does an intercooler add power.....(LONG)

OsoSlo z28 · Post by **OsoSlo z28** » September 13th, 2002, 11:34 am

this is a post that was put up on jaxracing.com. this is based on a supercharger, but a turbo is a form of supercharger that uses a turbine driven by exhaust gases rather than a belt driven application and the principles are the same...enjoy.There are a number of things that you need to take into account when you answer this question. If the supercharger is sized properly to the motor, and the intercooler is sized properly to the "system" then all things being equal (timing and a/f ratio) you will make more power. This is due to numerous reasons, not just the colder air entering the motor. The first point that I need to get across is that you must consider the whole supercharger/intercooler/engine system. Not just the intercooler. By system I mean everything from the nozzle (compressor) side of the impeller to the cylinders. When we measure pressure, we typically measure pressure at the manifold. One thing that is common to do is measure the pressure before and after the intercooler. This is NOT a valid thing to do. If the intercooler is properly sized to the "system" it will cause very little AVERAGE pressure drop. If you want to measure the pressure drop due to installing an intercooler you need to measure manifold pressure both before you install the intercooler and after you install the intercooler. For example, if you have a non intercooled supercharger making 15 lbs of boost and then install a properly sized intercooler you may only see .5 - 1 lbs change in manifold pressure. If you measure before the intercooler you most likely will see more than 15lbs of pressure. I will explain what causes this more further down. What I need people to understand is that for a given system where you add an intercooler that is properly sized to the system you will see little or no AVERAGE pressure drop and manifold pressure change will be miniscule. Measuring pressure before and after the intercooler is not a valid thing to do. (again I will explain this further down)The most important thing to understand about air and making power is Mass. The Mass of the air, while a function of Volume and Density, if kept constant, will contain the same amount of Oxygen. Mass = Volume * Density. While most newer cars use a Mass Air sensor to determine how much air is entering a motor, a Speed Density system works by using pressure and volumetric efficiency, using these two things we can compute the Mass of air entering the motor. Given the Mass of the air entering the engine, we can make a fairly good assumption on how much power you can make. Lets investigate the effects of Pressure and Temperature on volume and from that we can infer the change in Density due to the same effects.Lets take a look at what happens when you heat and cool air. Air is made up of molecules of nitrogen, oxygen, and other gases. The molecules are moving around at incredible speeds coliding into each other and all other things. As the temperature increases, the molecules move faster which means they push harder against their surroundings. If you fill a baloon with air, heating it will expand the baloon, and cooling it will cause the baloon to shrink. Try blowing up a baloon and putting it in the refridgerator. I think you will be surprised at how much the baloon shrinks. There is exactly the same amount of air in the baloon as when you started, but it is much denser. Charle's Law (V1/T1 = V2/T2) states that the volume of a "fixed" mass of gas at a constant pressure is directly proportional to its absolute temperature. What this means is that when temperature increases at a constanat pressure, the volume required to hold the air increases. When temperature decreases at a constant pressure, the volume required to hold the air decreases. Next lets take a look at what happens when you compress air. Boyle's law (P1 * V1 = P2 * V2) states that the volume of a "fixed" mass of gas at a constant temperature is inversely proportional to the pressure of the gas. Basically when pressure increases, the volume needed to hold a given Mass of air decreases, and when pressure decreases, the amount of volume needed to hold a given Mass of air increases. Looking at these two laws, we see an increase or decrease in volume given changes in pressure and temperature. Because the volume of our supercharger system is a constant volume (cooler, tubing, manifold, etc), and because Mass = Volume * Density, at a given Mass, pressure and temperature will effect the Density inversely proportional to the way they affect Volume required to store the same Mass. Given constant Mass, Volume, and Pressure, as we decrease the temperature of the air, the density increases. Similarly, given a constant, Mass, Volume, and Temperature as pressure increases, the density of the air increases. Now if you consider that the volume is a constant, and airflow entering the supercharger is a constant both with and without an intercooler, the resulting Mass of air entering the engine is exactly the same with and without the intercooler. The density of the air is higher after the intercoolers cooling of the air. For the Mass to stay the same, something else has to happen. Since density increases as the air temperature decreases, the result is a pressure drop at the outlet of the intercooler with respect to the outlet of the supercharger. This is not due to (in a perfect system) restrictions in the intercooler, but it is due to the air coming out of the intercooler taking less space. What you are thinking now is that I just proved myself wrong. One thing we havent considered in all of this is the physics of the supercharger. Superchargers make boost, and most people get caught up in boost numbers and