Turbo Glossary
Copyright © 2003 Eric Fahlgren
Last updated 2007-09-18 11:17 PDT
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All temperature calculations are done in absolute scales (Kelvin or Rankine). To convert Celsius to Kelvin simply add 273 (well, actually 273.15 but you'll be lucky to be dealing with values ±10%, much less tenths of a degree). Likewise convert Fahrenheit to Rankine by adding 460 (459.67, but who's counting).

Photos of components or descriptive graphics are indicated by the camera icon .

Photos and diagrams stolen shamelessly from other websites, without credit or permission.


The Turbo Calculator computes pressure ratios, temperatures and estimates torque, HP and injector sizes given engine and environmental parameters. See below for definitions of the input terms and resulting values.




A process that occurs without heat loss or gain from the surrounding environment is called adiabatic. No mechanical device works adiabatically as there is always some heat transfer, the measure of heat loss/gain is called efficiency. In the case of a compressor you might hear such terms "operating at 75% adiabatic efficiency," which is described mathematically under compressor efficiency.

Air Fuel Ratio (AFR)

Simply stated, AFR is the ratio of air mass to fuel mass in the combustion chamber. For typical pump gasoline, AFR is usually in the range 12:1 to 17:1, depending on conditions and engine tune.

For a given engine using gasoline, the relationship between thermal efficiency, air fuel ratio, and power is complex and depends on many factors including fuel composition, charge temperature and pressure, and combustion chamber geometry. Stoichiometric combustion generally produces neither maximum power (which typically occurs around an AFR of 12-13:1, also known as a "rich mixture"), nor at maximum thermal efficiency (which usually occurs around A:F 16-18:1, known as a "lean mixture"). The AFR is often controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust. The AFR is typically enriched during full throttle operation to maximize power and to reduce detonation.

There is a range of AFR where the power delivered is directly proportional to the amount of fuel in the mix, for example, a 12:1 ratio might deliver more power than a 13:1 ratio. This range is specific to the engine and its fuel delivery system, so no generalizations can be made, but the as the mixture goes beyond the rich end of this range, the specific fuel consumption goes up and ultimately power goes down.

Alpha-N Fuel Metering

Read about electronic fuel injection first, then come back here for details on alpha-N.

An alpha-N system is one whose load sensor input is from a Throttle Position Sensor (TPS), which it uses along with engine RPM to estimate air flow.

Ambient Pressure

The pressure, density and temperature of the atmosphere change with altitude and weather conditions. NASA has published a standard atmosphere model (typically used for rocket simulations as the vehicle gains altitude), which gives mean values for these quantities at a given altitude.

Translating the F90 code for the 1976 Standard Atmosphere, I've written a calculator giving the pressure ratio and resulting ambient pressure at a given altitude. Note that this is only applicable to elevations below 11000 meters (~36,000 feet) and the "density" does not take temperature into account.

A/R Ratio

The ratio of the exhaust inducer nozzle area (A) over the exhaust inducer radius (R; measured from the center of the turbine shaft to the center of the inducer nozzle). This number (usually from about 0.4 to 1.0) gives you an indication as to the tradeoff between lag and high volume efficiency for a given configuration. When the A/R ratio is low, then the small inducer area creates high velocity flow at low volumes, thus improving spin up. A high A/R allows high volumes of gas to flow through the exhaust turbine, making the turbine more efficient at those high volumes, but it will then be sluggish to spin up.

Since A/R ratio is a significant factor in turbo lag, some manufacturers have devised means for varying the A/R ratio to reduce lag while still maintaining high-volume efficiency. Such a scheme properly carried out would eliminate the need for a wastegate, which is indeed the case with the Garrett VNT (Variable Nozzle Turbocharger). The VNT uses adjustable vanes inside the exhaust scroll to change the angle of attack of the incoming gasses as they strike the exhaust turbine.


Atomization is the mechanical process of changing a body of liquid into smaller droplets of the same liquid. Ideally, no state change occurs, so the resulting suspension of liquid particles is at the same temperature as the original liquid. See vaporization for lots more.

Atomization is "good" for performance, smaller droplets ignite better and burn more thoroughly, and they do not consume much intake charge volume, as opposed to vaporized fuel, which consumes significant volume thereby reducing the quantity of oxygen in the charge. Vaporization may be good, too, depending on the type of liquid being evaporated and the medium causing the evaporation.



