I'm not sure I agree with the notion of sourcing damper holes as large as possible! How large do you want to go there? There's a very fine line between altering the damper effect and retaining the safe response of the suspension and knocking it right off the scale!
Fitting off spec. sphere pressures/damper ports/ volumes etc. might seem like you're making a BX float like a 50s DS but you'll soon find that the other components are not designed to run with this spring/damper range! And of course, like I said; it may seem sensational when you've just modified it all and it seems like you've got it right but as soon as you're carrying 3 passengers and stuff in the boot you'll enter the realm of MAD suspension where it's underdamped and wallowing like a basterd.
I wouldn't recommend fitting front (55bar) spheres on the rear because the pressure is too high and the damper is too weak (for the rear ratio of LHM displacement versus upwards hub travel). Hitting the bump stops is NOT the thing to aim for!
The trouble here is that the BX seems to generate a sense of disappointment where the ride sensation begins to feel mush more 'normal' through use. I spent weeks on my first car, a BX 14E, tring to cure what I though twas 'stickiness' n the struts - it was not stickiness - I'd just gotten too used to the ride.
My recommendation to anyone doubting the BX's ride sensations is to drive some modern 3door hatch thingy for a week and then go back to your BX and you'll notice how plush it feels.
Spheres and stuff aside - it's the set up of the BX's suspension parts that gives it's less 'DS-like' ride. Take for the example that the BX14E uses the self same spheres as the GS/GSA. Same size, damper and pressure and yet both cars feel totally different! Why is that? it's because the ratio of LHM movement to the displacement of the wheel hum, up and down is different and that the cars have different weight, power, weight of wheels and hubs and different anti roll bar diameters and moment arm(distance between the connection of the arm to the anti roll bar) to each other. different ride - same spheres.
Just slap on the BX petrol fronts in the 500cc size and you're good.
Could it possibly be that the diesels are a bit heavier at the front end than the petrols? Thats why you've experience the bottoming out?
Diesels are heavier over the front axle and additionally, with that you'll get more inertia too. But funnily Citroën chose to use spheres of the same pressure but firmed up the damper on the Turbo Ds. Cos don't forget, the beauty of the hydro suspension is that with increases in mass and hence pressure of the LHM holding it up, the gas in the sphere gets compressed further which increases the spring rate. If you choose a higher pressure sphere to start with (i.e. higher than the normal 55bar for the front) then you'll have softer springs for the same weight. Potentially this would be even more appropriate to the diesel at say, 60bar but you still have to have them 'gassed' to this pressure by a refilling place as the next available 'off the shelf' sphere of the 500cc size is the CX front and it's 75bar - I'd wager, too high a rest pressure.
O kok, I've gone on again... there are so many myths to address with spheres/ damper holes.
Er, I thought Hydractive centre spheres had no damper at all!?!? The damper valves for these are within the 'firmness regulator' that it's attached to.