Choosing the right size turbo for your application and knowing when you’ve used all of its power potential are key to building a reliable turbocharged car.

The tubocharger’s job is to compress air and force it into the engine, cramming more air into the cylinders so you can burn more fuel to make more power. Simple? Well…

Choosing the right turbo

The journey to running out of turbo starts as soon as you’ve decided which one you want/need – so how do we even make that choice in the first place? When researching you’ll always find forum posts about specific setups making a tonne of power and dyno charts that are say within 20-30% of each other – it’s never a cut and dry answer on what power the setup will make.

That’s where compressor maps come in, while forum advice is helpful to find a baseline idea of which turbos to consider, compressor maps tell us how much air can be flowed at a given boost pressure.

On the Left Side of the compressor map we have a pressure ratio – and to make this easy if you live at sea level the pressure will be 14.7psi or thereabouts. For the purposes of our tests we’re using ‘standard day’ which is also 14.7psi.

Compressor maps take into account the fact that as you increase in altitude the relative pressure decreases, which, if we maintain the same boost pressure, increases the pressure ratio.

So how do we work out what pressure ratio we need? And how does LBS/Min relate to horsepower, and how can we work out where the engine is operating in this map? When will the turbo start to spool? How responsive will it be?

Prepare yourself, for some maths – I’d recommend tea, yorkshire tea to be precise.

Horsepower Goals – The right hand side of the compressor map.

Now armed with the information above we can start to look at compressor maps with the idea in mind of how much horsepower we are looking to make.

Compressor maps display the amount of air the turbo will flow, either in CFM or LB/Min, a general rule of thumb is 10bhp per lb/min of airflow – however depending on how efficient your combustion is, how much timing and fuel you’re running this could be higher or lower.

To accurately work out the horsepower goals, we need to look at how much air the engine will move naturally aspirated. A good equation for this is;

Engine Size in CC: 1598

Volumetric Efficiency: 0.8 (note this is a ‘reasonable’ estimate)

Maximum RPM: 7200

Briefly…. Volumetric Efficiency describes how efficiently the air in the cylinder is used. Having a mass air flow sensor would allow us to better understand the exact airflow entering the engine, when coupled with the fuel required would give us the effective VE. However this is not a constant and changes with RPM and is also heavily impacted by the camshaft and overall engine design.

To make this easier, we’ll work in imperial measurements:

Engine Size in CC 1598 x 0.0610237 = 97.515 cubic inches

For a 4 stroke engine the calculation is as follows;

Engine Size in cubic inches x Maximum RPM (97.515 x 7200) = 702114.282

Divided by 3456 (702114.282 / 3456) = 203.158

This describes the amount of air the engine can physically pump, however we all know that porting heads, bigger valves, better intake and exhausts improve power – but they don’t make the engine breathe more than it physically can. What it does is optimise the available airflow potential of the engine i.e. its volumetric efficiency.

Divided by the Volumetric Efficiency (203.158 * 0.8) = 162.52

That gives us the CFM of the engine to work with. 162.52

Some compressor maps work in CFM, but most are in lb/min (pounds of air) so we’ll need to calculate that and as Cubic Feet is a measurement of volume, not of weight (or rather, density)  and the weight of air changes depending on static pressure, temperature and humidity we’ll need to use a standard day calculation.

The standard day is as follows:

Temperature: 15c

Pressure: 14.7psi

Humidity: 0%

This means that for every degree over 15c we’re losing power as the air is less dense for the same volume – i.e. it contains less oxygen.

We can calculate LB/Min by multiplying the CFM by 0.069 (162.52 *0.069) = 11.213

Naturally aspirated, with a reasonable volumetric efficiency of 0.8, on a standard day, the engine is moving 11.2 lbs of air per minute.

With this information to hand, we have a baseline to work from, we now need to decide how much air we need to flow, this is where guesswork, estimation and testing comes into play as there are a variety of factors here that are simply a ‘best guess’.

A rule of thumb is that for every 1lb of air we can generate 10BHP, some suggest that this should be 10 ‘Wheel horsepower’ which adds even more variables into the mix. We’ll use 1lb = 10BHP (at the flywheel) in our calculations below and adjust the volumetric efficiency of the engine to move expected airflow around.

So let’s calculate how much boost we need to run in order to hit our horsepower / airflow target – which in this case is 250BHP, or 25lbs, an accepted safe stock engine power level.