The back "half" of the compressor housing. It usually holds the turbine shaft bearings into the bearing housing, and is bolted onto the CHRA before the compressor turbine is installed.

Bearing Housing (Center Housing)

The bearing housing is the central structural element of a turbocharger. A bearing housing is called "wet" if it is plumbed into the cooling system of the engine, otherwise it is called "dry" and relies solely on the lubricating oil for cooling.


Turbochargers usually have one or two bearings in the bearing housing, which are composed of bronze. More modern designs use roller or, more recently, ceramic ball bearings. These newer designs can often spin up at rates up to 30% faster than old bushing-only models. Turbocharger bearings are most often lubricated by high pressure engine oil, but some turbochargers (like Aerodyne models) have a self-contained oil supply.

Blowoff Valve (BOV or Pop Off Valve)

Note: This term is often erroneously used in place of "Bypass Valve!" A blowoff valve is a pressure relief device used to save the engine when overboost occurs. It is actuated solely by overboost, with no other controls necessary. In a supercharger system, a blowoff valve can be used as a means of dynamic boost control (static boost control is achieve by choice of supercharger drive ratio). A BOV might be added to a boosted system to protect against backfire damage to a super- or turbocharger. It can also protect against a stuck wastegate valve by sacrificing the turbo to overspin.


See Brake Mean Effective Pressure.


The amount of pressure generated by the compressor of a turbo- or supercharger. It is measured either in absolute or relative (also called gauge) terms. When absolute boost is presented, its units are usually BAR; relative pressures are usually in psi. For example, you might interchange "2.0 BAR" and "15 psi" boost to mean the same thing (but you'd better be careful that your ambient pressure is 1.0 BAR). To be more precise, you should actually say "15 psig" (psi gauge) or "30 psia" (psi absolute). See pressure ratio and density ratio for more meaningful numbers.

Boost Controller

A feedback device that controls the amount of boost produced by the turbocharger. It usually does this by controlling vacuum and pressure to the diaphragm or piston on the wastegate.

Modern boost controllers are electronic, some with sophisticated algorithms that take into account engine speed, engine load and driving habits to produce maximum boost as early as possible.

Boost Creep

Boost creep occurs when the wastegate/turbocharger combination is improperly sized and cannot dump enough exhaust gasses. The symptom is rising boost as RPMs rise, sometimes in an uncontrolled fashion. To reduce this effect, either install a larger capacity wastegate, or an exhaust turbine with a higher A/R ratio.

Boost Map

A table used by an electronic boost controller to relate the RPM (and often other parameters such as coolant and intake charge temperature) to the boost to be produced. An Audi MAC-11B has a map which contains the following values:

RPM 1000200030004000500055006000>6375
Boost (BAR)1.041.35 1.4 1.4 1.41.42 1.4 1.4

Note that these are target maximums, which may not be achieved depending on the condition of the engine and turbocharger system.

Boost Taper

A phenomenon produced by flawed boost controllers, which causes boost to drop (or taper off) when it should remain steady. Note that this is not the same as a controller with initial "overboost," which creates an initial boost spike followed by a constant, but lower, boost.

Brake Mean Effective Pressure (BMEP, Effective CR)

MEP is the theoretical average pumping pressure required during the power stroke for an engine to produce a given power output. Brake MEP is an empirical measure of the same quantity, usually performed indirectly on a dynamometer. It is a measure of how effectively an engine of a specific displacement does work.

The following formula gives the BMEP number in psi based on measured HP and RPM. It assumes 100% volumetric efficiency.

            BMEP = ------------------

                    HP * 793,000
                 = -------------- PSI
                    RPM * CID
            HP  = measured engine horsepower
            CID = engine displacement in cubic inches
            RPM = engine speed

BMEP values of 100-200 psi are typical for normally aspirated engines; 200-350 psi are usual for forced induction systems. If you use the turbo calculator and see numbers at 400 psi or higher, realize that you probably won't be able to build the motor you are simulating.

Brake Specific Fuel Consumption (BSFC)

BSFC is the empirical measure of an engine's specific fuel consumption, usually performed on a dynamometer.

