Ok I am looking at upgrading my impulse engines, I currently have mk 10 hyper with full x3 for zipping around fast out of combat. The question I have is Combat impulse has a slower speed but says efficient at low power. There is no description however as to what that actually does. What does efficient actually do when your at high or low power?
I never fully understood the efficient at low or high power thingy. However, I'm pretty sure that it means that you get more bang-for-your-buck if you have your engines at less than 50 (for Combat) and at more than 50 (for Hyper).
Actually, if I'm reading that right, then the rules distill down to something like this:
The listed speed bonus of the engine is what you get with no skills and engine power set to 50.
For most ships (impulse modifier 0.15) with standard engines, going down to 25 power will slow the ship down by 3.75. Going up to 100 power will increase speed by 7.5. Full impulse will grant full power to engine + the full impulse bonus of your engines.
Combat and hyper engines have smaller or bigger multipliers depending on type. A combat engine has a 0.6 multiplier, so at 100, it only gets a 4.5 speed bonus, but in return, doesn't lose as much to low power either. The combat engine only loses 2.25 speed at 25% power. Hyper engines have a 1.35 multiplier. So at 100 power, they get 10.13 impulse speed. But they suffer as a result of low power. Going down to 25 power will reduce speed by 5.06.
This is with no ship piloting skills, both your engine's base speed and engine power bonuses get a small multiplier for your ship piloting skills (going up to 1.25 if you master all three skills for your ship type, apparently). So these bonuses and penalties will get bigger as your ship skills go up, but you'll be going faster in most cases regardless.
Deep space science vessels and escorts have bigger ship multipliers. 1.7 for the DSSV and retrofit, 2.0 for all escorts and birds of prey. The escort (and BoP) modifiers seem to look like this:
Combat: -3 (25%) to +6 (100%) speed
Standard: -5 (25%) to +10 (100%) speed
Hyper: -6.75 (25%) to +13.5 (100%) speed
The result is they go faster when cranked up, but are also slower when the engine power is turned down.