Taking the naturally aspirated maximum airflow of 11.2lbs knowing we want to make 25lbs the equation is: 25 / 11.2 = Pressure Ratio (2.23)  and if we assume the atmospheric pressure is 14.7 then a PR of 2.23 would be 18psi.

We know from testing and anecdotally that this number is achievable with less boost pressure which tells us that the volumetric efficiency of the engine increases somewhat considerably under boost.

But for the purposes of following what the maths tells us, let’s see what the minimum turbo we would need to make 250BHP reliably on a 1.6 Litre MX5 engine.

GT2554R Compressor Map

With this turbo we can see that a pressure ratio of 2.23 puts us in the highlighted area and in order to make 25lb/min of airflow we’ll be right at the far right of the turbo, effectively over the mapped area in sub 65% efficiency. This turbo in theory, may be too small for our needs, let’s quickly dive into why. (Anecdotally the most power we’ve made on a 1.6 is 210WHP (measured at the hubs) at 16psi, with that boost around 3700rpm).

Turbo Efficiency Explained

Calculator here: +

The turbo’s job is to compress air, when you do this that process creates heat and hot air is less dense, in our examples above we’re talking about air in the sense of weight i.e. 1lb of air moving through the cylinder every minute. 

That calculation is designed to ignore density as the ‘mass’ of the air is always the same. This isn’t the case in the real world.

Measuring the throughput in CFM (cubic feet per minute), that’s a measure of volume i.e. how much space is in the cylinder. If the air in that space is say 0c then the oxygen molecules will be far closer together, meaning we can fit more in the same space – giving us a more dense charge with far more power potential.

When a turbocharger takes in fresh air at say 15c ambient and compresses it, it will output that air at our pressure ratio of 2.23 at 65% efficiency at 128c

The outside air started with a density of 0.0012253g/cm³ and after it’s been processed by the turbo its density is 0.0019628g/cm³, this equates to 60% more air (oxygen) entering the engine, for 2.23x the airflow. You wouldn’t want to run this kind of hot air into your engine as it will massively decrease the VE of the engine itself and promote premature knock.

If we parse this air through an air to air intercooler at 85% efficiency with 15c ambient temps we’ll end up with air into the engine at 31.6c which has a density of 0.002505g/cm for a total increase vs naturally aspirated density of 104.44%. 

However as we’ve increased the density the pressure will have dropped out of the intercooler – this isn’t a bad thing as its usually only 1psi or so, but it’s a required trade off. It’s worth keeping in mind that now if we want to add that 1psi back in, we’ll be pushing, even more, air and working the turbo even harder.

Another point to keep in mind is that our horsepower goals are based on consuming 2.23x the amount of oxygen/airflow and 104.44% is only 2.04x – which means our actual airflow in terms of lbs/min is lower than expected after processing.

We’ll now be asking whats a reasonable lowest efficiency we should aim for in the compressor map. The answer to that isn’t all that simple as a 15% increase in efficiency only drops our system temperature down to 28.88c, not a big jump.

It does, however, bring the total density to 106.27% – a 2% increase in power potential could make the difference and just dropping your intake air temps will massively help the whole system, in reality, the air temps under the hood will be considerably higher than ambient.

While it is important to maintain a reasonable (65%+) efficiency with your turbo, its far more important not to run its RPM too high on the right-hand side of the map, the efficiency starts to fall off a cliff not far after the unmapped sections on the right hand side, killing horsepower and potentially the turbo too.

Anyway, back on to finding our choice turbo (note, all the turbos below are ball bearing in nature, not oldschool journal CHRAs)

GT2560R Compressor Map



Garrett GT2560R Turbocharger

836023-5004S / 466541-4 / 466541-5004 £679.99 inc VAT

With this turbo, we can see that if at a PR of 2.23 at 25lb/min of airflow we would be in a very efficient zone at 72%. We could fairly easily push this to 30-33lb/min as a reasonable maximum at that pressure. 14psi at 3700-3800rpm, positive pressure at 2500rpm or less.

GT2860RS Compressor Map

GT2860RS Compressor Map

We can see with this turbo that it’s more efficient than the GT2560 at 25lb/min and would be happy to run out to 34lb/min at the upper end of its airflow at 2.23pr. 14PSI at 4100rpm

GT2871R Compressor Map

With this turbo at 25lb/min at 2.23 we’re in a 77% efficiency range, but this turbo really wants to be operating between 35-40lbmin at 2.5pr so we would have a hard time keeping this in spool at the lower rpms.