Bump Hose (Hump Hose)

A hose with a "hump" or "bump" in it that allows it to flex significantly by varying its diameter and creating an accordion effect (these hoses are sometimes called "Michelin man" hoses for this reason). The final inlet hose that connects the intercooler to the throttle body usually has some sort of motion isolation designed into it to keep engine movement from applying extreme forces to the TB. Bump hoses should be used anywhere that lots of motion will be allowed.

Bypass Valve (Dump Valve, Anti-Surge Valve, Recirculating Valve)

(Note: Often erroneously called a "Blowoff Valve.") A pressure relief device actuated by post-throttle body manifold vacuum. The purpose is to reduce or eliminate high pressure in the intake tract between the turbocharger outlet and the throttle body, thus maintaining the momentum in the turbo for reduced lag. Another important reason to have a bypass is to eliminate pressure spikes, which can pop hoses, can rupture the intercooler and, worst case, can induce such high torque on the compressor that it breaks the turbine shaft.


CHRA (Center Housing Rotating Assembly, Center Housing or Center Section)

The turbocharger assembly sans housings and any wastegate mechanism, but including the bearing housing, bearings, cooling and lubrication systems, and the turbine shaft and wheels.


Clipping an exhaust turbine is the act of machining off the outer perimeter of the turbine blades. This has the same effect as raising the A/R ratio, by increasing the radial distance between the nozzle of the exhaust scroll and the tip of the turbine blades. Games are also played with the angle at which the ends of the blades are cut, so you will hear terms like "15° clip" and so forth.

Closed Loop Fuel Metering

Read about electronic fuel injection first, then come back here for details on closed loop EFI.


Compressor Efficiency (Turbocharger Efficiency)

Compressor efficiency is a measure of how well the compressor wheel uses its kinetic energy to compress air (the remainder of the energy is turned into heat in the compressed charge). In an ideal system, compression of the input fluid would raises its temperature adiabatically. This is never the case in the real world, so all calculations must take the compressor efficiency into account.

In order to calculate compressor outlet temperature, you must know compressor efficiency, pressure ratio and ambient temperature.

                     (PR^0.283 - 1) * Tambient
            Trise = ---------------------------

            Trise    = increase in temperature
            PR       = pressure ratio
            Tambient = ambient temperature (in an absolute scale, Kelvin or Rankine)
            Ec       = Efficiency of the compressor

The exponent of the pressure ratio arises from the molecular structure of the gas. Diatomic gasses (like N2 and O2) have seven degrees of freedom, five of which are excitable at STP. Thus gamma = 7/5 in the equation P(V^gamma) = constant, and we can derive an exponent of 1 - (1/gamma) = 0.285. For real air, containing non-diatomic molecules like CO2, a better value is 0.283. For more on this, see the gas thermodynamics section of Nuclear Weapons FAQ (!) (scan down to section 3.1.6).

So, for example, my Garrett T04E compressor running at PR of 2.5, Ec of 0.75 in the good old summer time with a temperature of 27 degrees Celsius (300 Kelvin) produces:

                     (2.5^0.283 - 1) 300
            Trise = ---------------------

                  = 118°
Compressor Efficiency Map (Compressor Map)

A compressor map is a graph representing compressor efficiency as a function of pressure ratio and flow rate. In the example, for a Garrett T04-54, we see that the pressure ratio is on the vertical axis and that flow in lb/min is on the horizontal. To use the map, you must compute these values and plot them on the map, thus determining compressor efficiency.

To size a compressor for a particular application, plot at least three points on the map:

  1. Plot the point at which boost first occurs (i.e., where about 1 psi boost is evident);
  2. plot the point at the torque peak of the motor (where VE is at its greatest);
  3. finally, plot the point at the top of the RPM range (where flow is at its greatest).

If you use the Turbo Calculator to compute this last point, you also should take note of the estimated injector size to see if your EFI system is sized appropriately to handle the expected requirements.

Compressor Housing (Cold-side Housing or Cold-side Scroll)

The (usually) aluminum casting that directs the incoming charge to and away from the compressor turbine.

Compressor Turbine (Compressor Wheel)

The impeller which compresses the incoming air. The shape and volume of a compressor turbine and its associated compressor housing (they almost always must be changed together) are described by their trim.


Density Ratio (DR)

The One True Useful Number. This is a number computed from pressure ratio, compressor efficiency and intercooler efficiency to provide the actual increase in pressure realized in the cylinder as a product of the compressor system. Equally applicable to supercharger applications.


