I have Grade 5's on all of my ships (15 engineered i think?). I'd be interested on seeing some data to help better understand what kind of rolls are needed to justify a Grade 5 drive over a Grade 3 drive in terms of optimal mass. The multiplier is transparent in what it means; the optimal mass is opaque. I feel it's bad game design when you can't simply eyeball changes and determine if it's better or worse.
At minimum there should be some way to see how a part will impact your ship before agreeing to a mod. One of many things FDev dropped the ball on imo. Given how long it can take to get these mats, you'd think they want to make sure we know exactly what we're going to get.
here is a thread giving you all infos on optimal thrsuter mass:
https://forums.frontier.co.uk/showt...plained-in-one-simple-table-(Rating-and-Mass)
for the sake of argument let us assume you are running a viper mkIII with 3A thrusters (optimal mass 120T).
your viper weighs 100T including fuel (that's already a lightweight build).
1. without engineering, your viper gets a speed and manouverability bonus of 3% (between 2-4,5 % according to the table linked above) for its 3A thrusters being at 83% optimal mass
2a. you do G3 dd tuning, with the following roll: optimal multiplier +20%, optimal mass -9% (e.g. both sides maxed).
2b. new optimal mass of your 3A G3 thrusters is 110T. you are now at 91% optimal thruster mass. you get ~2% from being below optimal thruster mass, and 20% from the new optimum multiplier = your viper is 22% faster.
3a. G5 dd tuning, roll: +27% optimum multiplier, -16% optimal mass (average increase on multiplier and heavy hit on optimal mass). new optimal mass 101 T.
3b. your viper sits at the optimal mass, so you loose any speed increase from that. you still gain 27% from the new optimum multiplier. in effect your ship is 5% faster compared to G3 tuning, and 24% faster than without tuning.
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this exampel differs widely depending on thruster size (thrusters optimal mass compared to ship mass). a ship like a vulture, which will sit below 50% optimal thruster mass before engineering, will gain more of the G5 multiplier (probably all), while a ship like a corvette (with that crippling class 7 thrsuters) while gain less - at least if it fits bulkheads etc.
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the maximum gain of speed/manouverability of a-class thrusters is +16% at 50% optimal thruster mass. with a heavy hit on optimal mass, this can be reduced to +12% with the same mass. the difference of 4% will be deducted from your G5 optimal multiplier gain. you will always gain speed manouverability, but you might gain less than expected.