Trials and tribulations of turbocharger sizing

I've spent the past month or so trying to decide on a suitable turbo upgrade. Back when I had the 300 it was much easier, just bang in the wastegate , crank the diaphram on the fuel pump up (hmm, or is that the other way around?) and voila!!! instaboost!!

The td5, however, is a bit more tricky, rudimentary overboosting techniques would send the computer into a limp mode, which is just no good, and that leaves me with one option, that is, purchase a boost bypass module that sends a dummy signal to the ecu to get around the limp and have the ecu remapped to compensate for fuel.

Mapping choice is an issue I'll deal with in another post once I can understand it better. So for now I'll keep it turbocharger related only.

Originally, So I started doing a bit more online reading into which one would be an (almost) direct bolt on. The stock unit is a garret GT20 with an A/R of .50 so I just said that I'd just clap a larger turbo on it, "yes, a nice ball-bearing unit for lightning spool times and be happy with that, after all, it's just an old landy". Then I thought to myself "Hang on, maybe I should do this properly" so I started traipsing the interweb forums for advice on turbo selection, and there wasn't much there really. Then I found a turbo sizing calculator over on squirrel performance's website.

I figured a GT28rs sounded like a good starting point so that's what I began with, and after crunching the numbers against the GT28rs with the 0.60 A/R through squirrel performance's calculator this is what came out;

Good lord, I may not be the sharpest tool in the shed, but that can't be good. My understanding of turbotech is that an ideal plot would be right through the centre of those islands, as close to the dotted line as possible. The left line of the map is the surge line, which the disco-potato edges dangerously close to, I know the diesel won't surge per se because there's no throttle plate to surge against, but logical thinking tells me that it would result in my air-fuel being way out of whack, and possibly even leaning out the motor (?).

Edging certain frustration with turbo selection, I thought let me just punch in the same parameters on the GT20 and see what comes out, and this is what followed;


Great, that's that then, I'll stick with the GT20 unit and probably just have it reworked a bit by the good folks of TD5alive and that suffice. I've just gone around the world in a massive exercise to find out that Landrover had done their homework properly to begin with.