In The High-Speed Internal-Combustion Engine, Third Edition, 1941 by Harry R. Ricardo, F.R.S, read Chapter 2 "Detonation," where on p.40 he says:

The phenomenon of detonation appears to be the setting up in the cylinder of an explosion wave. This occurs when the rapidity of that portion of the working fluid first ignited is such that, by its expansion, it compresses before it the unburnt portion beyond a certain rate. When the rate of temperature rise due to compression by the burning portion of the charge exceeds that at which it can get rid of its heat by conduction, convection, &c., by a certain margin, the remaining portion ignites spontaneously and nearly simultaneously throughout its whole bulk, thus setting up an explosion wave which strikes the walls of the cylinder with a hammer-like blow and, reacting in its turn, compresses afresh the portion first ignited. This further raises the temperature of that portion, objects in its vicinity, thus ultimately giving rise to pre-ignition. It would appear, therefore, pretty certain that detonation depends primarily upon the rate of burning of that portion of the charge first ignited, and it remains to discover what actually controls this rate.

Go get a copy of Design and Simulation of Four-Stroke Engines by Gordon P. Blair (SAE International, 1999, ISBN 0-7680-0440-3), where on p.411 in Chapter 4 - "Combustion in Four-Stroke Engines," you can read this:

Detonation occurs in the combustion process when the advancing flame front, which is pressurizing and heating the unburned mixture ahead of it, does so at such a rate that unburned fuel in that zone achieves its auto-ignition temperature before the arrival of the actual flame front. The result is that the unburned mixture combusts "spontaneously" and over the entire zone where the auto-ignition temperature has been achieved. The apparent flame speed in this zone is many orders of magnitude faster than that in conventional combustion initiated by a normal flame front, with the result that the local rise of pressure and temperature is significantly sharp. This produces the characteristic "knocking" or "pinking" sound, and the local mechanical devastation that this can produce on piston crown or cylinder head can be considerable. Actually, "knocking" is the correct terminology for what is a really a detonation behavior over a small portion of the combustion charge. A true detonation process would be one occurring over the entire compressed charge. However, because detonation in this strictly defined sense does not take place in the spark-ignition engine, the words "knocking" and "detonation" are used interchangeably in the literature, without loss of meaning, to describe the effects just discussed.


Electronic Fuel Injection (EFI)

There are four basic ways that electronic fuel injection systems use to determine the appropriate amount of fuel to be shot into the engine. One is the MAF system, second is speed density (SD), third is the alpha-N, and fourth is closed loop. Most modern fuel injection systems use more than one of these algorithms, and switches between them based on conditions and load (the first three are collectively referred to as "open loop" modes in systems that support a closed loop mode).

Most of this discussion assumes we are talking about modern electronic or electronically-augmented fuel injection systems, such as Bosch CIS-E, Bosch Motronic, Ford EEC or GM TPI.

When operating in closed loop or "cruising" mode (the engine at constant and modest throttle opening), both systems rely on an oxygen sensor to adjust the amount of fuel injected. In this mode the goal is to maximize fuel economy and minimize exhaust emissions, which is an operating regime where the oxygen sensor works best.

Open loop algorithms use directly or indirectly measured air mass to determine the fuel required. Each of the above three schemes uses a different method for determining the air mass.

  1. Alpha-N uses a transfer function (usually in the form of an interpolated table) to convert two variables, alpha or throttle opening, and engine speed, RPM, into a single volumetric efficiency number. The VE is then combined with intake air temperature and possibly ambient pressure to arrive at an estimated air mass.

  2. Speed Density uses the same basic scheme, but uses manifold absolute pressure instead of alpha as one of the lookup variables for the VE table. MAP is typically a better variable than alpha, especially for boosted applications.

  3. MAF system has a meter which measures either air flow (which is then converted to mass via density computations) or direct air mass.

Once you know air mass and know a desired AFR you can calculate fuel mass. Using this mass information with the fuel injector's flow rate, you can determine a pulse width to deliver the desired quantity of fuel.


Enthalpy is a thermodynamic quantity that is the sum of the internal energy of a mass and the product of its volume times pressure (i.e., H = U + pV). Also called heat content.

