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MK5 GTI Big Turbo – Which Size Is Best?

GTX2876R compressor map with plots for Mk5 GTI

In this article we are going to be using the calculators on StrikeEngine.com to find the optimum MK5 GTI big turbo.

MK5 GTI Big Turbo – Short Version

For our boost target of 22PSI….

First Place: G25-550
Second Place: GT3076R
Third Place: GTX2867R

There are other sized turbos available but these are the three I used in this article.

The Turbo Selection Process

First we need to find the volumetric efficiency (VE) of the engine

Second we choose our boost targets and specific RPM points

Third we plug our targets and VE numbers into the turbo size calculator to get our airflow numbers

Fourth we plot the airflow numbers onto turbo compressor maps to find the optimum MK5 GTI big turbo.

MK5 GTI VE

To get the VE for the TFSI engine we need dyno graphs which also show boost pressure.

Here is the power graph I used. Here is the boost graph I used. Not perfect but it is the best could I find.

Bolting the boost, power and RPM numbers into the VE calculator we get these VE numbers.

RPMBoostPowerVE
2800121200.8
3500121500.82
4500121800.76
550082050.84

MK5 GTI Big Turbo – Boost Targets

We obviously want to make some more power with the big turbo and for that we need to run some more boost. I’ll keep it sensible because the bigger turbo is going to help out the VE at the top end as well.

Targets

RPMBoost PSI
28000
35007
450014
550022
600022 (assuming 0.8 VE)

Turbo Size Calculator Flow Numbers

Putting the above numbers along with the VE into the turbo size calculator we get these airflow and pressure ratio (PR) numbers

RPMBoostVElbs/minCFMPR
280000.86791
350070.8211.51501.47
4500140.76182361.95
5500220.84314072.5
6000220.832.542232.5

With the airflow numbers and the PR numbers (right columns) we can plot these points onto some turbo compressor maps and see which is the best size of MK5 GTI big turbo.

Turbos

Garrett give excellent info on their turbos so these are the turbos we are going to use in this article

GT3076R

The GT3076 is usually the go-to turbo for 2 litre engines so let’s start with that.

GT3076R compressor map with plots for Mk5 GTI
GT3076R – Maybe a bit big?

G25-550

G25-550 compressor map with plots for Mk5 GTI
G25-550 – Every plot looks to be above 70% efficiency

GTX2867R

GTX2867R compressor map with plots for Mk5 GTI
GTX2867R – Lower RPM less than 70% efficiency

Best of the three

The G25-550 looks a good choice as a MK5 GTI big turbo upgrade. While the plots are towards the edge of the efficiency islands the G series of Garrett turbos have a higher efficiency all round which makes them look worse than they actually are. For example even though the low RPM plots are around ten islands away from peak, that still lands the plots on a 70% efficiency island. At high RPM we are bang in the centre of the compressor map. Out of the three turbos above, the G25-550 looks to be the choice.

Second place goes to the GT3076R. On first glance it looks a bit big for the TFSI motor but looking a bit closer the peak efficiency island in 77%. At 4500 RPM we are around 7 islands away from peak but that still means we are at 70% efficiency. At high RPM we are at 74/75% efficiency. The issue I see with the GT3076R is there isn’t much headroom is any to turn up the boost above 22PSI, we will be getting a bit close to the surge line. It will probably be fine and I’m sure there are thousands on MK5s out there using this turbo but for this article we are looking for the hypothetical optimum sized turbo and personally I don’t think the GT3076 is as good at the G25-550.

The GTX2867R. Peak efficiency is the lower than the other two turbos and efficiency at 3500 and 4500RPM is less than the others at around 67%. At high RPM we are in the sweet spot for the turbo but 22PSI is getting close to the limits of the turbo.

Summary – MK5 GTI Big Turbo Selection

All three turbos look fine for our 22PSI boost target.

My first choice would be the G25-550. Fine for our 22PSI boost targets and also looks fine if we want to up the boost to 29-30PSI

Second is the GT3076R. Fine for our boost targets but personally I wouldn’t go above 22-25PSI

Third the GTX2867R. Efficiency not as good as the other two turbos and even less headroom than the GT3076R.

Some Provisos

First of all the numbers in this article are hypothetical. This article is not a replacement for speaking with professionals and doing independent research.

Second and this is a big one. The VE numbers can/will change as we flow more air through the engine. For the numbers in the article to be in the ball park (in real life) we have to eliminate obvious bottle necks at higher horsepower. This could mean having to fit a freer flowing exhaust manifold, bigger intercooler piping, bigger air filter, bigger intercooler, larger diameter downpipe, larger exhaust, better camshafts and so on.

And remember, when we change these parts, in a “perfect” world we will do the turbo selection process again to see if we have the correct turbo. Not ideal in the “real” world after we have spent the money.

There is a potential for some tail chasing but hopefully this article will get you pretty close to where you need to be. Again, this article is not a replacement to speaking with professionals and owners who have done it already.

What we can do is analyse some tuned motors to see if their turbo selection is good but that will be for another article. Subscribe to our newsletter to keep updated!

This page was last modified Nov 10, 2022 @ 1:37 pm

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