Are you sure you just have the low connected? On the '90's Newell's with DDEC-2 8V92's with a single-stage Jake brake it activated all 8 cylinders. Jake's don't provide the retarding power on a two-stroke as they do on a four-stroke. Mine got better after the Detroit Diesel mechanic found a broken wire under the valve cover that was preventing the Jake on two cylinders from activating so I was only getting benefit of the Jake on six cylinders instead of eight.
Jakes work better at high rpm's so keep the engine speed up in the 1800-2100 range to maximize the affect of the Jake. Detroit Diesel did make 8V92's with 2 speed Jake's but I don't know if Newell ever used them.
In 1989, Jacob's released a Jake Brake Tune-Up Kit part Number 017057 that would retrofit the design used previously.
Performance data with the 017057 Jake on an 8V92 was:
1100 RPM 110 Retarding HP
1300 RPM 135 Retarding HP
1500 RPM 170 Retarding HP
1700 RPM 185 Retarding HP
1900 RPM 230 Retarding HP
2100 RPM 260 Retarding HP
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