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.
Bolting the boost, power and RPM numbers into the VE calculator we get these VE numbers.
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.
|6000||22 (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
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.
Garrett give excellent info on their turbos so these are the turbos we are going to use in this article
The GT3076 is usually the go-to turbo for 2 litre engines so let’s start with that.
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.
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!