In this article I am going to look for optimum size of a MK4 1.8T Big Turbo. To find the size I will be using the StrikeEngine turbo size calculator and the StrikeEngine engine volumetric efficiency calculator . (Also applicable to Audi A4 1.8T, Audi A3 1.8T, S3 8L, Seat Leon 1.8T, Ibiza, Skoda Octavia 1.8T, Golf MK4 GTI).

Short Version

The Garrett GT2860RS looks perfect up to 300bhp and 15PSI of boost

The Garrett GT3071R looks perfect up to 400bhp

Turbo Shopping

Buy your Garrett GT2860RS on ebay
Buy your Garrett GT3017R on ebay

First Step – 1.8T Big Turbo Selection

We need to know the volumetric efficiency (VE) of the engine at various RPMs. I have chosen 2000, 4000, 5000 and 6000rpm

To calculate the VE of the 1.8t, we need dyno graphs of engines which are stock or close to stock.

Luckily there is a page dedicated to 1.8T dyno graphs here. Not for stock engines but it should give us the ball park figures we need.

From the graphs we pull out the power & the boost the engine was running at 2k, 4k, 5k and 6k rpm. We plug these numbers into the engine volumetric efficiency calculator and it gives us the VE (right column) at that RPM

This is the data we get….

**Click on the images in the 1.8T dyno graph page. K03S is “item 4”, K03S b is “item 5” and K03S c is “item 7”

The VE Numbers (Right hand column)

At 2000 rpm we have quite a big spread of numbers due to the boost being different. Lets go for 0.8

4000 rpm 0.91 is the middle value of the three

5000 rpm 0.87, 0.87 and 0.91. We’ll call it 0.88

6000 rpm 0.65, 0.77 and 0.71. Turbo looks to be a restriction. For this reason we’ll bump the VE up a little bit. Let us say 0.78.

Getting the 1.8T Big Turbo, Pressure Ratios & Airflow Numbers

Now we have the VE of the engine we can use the turbo size calculator to get our pressure ratio (PR) and air flow numbers. We need the airflow (in lbs/min or CFM depending on the turbo manufacturer) and pressure ratio (PR) to analyse turbo compressor maps.

Targets

For this hypothetical example I am going to say we want to have 7PSI of boost by 3000rpm and 15PSI of boost by 4000rpm and we want to keep the same boost pressure until 6000rpm+

We measured the VE at 2000rpm and 4000rpm. We’ll split the difference to get the VE at 3000rpm. 0.85

Entering the RPM, our boost target at that RPM and VE at that RPM, into the turbo size calculator we get the following PR, lbs/min and CFM numbers.

For this example we are going to look at Garrett turbos and specifically the older GT Series turbos.

1.8T Big Turbo Analysis

Below are the turbo compressor maps for three turbos from the GT Series range. We will plot the PR and lbs/min numbers (that we got from the turbo size calculator, right hand columns in the table above) onto the turbo compressor maps to see where the 1.8T engine lies.

GT2860RS

GT3076R

GT3582R

Observations

GT2860RS

For the stock engine ie stock internals with the stock exhaust manifold the GT2860RS looks a very good match. If we were to fit a freer flowing manifold and big diameter downpipe our green plots would move to the right which would put us even further into the high efficiency area for this turbo.

The big issue we have with the 2860 is if we wanted to increase the boost from say 15PSI to 21PSI. At 2.5 pressure ratio we will be at the edge of this turbo, 17-18PSI is probably the limit if we want to keep the intake temps down.

Buy a GT2860RS on ebay

GT3076R

The stock engine is barely suitable for this engine. However with a big diameter downpipe and free flowing tubular manifold the plots will move to the right which would see good improvements in turbo efficiency.

The extra benefit the GT3076R gives us over the GT2860RS is the extra headroom we have to run more boost. At 22PSI (2.5 PR) we are still close to the meat of the peak efficiency island although still a little close to the left side of the map.

GT3582R

Even if we were to improve the VE of the engine with a tubular manifold and big bore downpipe I think the turbo still looks too big. To make this turbo work I think more radical modifications would be needed to increase the VE, I’m thinking ITBs and camshafts at a minimum. In short, for 99% of people, this turbo is a complete non starter.