Where is this useful? When you evaporate water in a water injection system, you can compute the evaporative cooling by knowing the change in enthalpy of the water when it transitions from a liquid state (Hf) to a vapor state (Hg). Enthalpy can be expressed as a function of temperature, as shown here (note that these functions are approximations which are only accurate for temperatures from 0-100°C):

            Hf(t) = 2.2801 + 4.0596 t + 8.7193E-04 t^2 J/g
            Hg(t) = 2501.4 + 1.8799 t - 1.3143E-03 t^2 J/g

An exducer is a gas outlet, and exists on both the cold and hot sides of a turbocharger, but as is often the case, common use refers to just one of them: the hot side outlet. You can see in this photograph that the exducer size varies quite significantly with turbine trim.

Exhaust Gas Temperature (EGT)

??? Reasonable temps, probe types, thermocouples, probe location. send me stuff

Exhaust Turbine Housing (Exhaust Housing, Exhaust Scroll, Turbine Housing, Hot Side Housing)

The part of the turbocharger that directs the hot gasses to the exhaust turbine and then to the exhaust pipes. The exhaust housing can be either tangential or center-fed.

Exhaust Turbine (often called just "Turbine")

The part of the turbocharger that converts the kinetic and thermal energy in the exhaust stream into mechanical energy.




Heat Soak

Heat soak occurs when a component reaches temperature equilibrium with the gasses flowing through it. In the case of an intercooler, this is a bad thing, and the efficiency of the intercooler drops significantly.


A pressed sheet metal component that protects the hot-end bearings of the CHRA from direct contact with the exhaust gasses. This shield can become covered with coke and crystallized oil, and seize against the exhaust turbine. This is most likely to happen when the turbocharger has been overspun and damaged.

Humidity (Relative Humidity)

Relative humidity probably affects pressure ratio quite a bit by changing gamma because of all those big H2O molecules mixed in with those little gas molecules. It almost certainly has some effect on evaporative cooling in a water injection system. If you have knowledge of anything related to this, please e-mail me.



The inducer is the inlet on the compressor housing. Its shape is determined by the compressor turbine.

Intercooler (Aftercooler, Charge Cooler)

An intercooler (more commonly called an "aftercooler" by diesel folks) is a radiator device used to reduce the charge temperature between the compressor and the engine. There are two common types: air to air and air to liquid. In automotive applications, air to air is usually more efficient, with air to liquid coolers used only when space or plumbing considerations are overriding.

An air to air intercooler looks like a coolant radiator, through which the compressed air passes to be cooled. Efficiency of air to air intercoolers is usually from 50% to as high as 80%. Undersized intercoolers are often prone to heat soak.

Typical automotive air to liquid intercoolers have two heat exchanger components. The first is a set of tubes passing through the intake plenum, carrying the cooling liquid. The second is a heater-core-like radiator, which dissipates the heat in the liquid. The liquid is circulated through the system by an auxiliary pump. Since there are two heat exchangers, each with a less than perfect efficiency, the overall system efficiency rarely approaches that of an air to air intercooler.

In specialized applications, the air to liquid intercooler can be made more effective than an air to air intercooler. A drag racer could use such a system where the radiator was immersed in dry ice or some medium where the ambient temperature is well below that of the atmosphere.

In a boat, you have the opportunity to cool the charge with water, which typically is better than air in two ways: it has much higher heat capacity than air, hence improves heat transfer, plus it is often cooler than the surrounding air. Coupling these two facts can result in a design that produces a very dense charge.

Intercooler Efficiency

The measure of how well an intercooler reduces the charge temperature. Efficiency is the ratio of inlet temperature over outlet temperature relative to ambient temperature.

            Tdrop    = drop in temperature
                     = (Tin - Tambient) * Ei

            Tout     = output temperature
                     = Tin - Tdrop

            Tin      = input temperature
            Tambient = ambient temperature of the cooling medium
            Ei       = efficiency of the intercooler

So, let's assume we have a system in which the turbocharger produces a charge temperature of 418°K at an ambient temperature of 300°K. With an intercooler efficiency of 70%, we get

            Tdrop    = (418 - 300) * 0.70
                     = 82.6

            Tout     = 418 - 82.6
                     = 335.4°K

Make sure you choose the correct ambient temperature (that of the medium in which the intercooler is immersed) for calculations, which is not necessarily the ambient air temperature.




See detonation.



