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Old August 19th, 2006, 04:18   #3
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Join Date: May 1999
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Default Part 1: Introduction

I’m sorry for being delayed in responding to this, although I had always intended to address this question. Unfortunately, I have not taken the time to search the Internet for a good and succinct tutorial on reading compressor maps, but I’m sure searching on Google will get you the same results as I would... so here is my “short” spiel about it. Because of the length of this, I will split this into a couple of separate posts, beginning with
1. Introduction and a brief note about the specific turbo being discussed
2. Important definitions for reading a compressor map
3. Plotting operating points between engine volumetric capacity and the turbocharger (the fundamental aspect of turbo matching), as well as further reading references.

First of all, I will use as a subject, a turbocharger that is one of the most hotly discussed for TDIs at the moment: the VNT-20, a.k.a. GT2052V or GT2252V. It should be noted that there are many, many variations of turbochargers within each Garrett product family. When you ask for a VNT-20 or GT2052V or GT2252V, you’re not just asking for one specific turbocharger, but rather there could be dozens of options to choose from among various OEM sources (there are no known aftermarket sources for VNTs per Garrett policy):

For now, however, the compressor map I’m using comes from a VW-authored paper on the 1997 R-TDI. It is a VNT-20 model, but the exact VW part number or original fitment of this turbocharger is uncertain. However, based on the 1997 date of the paper, and the shape of the compressor outlet of the VNT-20 unit available in VAG models at the time compared to that installed on the 1997 R-TDI, it is reasonable to deduce that this turbocharger is the same as or close to that installed in the top-rated 5-cylinder TDI engine of the period, that is, 074 145 703E pictured below and NOT 059 145 701E/-F/-G installed into the V6 TDI nor 145 703D installed in the LT II. See pictures below to illustrate compressor outlet differences.

VNT-20 turbocharger installed on the VAG V6 TDI and LT II. Note the direction of the compressor outlet is parallel to the inlet.

VNT-20 turbocharger installed on the VAG 150HP 2.5 5-cylinder TDI (AXG/AHY; 074 145 703E).

Rear view shot of the R-TDI engine. Note the compressor outlet direction perpendicular to the inlet and apparently simply clocked differently to the stock AXG turbo…

With that out of the way, on to the map:

It should be first noted that maps are also NOT generic within a turbo family but rather are specific to parameters such trim, A/R ratio, exducer diameter, blade count, etc., and OEMs can vary these options in order to get the optimum match to their respective engines. Therefore, using THIS map can be different from the map of another VNT-20, even from the same car manufacturer! Use this map ONLY as a reference and rough guess! In future, if there is interest, I might give a short explanation of these parameters and their effects, but your best resource is to refer to the books I list at the very end.

Everything discussed here onwards will reference to what is known about the R-TDI turbo and the T4 Transporter OEM version (074 145 703E).

TDIMeister's German-imported 1998 Audi A4 Avant TDI quattro

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Last edited by TDIMeister; December 15th, 2007 at 15:57.
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