The power of air density is one of the most overlooked factors in making power. Engine displacement, big turbos, large head ports and high lift cams only maximum air flow when the volume of air is at its highest. What factors are involved in the power of air density? Fine Line Imports has put together some useful information from research performed in our dyno room over the period of 3 years. All data was logged at the time of the actual dyno run.
Temperature, humidity and altitude are major factors when trying to achieve maximum power of an engine. Fuel quality, tuning and the level of Volumetric Efficiency (VE) of the engine also play important roles. VE is a different conversation all together. We will just be talking about air density in this article.
What important factors decrease air density? Number one is heat. A direct effect of heat through the intake manifold (metal) is heat soak. Heat soak occurs when the engines air charge stops and the metal surrounding the intake of the engine absorbs this energy, specifically the intake manifold. Once the intake manifold gets warm it is hard to cool it down, hence why intake air temperature is so important. The air entry of the engine is controlled by the throttle body, which is located in the middle of the intake manifold,.
The factory IAT (intake air temperature) sensor on any U.S. turbocharged Subaru is located on the intake tube near the air box with the mass air flow (MAF) sensor. This is a very poor placement and representation of actual IAT. You can have a difference of 15-
95 degrees between the intake manifold and air intake tube!
Most modern turbo charged cars come with an intercooler, which helps cool the air charge before it enters the engine. Remember the colder the air, the denser it is. The hotter the air, the less dense it is. This works with the combination of humidity. The more humid the air, the more dense it is and vise versa.
The two main styles of intercoolers are FMIC’s, (front mount intercoolers, mounts in front of the radiator) and TMIC’s (top mount intercooler, mounts on top of the engine).
Both have good and bad features.
FMIC
The major benefit of a FMIC, is the cooler temperature of air charge because of the location. The disadvantage of a FMIC is the extra lag time due to more intake piping which must be filled. If you compare a well-tuned FMIC vs. TMIC map on the same car with the same modifications using the same fuel, most of the time you will have higher horsepower and torque levels at higher rpm’s with a FMIC.. This is because you can run more boost at higher rpm’s due to the intake air charge being cooler. Keep in mind that if you do not have good fuel quality, a well-serviced engine and a turbo that can support the volume of air needed then you are wasting your time trying to make big power. If you

are looking for response, then you might benefit using a TMIC instead. As you can see by the following dyno graphs, the TMIC set up produces much more response in power but drops off near the top of the power band. If you are looking for a car that pulls hard at the higher rpm band but has a little bit more lag, then the FMIC is for you. As you can see by these examples which use a before and after on the same cars.
Power of Air Density 02 WRX SR40 TMIC vs. RFMIC Graph.jpg
Power of Air Density STI's 20G FMIC vs. TMIC Graph.jpg
TMIC
The most awarding feeling when street driving a turbo charged car with a TMIC, is hands down response. When you can build 14-20 psi around 2800-3300 rpm then you have all the response you will ever want in a car. Enough power, well that is a different story
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Street cars are in the 3500-5500 rpm power band on a daily basis. TMIC’s work well in street cars because of this. However, one major factor to lose air density is heat soak.
Heat soak occurs in bumper-to-bumper traffic, stop lights or anytime the engine is on but at an idle. In some cases the temperature outside is cold enough where there is minimal change in the IAT at the above conditions. However, just like the intake manifold, once metal of the intercooler end tanks and piping absorbs the heat, it is hard to cool them down which makes it harder to raise the air density of the intake charge. Here are some examples of turbocharged Subaru cars with TMIC's.
Power of Air Density STI 20G TMIC Graph.jpg
04 WRX 2.5L 20g Jun 256 cams pp fli heads vs 2.5L tdo4 pp fli heads stock cams1.jpg
Power of Air Density FXT 20G TMIC Cold vs. HOT Graph.jpg
RFMIC
RFMIC stands for reverse intake manifold front mount intercooler. On a Subaru boxer style engine, you can simply flip the intake manifold 180 degrees and help eliminate 1-6 feet of piping depending on the intercooler kit. This would decrease lag and support more boost at higher rpm’s. Sounds easy right? Not really, just like anything else with car modifications, all it takes is time and money. You will typically pay about double for the customization of parts and labor to install the RFMIC vs. a FMIC. However, you can create a happy medium between the FMIC and TMIC. For more info visit here: FLI
RFMIC Examples-hyperlink. As you can see by these dyno sheets, a RFMIC will make more boost (more power) at lower rpm's. The first graph is a non AVCS 2.5L 02 WRX with ported heads and upgraded camshafts vs. an STI with AVCS with a FMIC.
Power of Air Density 02 WRX Port head with Jun Cams SR56 RFMIC vs. 04 STI SR56 with FMIC Graph.jpg

