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How bad an upgrade is a K04 turbo on a 1.8T 20V?

Upgrading to K04 turbo on a 1.8T is common for the engines using a K03 turbo from the factory. But is a K04 turbo really an upgrade? Is it worth the time and money?

Lets analyse the performance characteristics of the K04 turbo to see what it can do for us on the 1.8T

Step 1 K04 Turbo 1.8T Analysis – Volumetric Efficiency (VE)

I have done an article on 1.8T big turbo selection before. I am going to use the VE numbers from that article here


At 2000 rpm .. 0.8
3000rpm .. 0.85
4000 rpm .. 0.91
5000 rpm .. 0.88
6000 rpm .. 0.78

VE numbers for the 1.8T engine

Step 2 K04 Turbo 1.8T Analysis – Pressure Ratios & Flow Rates

Again, using the data from the 1.8T big turbo analysis we get the following air flow and pressure ratio numbers for the following boost targets (when we bolt our VE and boost target data into the turbo size calculator).

Boost targets are: 7 PSI at 3000rpm, 15PSI at 4000rpm and 15 PSI and 6000rpm.


We’ll use the PR (Pressure Ratio) and lbs/min (air flow) numbers below

Step 3 K04 Turbo 1.8T Analysis – Plotting Points on the Turbo Compressor Map

Plotting the air flow and pressure ratio numbers from each RPM onto the K04 turbo compressor map we get this

Air flow and pressure ratio numbers plotted onto the K04-015 compressor map.

What do we see?

Up to 4000rpm the K04 turbo is absolutely perfect or as close as we can reasonably hope for.

However at 6000rpm efficiency drops to around 67% from 74%. Doesn’t sound like a big drop but peak efficiency for the K04 turbo is only 75% and anything less than 60% is not even plotted on the map.

Also at 6000rpm the turbo is spinning at over 160,000rpm.

In short, we will be pushing the turbo hard to get our boost targets.

When we are pushing the turbo away from the peak efficiency islands, the turbo is starting to heat the air more than pressurise it.

It would be better for the turbo to be getting closer to peak efficiency as the RPM increases. But the K04 is moving further away from peak efficiency as RPM increases.

How Much Power with a K04?

250-270bhp seems to be a common peak power number.

For example


Torque starts to drop off rapidly at 5000rpm. As we move above 5000rpm the turbo efficiency is dropping

K04 Turbo Dyno Graphs

This page has a selection of K04 dyno graphs. The pattern is the same for all of them. Torque is dropping of after 5000rpm. Peak power at the flywheel is around 250-270bhp.

& one more dyno graph

Is the K04 turbo worth it?

What power do people get from the K03S turbos? 190bhp? 210bhp?

Is it worth the hassle and cost of fitting a K04 to get an extra 50bhp?

When you take into consideration the supporting mods you will need to do to get the most out of the K04 ie injectors, downpipe, exhaust, intercooler, MAF, program, time/labour. Wouldn’t it be better to just pony up a few hundred extra notes to get into a GT2860 or a GT2871?

The GT2860 will get you to 300bhp with less strain on the engine because you will need less peak torque. You can achieve more power by maintaining lower torque but to redline.

Power from torque calculator here. See how much extra power you can make if you could maintain your peak torque to 6000rpm.

Is the K04 a worthwhile upgrade? It depends what your goals are. From my point of view, for the sake of a turbo manifold and bigger turbo, I think I’d spend the extra few hundred to get a GT2860 instead. I’ll be comfortably into the 300bhp range and also giving my engine an easier life because its making the torque and power at a higher RPM and I can run slightly lower peak torque to get the bigger power numbers. (If you have a built engine then the GT3071 has got to be on your radar instead)

Find performance parts on ebay

This page was last modified Apr 28, 2022 @ 4:56 pm

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