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How to Choose a Turbo Size – Things you need to know

This guide will show you the numbers you need, how to get these numbers and how to choose a turbo size for your engine.


Calculator Quick Links

Turbo Size Calculator

Engine Volumetric Efficiency Calculator


In a Nutshell

-We need to know how efficiently a turbo can flow X amount of air at Y pressure ratio > (turbo compressor map)

-We need to know how efficiently our engine can flow air > (engine volumetric efficiency)

-We need to know how much air our engine will be flowing at our boost pressure (Pressure Ratio/PR) target at our chosen RPM > (turbo size calculator)

When we have the air flow and PR numbers, we can analyse turbo compressor maps and find the optimum turbo.


Turbo Compressor Maps

The fundamental thing we need are turbo compressor maps.

If we don’t have a turbo compressor map for a turbo, eg the turbo manufacturer does not have one, it is impossible to know if the turbo is good for our engine or not.

About Non-Existent Compressor Maps

There are turbo manufacturers who supply turbos with the only spec being “this turbo is good for X bhp”. But it leaves a couple of key questions unanswered.

  • At what boost does the turbo make that power?
  • At Y airflow and Z pressure ratio, how efficient is the turbo? How much heat is the turbo putting into the intake air?

An example of the problem with just having a X bhp rating for a turbo

We have Person A with a K20 (2 litre) and we have Person B with a 4 litre V8 and both want around 600 bhp.

Can both people buy the same “600 bhp” turbo?

And will the turbo perform the same on both engines?

Will the turbo make the same power at the same RPM & boost on both engines?

I think we all know the answer to these questions.

And on top of these issues, it makes it impossible to compare turbos.

How can we compare a turbo from Garrett (who supply a compressor map with their turbos) with a turbo from a manufacturer which does not supply a turbo compressor map?

Of course, we can’t.

There is no way to know if the unknown spec turbo is better or worse than a Garrett. Unless we test both turbos on the engine, map it and drive it.

But I think it’s a bit cheaper and a bit easier just to compare compressor maps.

Turbo BHP Rating – Summary

In short, the power capability of a turbo, in my opinion, is next to meaningless when it comes to picking the right turbo.

OK, So What Data Do We Need & How Do We Get It?

Boost Target

We need to know our target boost for various points in the rev range eg 3000, 5000, 7000

Engine Volumetric Efficiency

And we need to know the volumetric efficiency of our engine at these same RPM points. (Don’t know the volumetric efficiency? Use the calculator to find out, click here)

Turbo Calculator

With this information we can use the StrikeEngine turbo calculator to get our pressure ratio (PR) and the CFM & lbs/min flow rate (at our RPM points).

When we have our flow rate and PR points, we can plot the points onto turbo compressor maps to see if a turbo is suitable for our engine and/or compare turbos to see which is most suited to our engine. I’ve done an article on turbo compressor map analysis here.

How to Choose A Turbo – Worked Example

I’m not going to reinvent the wheel in this article. I have already done a worked example showing the turbo choosing process on the legendary VW 1.8T 20V motor. You can read the article here. And you can see the accompanying video here.

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This page was last modified Dec 3, 2023 @ 4:11 pm

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