Lag is the delay between increased flow from the engine to when the exhaust turbine achieves rotational velocity appropriate for that flow. The rotating components of the turbocharger have a certain amount of inertia, and the bearings produce friction opposing the spin up of the turbocharger, so when you "stomp the pedal" the turbocharger cannot respond immediately. For turbochargers with a large A/R ratio, the lag can be quite noticeable.

An unsophisticated boost control mechanism (say, one with the wastegate diaphragm directly connected to the manifold boost) usually begins opening the wastegate long before full boost is realized causing lag. Various pressure valve schemes or electronic boost controllers (such as those manufactured by HKS or APEXi) intercept the manifold signal and keep the wastegate closed until the last possible moment, thus building boost much faster.


Manifold Absolute Pressure (MAP) Sensor

A sensor that measures absolute pressure, nominally in the intake manifold of your engine. It is often a strain gauge built on top of a reference chamber at known pressure. Its electrical output is proportional to the absolute pressure applied to the measurement port, and thus is the same irrespective of ambient pressure. So, for instance, at sea level you might read 100 kPa of pressure by exposing the port to ambient conditions and if you could trap that pressure at the port and take it to the moon, it would still read 100 kPa. (Contrast this with "gauge" pressures, which are relative to ambient conditions; play with pressures here.)

When the engine is not running the MAP sensor indicates the barometric pressure, and hence is often sampled by the ECU before startup to do barometric and altitude correction.

The output is often a proportional voltage, but some MAP sensors produce a varying frequency signal, again proportional to absolute pressure.

Mass Air Flow (MAF) Fuel Metering

Read about electronic fuel injection first, then come back and read about MAF systems.

A MAF system is one whose load sensor input is from an air flow meter (AFM), which it uses to directly measure the actual amount of air that is being consumed by the engine. The great strength of MAF systems is their ability to adapt to widely varying conditions and engine tune. If you hot rod a MAF-equipped system, the MAF adapts quite well so long as the AFM and injectors are big enough to handle the increase power; MAF simply sees more air going by and dumps in more fuel. Contrast this with a speed density system, which typically requires retuning after every major (and sometimes not-so-major) tweak.

Mass Air Flow (MAF) Sensor (Air Flow Meter or AFM)

MAF sensors come in many forms: old systems had a "flapper" valve in the air stream, often connected to a potentiometer to turn the displacement into voltage. More sophisticated MAF sensors use a heated wire or film, and impose much less restriction on the intake flow. Electronic AFMs have a means for temperature compensation, to take into account the varying density of the intake air.

AFMs can run into problems with performance engines, especially under low-flow conditions such as idle and even cruise. A "big cam" causes lots of reverse pulses into the intake tract and can fool the AFM into reporting more (or less) air flow than is actually being used.

MBT (Minimum Spark Advance for Best Torque)

The least ignition advance timing that produces maximum torque while holding all other variables constant.




Overboost can be either intentional (in the case of a boost controller "overshooting" the maximum steady-state boost) or unintentional (your wastegate is stuck shut). In the case of unintentional overboost, the result can be engine parts scattered far and wide.


Overspin occurs when the turbocharger exceeds its published "redline" RPMs. Typically, the initial damage is to bearings and seals, which allow oil to pass from the center housing into both the intake and exhaust housings; subsequent damage may result in total destruction of the turbocharger if one of the turbine wheels touches its housing. A mildly overspun turbo usually produces a tell-tale puff of smoke when the engine is first started.


Pressure Drop
Pressure Ratio (PR)

The ratio of outlet pressure over inlet pressure, in absolute pressure values.

                  Pa + Po
            PR = ---------

            Pa = inlet (ambient) pressure
            Po = outlet pressure

If you are at an ambient pressure of 13.5 psi (about 700 m ASL) and your boost is 19 psig:

                  13.5 + 19
            PR = -----------

               = 2.41

Note that an absolute boost level (i.e., one that is altitude adjusted) has a widely varying pressure ratio, depending on the ambient pressure (which depends strongly on altitude).

Note also that the outlet pressure of the compressor is often quite a bit higher than the pressure measured in the intake manifold, due to intercooler and plumbing constrictions.

Finally, note that pressure ratio is an intermediate value, and is not particularly important in power calculations. Density ratio is the number that actually tells us how much air the engine can consume so is the factor you must consider when determining system performance (PR, along with a compressor map, should be used to determine appropriate sizing of the turbocharger, not for calculating resulting HP).



