I came across this article while researching to find out whether I should bother decompressing an NA diesel motor when turbo charging it :
The short answer is that its not necessary but higher friction losses will result...I found the acticle interesting tho, The result wasnt what I would have expected.
Its interesting that there is alot of mis information out there on this particular topic, and I found a huge variation in what I have seen posted in various diesel engine forums.
It is true though that the RF-N uses a CR of 22.7:1 which is on the high side even for a pre chamber motor.
I would not use a spacer to lower CR as I believe this would alter the combustion chamber shape too much which would have a huge impact on how well the fuel burns, though changing the swirl pattern.
My best option would be RF-CX pistons (which would mean RF-CX rods also thanks to a larger small end bearing, a bonus in a sort of a way)
IMO the slight increase in friction losses through a high CR is probably better than the losses from a spacer (an the associated problems fitting it and stopping it from leaking)
If I can easily get my hands on some RF-CX pistons and rods I might look at fitting them, but other wise just bolt the turbo straight on to the RF-N.
The direct injection RF-T uses a CR of 18.8:1, but apparently the rules are a little different for IDI motors where cold starting often is the reason for a higher than ideal CR, thanks to the surface area/volume ratio of the pre chamber and associated heat loss.