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K04-064 Turbo – Too Small for a 1.8T?

The project Octavia VRS has a K04-064 turbo. An issue I am seeing is that boost is dropping above around 4500 RPM. In this article I want to see if the turbo is the problem ie am I asking for too much? Or is the problem with the setup on the car?

Concerns with the Setup

My main concern would be a restriction in the engine. Prime suspect is the downpipe being 70mm diameter. At the end of the article, I’ll hopefully know if the turbo is too small or if there is a restriction.

To find out if the turbo is a restriction, I’m going to follow the standard turbo selection process.

Step 1

First step is to find out the pressure ratio (PR) and airflow at different RPMs.

Using the 1.8T turbo selection article & video I did before, I can get the engine’s VE. In that article I was looking at 1 bar of boost. On the project Octavia we are running 1.5 bar of boost peak. And the boost is dropping to around 1.2 bar at high RPM. I want to maintain 1.5 bar to redline.

Using the VE numbers from that article and the new turbo boost targets and plugging them into the turbo size calculator we get the following PR, lbs/min and CFM numbers.

RPMBoostVEPRlbs/minCFMBHP Predicted
The numbers in the PR, lbs/min, CFM and BHP columns come from the turbo size calculator.

Step 2

Now we have the airflow numbers & PR, we can plot the points onto a turbo compressor map.

The big issue I have is getting a reliable turbo compressor map for the K04-064 turbo. This is the map I found, hopefully it’s accurate.

Image Source: check out this compressor map and tell me what you think | Page 40 | VW Vortex – Volkswagen Forum
I’m presuming the blue is the 064 turbo.

Plotting our Points on the Compressor Map

The green dots show the plots with the numbers from the table above.

I’m a bit suspicious of the accuracy of the map because it’s showing that the K04-064 turbo will be okay up to around 26 PSI.

The map is also telling us that 314 bhp is no problem for this turbo. And BorgWarner state it’s good for 325bhp and that does add up according to the BorgWarner catalogue.

Is the K04-064 Turbo Too Small?

Again, assuming the map is correct, or very close to being correct (god knows where the poster found it) the turbo should have absolutely no problem maintaining 1.5 bar to redline.

In short, the K04-064 is probably not the restriction on the Octavia project. According to this compressor map (above), if it’s accurate, the turbo can move the air for 314 bhp efficiently. The turbo is actually more efficient at 5000rpm and 6500rpm than it is at 4000rpm.


To find the boost tapering problem I need to look elsewhere. I could look to see where the wastegate signal is coming from, but given that the N75 valve controls boost, 99% that is not the issue. The exhaust downpipe could be the restriction, the PCV valve could be leaking, the intercooler piping could be too small or perhaps the air filter is restricting flow. Those are the targets at the moment. Subscribe to the newsletter below to see what happens.

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This page was last modified Dec 30, 2023 @ 12:46 pm

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