This turbocharger favors high-pressure ratio use, at 2.23 aiming for 25lb/min we would be barely waking this unit up and it would be very efficient at 72%. 14psi at 4300-4400rpm

GTX 2860R GEN 2 Compressor Map

GTX GEN 2 2860R

At 25lb/min at 2.23pr we would be sat at 73% efficiency in a reasonable area of the map, with scope to push the pr up to 2.5 to achieve 30lb/min but not much more than 33-35lb/min before this turbo becomes inefficient. 14psi at 3900-4000rpm

EFR6258 Compressor Map

EFR6258 Compressor Map

This turbo has an incredibly broad range of use, we can see that this one starts to make the pressure even down at 5lb/min of airflow. At our target of 2.23 at 25 we’re in 74% efficiency, and pushing this to 31lb would bring the efficiency up to 76%! Tracking all the way to ~40lb/min 14 psi at 3800-3900rpm

EFR 6758 Compressor Map

This turbo favours higher pressure ratios compared to the 6258 and has a far wider map, supporting 48-50lb/min at the edge of its efficiency range, 14psi at 4000-4100rpm.

EFR 7163 Compressor Map

EFR7163 Compressor Map

This turbo is very much looking for higher boost pressures of 3-3.2PR and will deliver ~55lbmin.  However this could potentially make 14psi at 3900rpm.

Using All Of the Compressor

We’ve identified a number of turbos we could use to meet our overall horsepower goals, but that’s only part of the requirement here.

We need to think about the use case of the car, where is it going to be spending most of its time with respect to the RPM Range. A street car needs to have good throttle response and pickup from as low as reasonably possible. A track car spends most of its time above 4krpm and needs to make consistent, efficient power at a constant high load.

With this in mind we’ll need to work out the airflow of the engine at each RPM point we’re interested in, here’s a handy calculator for that:

Use this calculator to work out at what RPM your engine is making the airflow at the pressure ratio we’re aiming for, check the compressor map – track where the map hits a PR of 2.0 and check the airflow, then compare that to the RPM of the engine – that’s roughly where it’ll make that pressure on that turbo, note VE makes a HUGE difference here.

Intake Air Temps

Most of the time we’re only interested in what the temperature of the air actually going into the cylinders is because that’s what we’re combusting and using to make power.

However, in the case of an intercooler car we are not seeing the temperature of the air as its generated out of the turbo, depending on how efficient our intercooler is we could be much further right of the compressor map than we think because the air coming out of the turbocharger could be far hotter than we expected.

This would put the turbocharger in an island that isn’t even mapped in most cases, potentially causing premature wear to the turbocharger.

In order to measure this, we would need to have an additional and well-rated air temp sensor in the airstream at the turbo outlet. With an additional sensor at the air inlet as well, we can then understand the efficiency of the turbocharger as well as the intercooler and this would be another datum point for us to confirm our position on the compressor map.

The typical rule of thumb is if you’re maintaining a set boost pressure and the intake temps are increasing with engine RPM then the turbo is starting to become inefficient, if the boost starts to drop off as well then it’s starting to really fall out of efficiency.

This can be tracked in compressor maps, we can see tracing from left to right the tested traces move in a linear fashion to the center of the island and then fall sharply the further right we go. These traces are a set compressor RPM, which infers the same amount of work being done to the turbine side, it just becomes unable to process the air.

Limitations of Speed Density Calculation

If we truly want to know where we are on the compressor map, we need to know exactly how much air is being processed by the engine.

We measure this with volumetric efficiency, which is often made simple by standardizing this to 80%, we’re assuming that at the engines best and its worst it’s pushing 80% of the maximum airflow it can (based on its size).

This is rarely the case, OEM engines are typically made to be most efficient in the middle of the powerband, giving best economy at cruise and a generally flat torque curve falling off quite dramatically after 5.5-6krpm (this is certainly the case with the MX5 engines). 

In reality what happening here is the engine can’t remove all the gas in the chamber from the previous combustion event which leaves the chamber with dead gas, lowering its volumetric efficiency.

Let’s look at an example of how much difference VE can make to the total power output at 7200 RPM.

Volumetric Efficiency at 0.8

Engine RPM 7200
Naturally Aspirated CFM 187.953
Naturally Aspirated LB/Min 12.969
Boosted CFM 545.064
Boosted LB/Min 37.609
Pressure Ratio 2.900
Volumetric Efficiency 0.8
Expected Horsepower 376.09

Expected horsepower assumes the age-old ‘1lb of air = 10bhp’ its a rule of thumb rather than anything reliable – but useful to demonstrate the changes ve makes.

