XRAY - Model racing cars

??
but I notice some european drivers prefer
1.5 or 1.6 drilled holes in the rough using thick oil

Piston & oils is often down to personal choice, how you drive, shock tower wishbone position, springs & just generally what you want the car to do with the conditions in front of you.
THe US guys generally have higher grip tracks & larger jump compared to Europe, so run harder springs for example silver or the grey blues & allot of the time 1.3 pistons with an oil range of 400-450cst setup
xform/index.php?act=view&ID=759&setup=xb808

For the UK our tracks do not groove up as much or are prepped as well so grip is poor. We find the grey springs for us work very well & use Front 1.4 & 600cst, Rear 1.5 & 700cst. For us the 1.3 pistons took away grip so run bigger holes & heavy oil with this setup
xform/index.php?act=view&ID=986&setup=xb808

On the forum we can always give advice but for what is best is down to personal preference as every one will have a different choice & style.

Bren

The theory behind smaller holes and a little lighter oil, is to achieve similar static/slow dampening so you can maximize weight transfer and chassis roll, which generates your grip, and minimizes chassis slap off of jumps and big bumps.

To get the same pack (how much the shock 'stiffens up' on hard hits due to the incompressible nature of a liquid) one would have to compensate with oil weight in a larger holed shock piston to the point where the shock would actually work slower at low compression/rebound speeds than it would with the smaller holes.

I've gone to the track with multiple sets of shocks with the all different sized piston holes, and noticed my car was best with 1.4 6hole or 1.3 6hole in the front and 1.3 or 1.3 5hole depending on how the bumps and jumps/landings are.