Turbo Compressor Efficiency Calculator – Is your turbo correct?
June 3, 2025Use this turbo compressor efficiency calculator to discover if you are using the correct sized turbo for your engine, boost pressure and RPM targets. Log turbo compressor inlet and outlet, temperatures and pressures, at various operating conditions. Enter the data into the calculator below to get a ball park figure of your compressor efficiency, at your chosen operating conditions.
Turbo Compressor Efficiency Results
Ideally we want our turbo compressor to be operating at at least 70% efficiency at the RPM and boost levels where we use our engine.
If our turbo is inefficient at key load/RPM sites there could be a number of causes.
Boost
Too much boost. Are we asking the turbo to make more boost than it is designed for?
Some turbos can operate at much higher boost levels than others.
If turbo is inefficient everywhere, we may not be asking the turbo to produce enough boost.
Engine Size
Depending on where the turbo is inefficient.
Is it bad at high flow conditions or low flow?
Bad at low flow suggests turbo is too big. Bad efficiency at high RPM? Turbo may be to small.
If we think we are asking for reasonable targets but the turbo is inefficient, the engine capacity may be too big or too small for the turbo.
Setup Issues
Logging the data can highlight issues not connected to the turbo.
Intake
For example, is the compressor inlet, absolute pressure below ambient pressure? If so this suggests we have a restriction in the intake system somewhere.
Not so much related to the calculator above but if we are seeing high intake temps, this suggests we have a problem with the routing of our intake system, it is getting heated or it is drawing in hot air?
Intercooler
The logs may highlight intercooler performance. We make think our intake temps are far too high but if we see our turbo outlet temps are far too high and the turbo is inefficient, we may have a different perspective on our intercooler performance.
Is the intercooler actually doing a superb job and the real issue is we are operating our turbo way outside of its peak efficiency?
Compressor Outlet Pressure
Logging compressor outlet pressure (absolute). We may see it is much higher than we are seeing from our MAP sensor. This may be normal or it may indicate we have a big restriction in the intercooler piping somewhere, perhaps the pipes, perhaps too many bends, perhaps the intercooler.
Build Our Own Compressor Map
Unfortunately many turbo manufacturers do not publish compressor maps to accompany their turbo. Some simply state that their turbo is good for X bhp. Only giving a power rating for a turbo is pretty meaningless. A one litre engine will need to run much more boost to make the same power as a boosted four litre engine. So the power rating is for what sized engine?
If we have a turbo on our engine for which we don’t have a compressor map, we have the potential to build our own map.
Why would we do that?
In short, it can help us identify how our turbo is performing, if the turbo we have has potential that we are not tapping into (and likewise if we have a turbo we have maxxed out).
Where compressor maps are supplied by the manufacturer the data will cover all conceivable operating conditions. We don’t need to do that. We are only interested in the areas of the map where our engine operates. Do three pulls for low RPM to redline at say, three different boost levels. That should give us a rough picture of where our turbo is.
We can more than likely log RPM, mass airflow and manifold absolute pressure with our OBD scanner app. And log our compressor inlet and outlet numbers with an external datalogger. Our boost gauge will help us to keep constant boost through the pull.
The RPM and mass air flow numbers will give us the points to plot on our DIY compressor MAP.
Our compressor inlet and outlet numbers will tell us the efficiency numbers that correspond to the plots we have put on our compressor map.
Into the weeds
We can go pretty deep on the logging if we want to but if we just want a rough idea of how our compressor is performing, twenty pulls at different boost levels should do the job.
Permanent Install
We probably won’t need to be logging/monitoring compressor efficiency all of the time. Getting a base line of where we are now is enough. And we’d only need to measure again if we change something.
Turbo Compressor Efficiency Logging – Summary
Not only can the turbo efficiency calculator help us identify if we are using the correct turbo compressor and/or using the turbo we have in the correct way. It can also help us identify problems in other parts of the engine from the air filter through to the cylinder head.
We may find we are operating our turbo, for most of the time, in good efficiency islands but we may also find numbers which could be better (eg vacuum in intake system). Even though our turbo is happy, logging compressor data will also tell us if we can improve things, if we are leaving power on the table.
