KNOW YOUR ENGINE DYNAMICS
How many KGs of air do you think your engine consumes? 
EQUATION USED (for a 4stoke engine): AIRFLOW = 0.5 x Engine Size x RPM x % V/Efficiency 
The chart below shows the quantity of air(kgs/min.) passes into the engine (16.5 litre) under different engine speeds. We assumed the engine's Volumetric Efficiency is 80% and Air Density = 1.2g/M³ (25C). The engine is a naturally aspirated type and operating at sea level. A typical two litre engine at 6000 RPM (red dot) consumes 5.64 Kg of air per minute, if water was injected at a rate of 250cc/minute (0.25kg), the water/air ratio is only 2.9% ! The percentage will drop further if fuel is included as a total mass in the calculation. 
* * * * * * * CALCULATING THE TEMPERATURE RISE Pity that temperature rise is proportional to pressure increase 
EQUATION USED:
T2={T1[P2/P1]^0.283} x n(65%) T1=inlet T2=outlet n=compressor efficiency 
* * * * * * * CALCULATING THE DENSITY RATIO We have to obtain density ratio first before calculating Mass Airflow 
EQUATION USED:
Density=(T1*P2)/(T2*P1) T1=ambient+273 T2=outlettemp.+273. P1=atmos P2=boost 
Now that we know that the temperature rise, we will calculate the air density ratio next and then we can obtain the mass airflow. Chart below shows the difference in temperature rise due to pressure. As the compressor efficiency drops, the density ratio also drops. Even if you have a turbocharger that has 100% efficiency, there is still a density ratio drop of 15%. At 60% efficiency the density ratio falls by 30% ! 
* * * * * * * THE DENSITY INCREASE AS TEMPERATURE DROPS We have to obtain density ratio first before calculating Mass Airflow 
EQUATION USED: % DENSITY INCREASE={(T1*P2)/(T2*P1)*(P2/P1)1}*100% T1=ambient+273 T2={T1*(P2/P1)^0.283)Td} Td=Tdrop P1=atmos P2=boost 
The range selected should be reasonably accurate up to 300°C, taking into account that the compressor is working close to its full pumping capacity.

MASS AIRFLOW FOR A 2litre TURBO ENGINE
Due to ease of calculation, air density is kept constant at 25C through out the calculation. 
EQUATION USED:
Mass = Density x [flow x density ratio] Density of air (at 25C) = 1.2 (uncorrected) 
At last, we can plot the mass airflow chart of a typical 2litre turbo charged engine, notice that the mass airflow doesn't double up when the pressure ratio is 2. The blue dot indicates the nonturbo engine.

CALCULATING THE EFFECT OF WATER INJECTION
Water flow=250g/min (25C), Airflow =8.64 kg/min (124C) 
EQUATION USED:
MaCpT1 + MwHf = MaCpT3 + MwHg T1=124C, T2=25C, T3=final Cp=Specific heat of air=1.005 Ma=mass of air/s, Mw=mass of water/s, Hf=Enthalpy of sat liquid, Hg=Enthalpy of sat vapour (obtained from steam tables) 
Air mass=8.64/60 = 0.144Kg/s. We need to guess the final temperature of the mixture to look up tables: Let T3=(T1+T2)/2 =74.5°C. say 75°C. From the steam tables: Hg @75°C = 2635.3 kJ/kg Saturated vapour Hf @25°C= 105kJ/Kg Saturated liquid rearrange the equation: MaCp(T1T3)= Mw(HgHf) to: T3= 124(2635.3105)/(0.144*1.005)Mw = 17547Mw Substitute Mw to obtain final temperature: For Mw=0.00333Kg/s (200ml/min): T3=12457.91=66.09°C For Mw=0.00417kg/s (250ml/min): T3=12473.12=50.88°C For Mw=0.00500kg/s (300ml/min): T3=12487.74=36.27°C Now that we have obtained the results for three different flow rate, a chart can be plotted to predict the intercooling effect of water. 
COMPARISON TABLE FOR WI AND IC

COMPARISONS 
INTERCOOLER  WATER INJECTION 
EFFICIENCY COST SIZE FITTING MAINTENANCE 
25%  75% HIGH BIG COMPLEX NONE 
0  110%+ MEDIUM SMALL 23 HRS (1s) WATER TOP UP 
Despite the comparisons, both systems gave good results. It is in our opinion that intercooler should be your first choice unless it is physically impossible. Water injection should be your preferred choice to upgrading your existing stock intercooler. Our reason for suggesting the above preference is purely based on common sense, it is best to dump the unwanted heat into the atmosphere via the intercooler rather than spending some of the cooling properties of water which is essential for incylinder cooling and detonation control. The best of both world can be combined if affordable, imagine injecting superchilled water immediately after the intercooler, the density gain is unimaginable ... It is worth noting that water injection is originally designed for incylinder cooling, we are delighted with the theoretical results. 