The Boosted LB/min describes how much air the engine is now able to pump with the added boost pressure, assuming 14.7 atmospheric in this example would be 28psi of boost pressure. This is also the calculated airflow, with the VE multiplier.

Volumetric Efficiency at 0.82

Engine RPM 7200
Naturally Aspirated CFM 192.652
Naturally Aspirated LB/Min 13.293
Boosted CFM 558.690
Boost LB/Min 38.550
Pressure Ratio 2.900
Volumetric Efficiency 0.82
Expected Horsepower 385.50

For a 2% change in VE we’ve picked up 9.5BHP and 0.9lb/min of airflow, if we’re chasing the edge of the compressor map this is quite a jump to compensate.

But let’s assume that we can expect a little higher than that.

Engine RPM 7200
Naturally Aspirated CFM 202.049
Naturally Aspirated LB/Min 13.941
Boosted CFM 585.943
Boost LB/Min 40.430
Pressure Ratio 2.900
Volumetric Efficiency 0.86
Expected Horsepower 404.30

For another 4%, we’ve managed to crest the 400BHP mark for a total overall gain of 28BHP and 2.8lb/min of airflow.

This is with the exact same boost pressure and pressure ratio, we’re simply moving further right on the compressor map as we make the engine more and more efficient, perhaps by changing the ignition timing in this instance – for bigger changes, you’d need a different camshaft.

Speed density is the algorithm used to work out the airflow in the system, it does this using pressure and temperature, however, it is rarely accurate enough to definitively calculate the air entering the engine.

Mass Air Flow (MAF) sensors measure the air volume itself, they physically sit in the airstream and at certain power levels become a hindrance but their fast reaction times and accurate readings allow us to better understand how much air the engine is consuming.

With that being said, the speed-density algorithm does a reasonable job at providing an accurate calculation of volumetric efficiency, which is then used as the basis for the fueling of the engine – as most cars are seemingly in a constant state of tune, changes to hardware often net a VE change that has to be remapped in, rather than increasing engine airflow which the MAF system would already account for.

A combination of the two, using an unrestrictive MAF sensor at the air intake would allow us to have the fast response and reliability of speed density while at the same time provide very specific information on the impacts of hardware changes and give us a datum point for our compressor map.

For now, we need to take the calculated VE table in your map with a little pinch of salt and use a reasonable average. Hence the 0.8 that most use as the basis for horsepower calculations/bench racing – as we’ve shown above, just a small percentage increase can make a huge difference to engine output overall, it may not be worth chasing an extra 2psi of boost pressure when a 3% increase in VE would net the same result (anecdotally).

How do we improve the VE of the engine?

Keep in mind that this is just a calculation, our engine above should be able to make 203cfm if everything was perfect, in reality, it’s more like 162cfm i.e. 80% of its potential.

Camshafts with higher lift and duration will give the cylinder more time to fill, larger valves will allow more air to flow and better runners and ports will support that additional flow – this is why headwork becomes expensive, but justifiable so long as the increase in VE helps you to meet the horsepower goal – it also provides a side benefit of considerably better response everywhere in the rev range.

Final Thoughts

It’s largely our opinion that if you’re building to a power number, pick a turbo that will there or thereabouts run out of the airflow at that power number, use all of it. It’s only in the case of the expensive EFR, GTX Gen 2 and G Series turbos with their incredible compressor designs that allow for the best of both worlds, i.e. quick spooling, great response and higher power potential than you’d want.

In every other case, the bigger the turbo, the slower the transient response the later the boost threshold and the slower your torque builds.

Volumetric efficiency makes a huge difference to the power output, it allows you to do far more with the same system, optimizing this will net gains everywhere in the powerband, in places you’ll really feel. Improve the flow into and out of the engine for best results.

Air temperature is pretty important if you can drop your intake air temps to near ambient you’ll be making the most of what mother nature gave you.

Fuel takes up space in the cylinder, you’ll never fill it with just air. 

We’ve not talked about compressor vs turbine sizing, different a/r and how that impacts spool and top-end power – as that’s part of the process of optimizing your VE, and that’d add too many variables for this post. However, a good rule of thumb is that the larger the A/R the better the engine will breathe at high RPM, which keeps the VE high. Lower A/R provides faster initial spool (although again, it’s more about VE and how well the engine breathes which it does do better at higher A/R at all RPMs) and will almost always fall off at the top as the VE starts to rapidly go down – there is a happy medium but it depends on your engines airflow requirements.


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