Power of Air Density 02 WRX Port head with Jun Cams SR56 RFMIC vs. 04 STI SR56 with FMIC Graph.gif
02 WRX FLI 2.5 Stage 1 block, RFMIC, 20G turbo, PE 800 cc, Cobb 3 inch TBE, Cobb
70mm short ram, APS header, Sae is.jpg
ELLIOTS RFMIC GT35 vs. JW GT35 FMIC-not done yet
COLD vs. HOT Dyno Runs
Here are many examples of hot vs. cold dyno runs back to back using the same cars, on the same day with the same gas and ECU map. Please read the description for IAT
(taking by external reading on the intake manifold) and temperature with humidity comparisons of the dyno room. As you can see, heat kills power.
Power of Air Density 98 Forester Hot vs. Cold Graph.jpg
Power of Air Density FXT St2 Hot vs. Cold Graph.jpg
Power of Air Density FXT 20G TMIC Cold vs. HOT Graph.jpg
HIGH OCTANE RACE FUEL
It is no myth that the higher the octane fuel, the more power you will get from your engine with the proper tune. However, you will pay between $7.50-10 a gallon for 100-
109 Octane fuel. Here at FLI, we map cars all the time with race fuel. Ranging from customers who mix 101 with 91 Octane to achieve a 95 or 96 Octane rating to others who run straight 101 or 104 Octane fuel. The reason why you can get more power from higher octane fuel is because normally there are more carbon-carbon bonds which contain more energy than carbon-hydrogen bonds (which are in lower octane fuels). Hence a fuel with a greater number of carbon bonds will carry more energy. Keep in mind that you must have a well tuned engine that is operating at a respectable VE percentage. Here is an example of increasing power with running higher octane fuel.
04 STI FLI Stage 2 KN Typhoon intake 91 Octane fuel IAT 70 vs Same Car with 104
Octane fuel IAT 106.jpg
Running race fuel does not mean that you do not get heat soak. Actually, you will see the same heat soak rules apply to higher octane fuels. Here is an example.
Power of Air Density 04 STI 104 Octane IAT cold 70 degrees, temp in room 70 hum is
30 vs hot IAT 113 degrees 75 roo.jpg

With the IAT increasing over 50 degrees, you can see that even with race fuel you still can have heat soak. Actually, this particular car made less power on 104 octane with high IAT's then his cold 91 octane map! How do you solve this problem of heat soak and rising temperatures in the combustion chamber? Methanol/Water injection kits.
METHANOL/WATER INJECTION
You now know that one of the keys to making power is Air Density. What happens when you live in a area that usually has hot summers or stays warm all year round? You still run into heak soak no matter what you do and intercooler water spray kits only do so much.
A good solution to heat soak when using what ever Octane fuel is a Methanol/Water
Injection (Meth/H20) kit. The purpose of this kit is to lower the intake air temperature and more importantly, combustion chamber temperatures. The mixture is usually 60%
Meth and 40% H20 because water is much denser then Methanol. The kit that FLI has done extensive research and development on is the Aquamist HFS-5(hyperlink to the
Aquamist HFS-5 on site). This kit uses a unique load measurement to activate and spray the Meth/H20. The HFS-5 kit uses fuel injector duty cycle (FIA unit) as the load vs.
MAP (manifold air pressure) or boost. This is a much more precise method of injection.
The mixture sprays 1:1 with the injector duty cycle. This creates a nice linear curve.
The HFS-5 kit also has a mechanical fail safe feature which switches the wastegate actuator to whatever the spring is set for on the turbo if there is a clog detected in the line or if you run low on the Meth/H20 fluid. For example if you have a factory U.S. STI turbocharger, then the actuator has a spring pressure of around 13.5 psi. If your custom map has boost targets of 20 psi tapering to 14 psi and the system detects a clog or you run low on fluid, the maximum pressure mechanical allowed will be 13.5 psi. This will save an engine!
Now here is where the power of air density gets interesting. The Meth/H20 nozzle is usually mounted somewhere near the throttle body, which allows it to spray in the intake manifold. FLI performed a test with the HFS-5 kit on a 2004 STI that has a custom FLI
Stage 2 map, catted turbo back exhaust, factory turbo and intercooler kit. The cold run temperatures were: Dyno room = , Dyno room humidity = , IAT (intake air temperature)
= 66 degrees. In these conditions this STI made 335 wtq (wheel torque). And 279 whp
(wheel horsepower). The hot run temperatures were: Dyno room = , Dyno room humidity = , IAT = 101 degrees. In these conditions this STI made virtually the same power as you can see in the graph below. This means in the summer time, on a race track or anytime else the heat is high, you can actually make almost the same power as you do in the winter!

Power of Air Density 04 STi FLI Stage 2 T85 H40 With Meth vs FLI Stage 2 no Meth
IAT 65 T 65 H50.jpg
Power of Air Density 04 STi Stage 2 With Meth comparison with IAT change IAT
100 and IAT 65 I65 H50.jpg
CONCLUSION
To make good power, you must start off with a well tuned, well maintained engine running good fuel (minimum good quality 91 Octane). This engine must also have all the mechanical upgrades to support the movement and volume of air/fuel mixture to make power. Making big power does not always mean you car will run better. You must also think about the power band you want. There are many other factors when making power that are not mentioned above. However, air density is one of the most overlooked factors when comparing dyno sheets, turbochargers/exhaust combinations, compression ratio, engine size and cams or head packages. Next time you read a dyno sheet from one of your friends, ask them what was the temperature and humidity in the dyno room for a better baseline of comparisons.