Been working hard this last few days so haven't even had much chance to play, so I'm still a total newb. One thing I did try and check out though, is whether there are any variations in engine design effects. But it appears that there aren't: max speed depends on volume of engines, and the thrust produced by your engines is proportional to their volume, resulting in an acceleration that's determined by the volume of the engines and the mass of the ship.
As a result, it seems (I may of course be wrong) that the maximum speed of a ship will go up as its size increases (if you scale the whole thing up consistently); for the same composition of ship, the volume of the engines will increase at the same rate as the mass, but the mass increase doesn't reduce max speed.
This seems 'counterintuitive' from generally expected tropes of space combat, where the larger ships are lumbering and slow, whereas smaller ships are nimble, faster by possibly orders of magnitude (in realspace at least).
Might it be worth considering not using 'total drive volume' as the determinant of the numbers for a ship's engines. Obviously, the mass of the engines should be proportional to their total volume, but how about making the thrust proportional to the 'cross sectional area' of the engine, and max speed proportional to some factor of the length of the engines.
So if you have 9vol of engine space, you could build it as a single 9-long, one wide engine and go fast (but take an age to get there) or as a 3x3 array one deep, and have 9x the thrust, for a (greatly) reduced max speed.
Straight away, you also get a disconnect between ship mass (increases as a cube law) and thrust (increases as a square law) for simply upscaling the same design. Big ships will either have wider mouths to their engines, or they'll accelerate less quickly than their littler brethren.
Scaling max speed to engine length is a little more complex. If you just take the length of the longest engine, it's fairly simple to game the system by adding one long skinny engine to your designs to access the high speed mode. Perhaps having the max speed factor be determined by some root of the average of the squares of the lengths of the fitted engines would work. Doesn't need to be a square root; natural logarithms work pretty well for this sort of design curve building.
Perhaps something like this has already been tried? Perhaps I've got the wrong end of a stick, at which point, just read the Subject as "How do engines work, again?"
Can anyone tell that the mucho work I've been doing has involved a fair few hours of unchallenging driving...?