Relative Humidity

See Humidity.

Rod To Stroke Ratio

???, qplots re mechanical advantage, before frictional side loads, overwhelm improved geometry at peak pressure.



A term used to denote either the compressor housing or the exhaust housing due to their shape.

Specific Fuel Consumption (SFC)

SFC is the measure of how effectively an engine takes advantage of the fuel in its combustion chamber. SFC is usually measured in lb/hr/HP or cc/min/HP, and traditionally good values are 0.40 lb/hr/HP (4.2 cc/min/HP) or lower; newer engines with better airflow numbers, and fuel and spark management have lower numbers. As the number gets bigger, the engine is using more fuel to make less power, so you want this number as small as possible. (Turbo and supercharged engines are notorious fuel gulpers and often have SFC numbers of 0.55-0.60 lb/hr/HP.) Note that improvements in SFC affect power output in a direct linear proportion. If at a given flow and AFR you can improve SFC from 0.50 to 0.49, you have increased power by 2%.

SFC is a function of many factors, not the least of which is static compression ratio. Increasing CR increases thermal efficiency, which directly affects SFC. Lowering the rod to stroke ratio can improve SFC to a point due to combustion pressure being applied when the crankshaft at a more advantageous angle (but it also induces greater frictional losses from piston side forces). Use of cast iron or thermal barrier coated cylinder heads, instead of bare aluminum, keeps heat in the chamber and lowers SFC. Likewise, use of thermal barrier coatings on valve faces and pistons drops SFC. Reducing internal engine friction and pumping losses improves SFC, so port your lubrication and cooling systems.

Combustion chamber design has a huge impact on SFC. A chamber with stratified charge (one where AFR varies at different places in the chamber, typically rich near the plug, but lean away from it) or lots of swirl usually was designed to improve SFC. Small, compact chambers which cause the charge to burn rapidly often have a lower (better) SFC than extended, flat ones. Placement of the spark plug in the chamber is important: a central location allows for a faster more efficient burn than an off center one.

Speed Density (SD) Fuel Metering

Read about electronic fuel injection first, then come back here for details on the SD algorithm.

An SD system is one whose load sensor input is from a Manifold Absolute Pressure (MAP) sensor, which it uses along with engine RPM to estimate air flow. ???

Spin Up

The process of changing from a zero-boost condition to a boosted condition. If spin up takes a long time, this time delay is called lag.


A chemically balanced air fuel ratio in which perfect combustion is theoretically possible. The A:F ratio is about 14.7:1 for a typical non-oxygenated gasoline. A rich mixture has proportionally more fuel than oxygen; a lean mixture has less.

Note that when you burn a stoichiometric mixture you are not assured that all participants will actually combine in a chemically ideal fashion. There are usually free HCs, CO and other pollutants remaining when burning gasoline stoichiometrically, due to the fact that the liquid fuel particles were too big and did not burn completely.


The point at which flow at a given pressure ratio drops below a certain level and the flow becomes unstable. This results in surging boost, where the compressor seems to be "gulping" air. Surge can be quite harmful to the compressor, so is to be avoided at all costs.

Surge Limit (Surge Line)

The line on the compressor efficiency map at which compressor drops into unstable flow or surge, due to lack of flow at a given pressure ratio. When presented with a compressor map, which has no defined surge line, first determine the pressure ratio of interest, then draw a horizontal line on the compressor efficiency map at the given PR, moving from right to left along the line until it reaches the 60% efficiency iso.


Thermal Efficiency

The efficiency with which the engine turns the combustion process into usable work. For a normally aspirated piston engine the thermal efficiency is approximated by the equation below.

            TE = 1.0 - (1.0 / CR)^0.172

            TE = thermal efficiency
            CR = compression ratio

You can see from the table why diesel engines get better fuel economy than spark ignited engines. Combine this with the fact that these efficiency numbers relate to the actual effective compression ratio (which only occurs when the throttle is wide open) and you will see even further why diesels with their fuel throttling compare well with spark ignited motors with air throttling. Note also that as the CR rises, the efficiency rises more slowly.

As a general rule of thumb pumping losses overcome thermal efficiency gains at about 17:1 CR for most piston engines.