fail to realize how much efficiency comes into play. Boost in this case, is a measure of the dynamic pressure between two air pumps. One important thing to look at is the effect of increasing the Volumetric Efficiency of the motor on boost. Adding a set of heads, intake, and camshaft to a typical 5.0 motor will increase the volumetric efficiency. Given a properly sized supercharger, the boost will drop as the volumetric efficiency increases. Because the volumetric efficiency of the engine increased, even with the resulting drop in boost, horsepower is increased. An example would be a stock motor with a 10psi non intercooled supercharger from any of the various manufacturers may make 300rwhp. Add a set of heads, a cam, and an intake while changing nothing else your boost may drop to 8psi but the motor will make a substantially increased amount of horsepower, say 400rwhp. Given the same impeller speed, the superchager is moving much more air.To clarify this a little, let's look at what is called a compressor map. An example can be seen at <A HREF="http://www.mercurycapri.com/technical/e ... tratm.html" TARGET=_blank>www.mercurycapri.com/technical/engine/t ... tm.html</A> A compressor map is a 3-D plot of a superchargers efficiency given a pressure ratio, Mass air flow, and the rpm of the impeller. In our case we want to compare apples to apples so we will keep the impeller speed a constant and look at the effect of pressure ratio and air flow. Pressure ratio is defined as (Boost + Atmospheric pressure)/Atmosphereic pressureFor example, if you are making 15lbs of boost, your pressure ratio would be 24.7/14.7 = 2.02. This is given Atmospheric pressure as defined at sea level which is 14.7psi. If you look at the compressor maps at the site above, you will see pressure ratio to the left. Now these compressor maps are for a turbocharger which spins at a substantially higher rate, but the principles are the same. Lets take the point where the impeller speed is 132,800. If you look at both of the compressor maps it is easy to see that if you keep the impeller speed constant and lower the pressure ratio, the point at which you plot the efficiency moves twards the right away from what is called the surge line thus increasing air flow. The surge line is a line on the compressor map where at a given pressure ratio, air flow, and impeller speed, the air stalls inside the compressor and actually stops moving. Idealy you want to be in what is called the island of efficiency. This is where the pressure ratio and air flow combine to generate the most airflow at a given impeller speed without generating extra heat. Typically within an rpm range you will be very close to the surge line at the lower RPM range and move up into the island of efficiency as airflow increases. That ofcourse is only if the supercharger is properly sized to your application. If the supercharger is too big, then you will operate to the left of the surgeline and actually lose power, if it is too small you can operate in what is called a choke condition. When you choke a supercharger, you are actually trying to move more air than the supercharger is capable of moving and the Mass airflow will stop increasing. In the efficiency island, not only is the temperature cooler, but the power it takes to sping the supercharger is lower.(One interesting thing to note here is that if you blow the supercharger off into the air, and not into a motor, you move very far to the right of the island. The supercharger will move a tremendous amount of air but at a high temperature and will also take a substantial amount of power to spin. That is why you can't measure horsepower used by a supercharger by just venting it into the air and dynoing the motor while spinning it.)Without exception a drop in pressure ratio, but an increase in Mass air flow will move the point on the compressor map to the right, further away from the surge line. If the drop in pressure is too severe you may end up in a choke condition where your airflow will fail to increase, and the temperature will continue to rise. (This will also happen if your intercooler is too small. Once you reach the max flow of the intercooler, your temperature will start to increase with little or no gain in airflow). As you can see, pressure ratio has severe effects on airflow and efficiency. The interesting thing here is that the supercharger sees the WHOLE system, from the volute to the cylinders when moving air. So looking at the whole system, what can we do to reduce the pressure ratio? Bigger heads, bigger motor, cam, intake, or making the air that you are pushing through all of those things smaller thus allowing more Mass. As I said above, the supercharger sees the Mass air flow through the whole system as well as AVERAGE pressure across the whole system. So if pressure drops and airflow remains the same, you move to the right of the map, if pressure stays the same, and airflow increases, you move to the right. And if you have both, you move to the right. One neat thing is that as you move more twards the efficency of the supercharger, boost will increase, temperatures will drop, and the power it takes to spin the supercharger will drop. Going back to a comment I made above... "If you measure before the intercooler you most likely will see more than 15lbs of pressure." If your intercooler is sized properly, and your supercharger is sized properly, adding the intercooler will make the engine look bigger tot he supercharger. This, in a perfect world, will move the supercharger closer to it's max efficiency thus increasing boost. One neat thing to do is measure the discharge temperatures of the supercharger at the volute both with and without the intercooler. You may find that the temperature at the volute drops when