SRP Motor Ratings and Specifications. To hit the correct motor ratings, i've decided to add another letter to the call out. For instance, if a motor is calculated to be a G70, instantly the motor will be downgraded to an F. Therefore, the callout would be G70F. I do this because our fuel does not have the same density as the calculator thinks. That's why I down size it in this way. Also, if a delay is used, and the calculation was for a G70F-4 delay, but only achieves a 2 second delay, then the callout would be a G70F-4-2. Until I perfect the delays and motor impulse rating, I will use this type of designation.