7.0   28.44%   13.0   35.67%  
8.0   30.07%   14.0   36.49%  
9.0   31.47%   16.0   37.93%  
10.0   32.70%   18.0   39.17%  
11.0   33.80%   20.0   40.27%  
12.0   34.78%   22.0   41.24%  

See also Volumetric Efficiency.

Throttle Position Sensor (TPS)

A TPS is a potentiometer attached to one or more of the throttle valve shafts that produces a resistance proportional to throttle position (alpha). Most EFI systems use one to detect changes in throttle position (delta TPS) for enrichment calculations, and the alpha-N system also uses it as the primary load sensor.

Note that most TPS potentiometers have linear scaling, i.e., the resistance changes in direct linear proportion to alpha, but some use a non-linear scaling. The Ferrari F40's twin-turbo V-8 uses a non-linear pot, which gives greater sensitivity at small throttle openings. Most audio pots have logarithmic scaling.

Trim (Compressor Trim)





A chemical process where a substance in its liquid phase changes state into a gas. This state change requires energy, thus the resulting gas is colder than the liquid. The gaseous form of a substance also consumes much more volume than its liquid form, so vaporization is generally a bad thing for fuel and injected water to undergo in the intake tract (it "steals" space from oxygen-bearing air). On the other hand, the cooling reduces the volume of resulting gas, so there is often a net gain (especially with water or methanol). Contrast with atomization, and see also water injection.

Volumetric Efficiency (VE)

Volumetric efficiency tells us how well an engine pumps air. A 594 CID engine will theoretically pump 594 cubic inches of gas in a single cycle (two revolutions). At 80% VE, the same engine only pumps 475.2 cubic inches, which proportionately affects the amount of power produced.

The torque an engine produces is actually just a function of VE times engine size, so a torque curve closely approximates a VE curve with a different scale. The VE of an engine is dependent on many factors, including port size and shape, valve sizes, lift and timing, and engine speed. Typically, street engines are tuned to have a wide VE (torque) curve, with a peak efficiency around 65-85%. A race engine, on the other hand, has its induction tract, cam timing and exhaust system tuned to resonate in a narrow RPM band thus producing a very high, but narrow peak in the VE curve. VE numbers can reach well above 100% for a well-tuned motor.

When you "make your engine breath" you are improving its VE. Typically, improvements are found by porting the cylinder head and manifolds, performing a multi-angle valve job, unshrouding valves, installing headers and reducing exhaust backpressure. Note that these things are just as important on a turbocharged motor as they are on a naturally aspirated motor, possibly even more so!

See also Thermal Efficiency.


Wastegate (Exhaust Dump Valve)

The device used to vent excess exhaust system back pressure, thus controlling the boost generated by the turbocharger. There are "internal" and "external" wastegates, the former being attached directly to the exhaust turbine housing of the turbo, the latter being a stand alone device, usually connected to the exhaust manifold.

They are usually constructed of a spring loaded piston, to which boost pressure is applied. The piston pulls an exhaust valve open as the boost pressure rises.

Wastegate Frequency Valve (WGFV)

A part used in the boost control system of Audi turbocharger systems. It is a solenoid that is modulated by a controller to adjust the bleed rate on the control line to the wastegate. An interesting diagnostic is to put a dwell meter (which is really a duty cycle gauge with weird calibration) on the signal lines to the WGFV and see when the boost controller is opening and closing the valve.

Water Injection

??? rework completely, this is a mess and contains falsehoods... Until I get the rewrite done, read these: Thermo stuff and Vaporization and fugacity.

Water (or water/methanol) injection has long been used to suppress detonation in engines and is often considered only in this light, but a properly designed injection system can be nearly as effective as an intercooler at reducing the input charge temperature. When considering a water injection system, you must examine the two following properties:

  1. Water vapor in the combustion gasses damps the burn rate, reduces flame front speed and thereby reduces HP (bad) and tendency towards detonation (good).
  2. The vaporization of liquid water "absorbs" a large amount of energy and can be used very effectively to reduce charge temperature (very good).

This last effect is used in the turbo calculator to show quantitatively just how much benefit can be derived from a water injection system, irrespective of any pleasant side effects on combustion.

Now here's the really good part, the vaporization of water requires very large amounts of energy, which we all know intuitively from being sprayed with a light mist on a hot day. See Water Injection Thermodynamics for the equations used in the turbo calculator to model the behavior of a simple water/methanol injection system. Read about the chemistry in Robert Harris's e-mail.




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