For s***s & giggles. Stock motor GTX3582

1.8T Big Turbo – An Alternative

So the GT2860RS looks like a winner for the stock engine, stock manifold, normal bore downpipe but its limited if we want to run more boost.

The GT3076R looks much better for higher boost with exhaust mods like the manifold and downpipe. But maybe the plots would still be a bit close to the left.

Lets take a look at a slightly smaller turbo, the GT3071R

GT3071R

The GT3071R looks to hit the sweet spot. Even at 3000rpm and 7PSI we aren’t in a horrible place and at higher RPMs we are around the peak efficiency islands. With the downpipe and manifold mods, again, moving the plots to the right, things are going to get even better and if we up the boost to 22PSI (2.5PR) we are still in a sweet spot.

All-in-All

The GT3071R looks to be really well suited for a stock and a modified 1.8T. For sure we are going to be giving up power and response from 3-4K RPM and down but we get a big payback at the top end.

GT3071R, big numbers, no problem

Tuning a GTX3071R

Big Turbo Extra Power – How Much?

NOTE: All the power and torque (flywheel) figures below assume we can maintain VE at higher horsepower. This may entail having to fit cams, bigger intercoolers etc. The article focuses on the turbo sizing only, not the supporting mods needed for the turbo.

Let’s use another StrikeEngine calculator to find out.

Going back to our dyno graphs for the 1.8T. If we look at “K03S c/item 7” we can see the engine can’t maintain torque to 6000rpm, presumably and largely because the turbo is choking the engine.

The bigger turbos on this page will all be able to maintain boost to 6000rpm (and to the redline). If we assume that they will also be able to maintain torque to 6000rpm, let us see how much extra power the “item 7” engine will be making with the big turbos.

Stock Turbo

With the K03S turbo it makes 230bhp peak power at 5000rpm. But peak torque is 304lbs/ft at only 3500rpm.

How much power would the engine make if it could maintain this 304lbs/ft torque at 5000rpm & 6000rpm?

Big Turbo

Bolting into the calculator below the 304 lbs/ft number and 5000rpm. It gives us a peak power number of 289bhp.

A 59bhp increase.

Bolting in 304lbs/ft @ 6000rpm and we get a bhp number of 347bhp,

A massive increase of 117bhp!

(Even the GT2860RS is theoretically suitable for this type of power level at 15PSI)

And remember, because the torque is the same (“item 7 car”) we are not putting an extra stress into the engine components (although we will definitely be using the engine at higher RPMs much more of the time to exploit this power and this will wear things out faster)

GT2860RS taken to the limit (and beyond? 26PSI)

Increasing the Boost

The GT3071R also gives us the possibility of increasing the boost, at high RPM, giving us even more power.

Buy a GT3071R on ebay here

Let say we increase the boost to 21PSI from 15PSI (a 20% increase absolute pressure). Increasing the torque number by 20% at 6000rpm gives is 364lbs/ft and using the calculator below, we get a power figure of 415bhp @ 6000rpm.

A 180bhp increase over the K03S, for a 7PSI increase in boost. The difference over the K03S being we are able to maintain boost and torque to redline.

But for sure, we would be at the ragged edge of con rod capability and the engine will need properly sized supporting mods to maintain the VE at high RPM eg intercooler, intercooler piping.

MK4 1.8T Big Turbo Wrap Up

Out of the turbos on this page I think the GT3071R looks the best match as a 1.8T big turbo kit upgrade turbo. It should just about work with a stock engine and gives us plenty of possibilities to increase boost when we have upgraded our turbo manifold and downpipe.

And an honourable mention goes to the GT2860RS. We are limited to 15-18PSI but it should maintain efficiency to redline at this pressure. The bonus of using the GT3071R over the GT2860RS is we can run 22PSI, maybe up to 25PSI, with the turbo still being efficient.

I guess the question we need to ask ourselves is how far do we want to go? Is circa 340bhp enough? If so the GT2860RS should be good. If we want more, while keeping the turbo outlet/engine intake temps reasonable, the GT3071R looks the choice.

DISCLAIMER

Remember, this is just a guide to give an idea of the possibilities of using a 1.8T big turbo upgrade. This guide is not a replacement for speaking to an experienced tuner and doing lots of personal research.

Stock Manifold(!) GT3071R. 22PSI and around 380bhp.

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