
11.1 gr/in = 172 fps
I draw right at 28" and shoot split fingers with a glove.
Not much difference in speed, seems the most efficient combo is the longer riser with lighter poundage. The highest speed was with the 66" 36 lb limbs @ 9 gr/lb.
All limb and riser combos are very quiet have and zero hand shock.