adding the intercooler. This happens because the supercharger is becoming more efficient.SO, given all this babling, let's answer the question. Does an intercooler in and of itself add horsepower whithout adding timing to the tuneup?Yes and No. Isn't that a great answer.Let's give a couple of conditions for Yes first :Given a perfectly tuned non intercooled setup where the blower is in the middle, or to the left of the efficiency island, adding a properly sized intercooler will make more power. This is because the entire system will look larger to the supercharger thus moving you to the right of the compressor map. This is especially noticed at low rpm where you will actually experience more boost because you are closer to the island and the supercharger becomes more efficient.Given a non intercooled combination where timing is a little too advanced, not to the point of damaging parts, but enough to where one more degree doesn't make anymore power, adding a properly sized intercooler will add power. As you approach knock conditions, the cylinder pressure "during" the compression stroke will rise to the point of taking away horsepower. A colder intake charge will slow the increase in cylinder pressure at the start of ignition, thus preventing the over buildup of pressure on the pressure stroke, thus increasing horsepower.Given a non intercooled system where the supercharger is a little too big for the motor, adding an intercooler will add horsepower. The colder air is denser, and thus you can move more oxygen through a smaller opening. Because you can do this, your pressure ratio will drop, thus moving you closer to the island in the compressor map. As you move closer to the island, efficiency increases and boost increases. This one is hard to understand so I will give an example. I had a 327 outlaw motor that made 17lbs of boost with one supercharger. I changed to a Procharger D3M. This was the same supercharger that made 30lbs of boost and went 7.70 in a Pro car with a 406ci motor. When I put it on my motor, the most it would make was 17lbs of boost. After contacting Pro-Charger, they sent me a D3C. This is the exact same blower with just a scaled down impeller. I was told to spin it much slower than I was spinning the D3M. The result was 32lbs of boost. I went with a smaller supercharger and spun it slower and picked up 15lbs of boost. The reason is that the supercharger was too big for my motor and was surging. An intercooler would have helped me to some extent.How bout a couple for No :Given a perfectly tuned non intercooled setup where the blower is to the right of the island, adding an intercooler may actually cause you to lose peak power but may still increase power in the lower range. This is because you are moving out of the island twards a choke condition. The supercharger becomes less efficient.If you size the intercooler too small for the system then two things will happen. The cooling effects of the intercooler will be minimal, and you end up moving twards the surge line of the supercharger. Also the air will back up in the intercooler causing the system to look smaller with also moves you closer to the surge line. The result may be no increase, or possibly a decrease in power.Other things to consider :The above discussed what happens as a result of pressure and cooling effects. One other thing that you end up with when you have a properly sized intercooler is a ramming effect. As the air cools, there is more space for the incoming air to fill. The air rushes in behind the cooling air and actually causes a raming effect which in turn helps cylinder filling.Colder air is more efficient when used to make power for the same reason that adding octane allows you to add more timing. It slows the rate of initial combustion. If you can get more air into the cylinder without rasing the heat of the air, it takes longer for the combustion to start. Therefore you end up with less cylinder pressure on the compression stroke but still have the same amount of oxygen as the combustion accelerates. The resulting charge on the power stroke is the same but with less power lost on the compression stroke. For this reason, you can also add timing... but that wasn't allowed in this discussion... Intercoolers are just plain old cool!!!!So needless to say, there are several ways to answer this question, and a million things that I left out. There are a number of things to consider when answering this question that I did leave out, mainly engine dynamics. What I describe above are given perfect described conditions.Im brain dead now... hope this was interesting reading!!!!Michael Freedman__________________ 94 Mustang Cobra Renegade Car 8.7@160

Psyrg · Post by **Psyrg** » September 15th, 2002, 12:10 am

Good job on that essay, I believe it is all correct as my understanding of the laws of gas flow and thermodynamics go.Perhaps it would have been easier to discribe the system not so much in mass flow, but in 'head loss' terms. If you dont know what head loss is, think of 'head' as the distance water would squirt in the air at a given pressure, commonly measured in metres. Most pipe and restrictive devices you can buy on the market are measured in terms of head loss.Another interesting law you could have given is the ideal gas law, which allows one to link the concepts of pressure, volume and temperature to moles (a molecular measure) It is written as:PV=nRTwhere R is a constant and n is moles.Rearrangement can show:n=PV/RTas a rough guide to how both pressure and temperature can affect the number of oxygen molecules are admitted to an engine.

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does an intercooler add power.....(LONG)

does an intercooler add power.....(LONG)

Re: does an intercooler add power.....(LONG)