"Boost 101"...from TM.com

Discussion in 'Fox 5.0 Mustang Tech' started by TT91, Apr 15, 2004.

  1. Thought this might help out alot of folks here:

    Boost Is Not a Power Adder

    As I speak with more and more people about the subject of boost it seems to be a recurring myth that boost makes power. Now I know that boost does make power but in the form of cfm. You really have to close your eyes and think about the reason boost makes power AND what kind of boost makes the best power.

    Boost is nothing more than a measurement of pent up air in an engine. Lets take a look at how boost works and why boost does not make power. Take a bone stock 302 or 281SOHC. Add ten pounds of boost with a small blower like an S-trim. For sake of argument let’s leave out the intercooler and fuel octane for the time being. Now with that said, let’s say the little blower is whizzing away and producing an extra 100hp on the 302/281. We have 3-inch inlet (suction side) feeding the blower. What happens when you change the inlet to a bigger one? Say a four inch power pipe? A slight increase of power across the rmp and a little more boost would result. More boost equals more power right? Wrong. The increase boost is just a byproduct of giving the blower more room to work. With the little inlet the blower was actually adding some parasitic losses through the crank due to the higher vacuum on the inlet tube. The blower took power away from the crank because it was harder to suck air into the blower. Increase the CFM capabilities to the inducer (impeller face) and the blower will produce more boost at the same impeller rmp because it has more air to compress. The reason for my example to have a blower in it is because of the ability to describe impeller speed as a constant.

    The easier you make things the more power you will make. So we learned that the bigger air inlet the better right? Here is where things get fun. Using a turbo instead of a blower, we change from a 3” to a 4” inlet. This time the power goes up but the boost does not. Since we have the waste gate regulating the boost to 10psi we do not see an increase in boost. But we see a increase in power. Simple really, the turbo is taking some of the parasitic losses and giving them back to the crank. Giving the turbo more room to breath has allowed the turbo to make the same boost but at a lower impeller speed. Because the impeller is spinning slower the exhaust gas pressure is reduced. Anytime a piston has to push it takes power away from the crank. The higher your exhaust pressure before the turbo is the more power the piston uses to push the exhaust out of the engine. Think of it this way. The turbo did not have to WORK as hard to make 10psi of boost. Less work, more power.

    Back to the blower and the impeller speed vs. boost. Now our S-trim is making 10psi of boost. The engine we have is stock. We swap engines for one with H/C/I. What happens? Boost drops. The reason for the drop in boost is directly correlated to the volumetric efficiency (VE) of an engine. Because the engine has added VE, the blower, still spinning the same rpm as with the stock engine, is simply not moving the cfm of air to produce the 10psi of air back-pressure in the engine. At the same time as the boost drops the power goes up. LESS BOOST, MORE POWER??? The blower is still moving the same speed. It is moving the same air too. Here is where it gets real interesting. Because the blower is not pushing as hard to move that same air into the engine less power is sucked from the crank. Remember less work, more power.

    There is another reason there was added power output as well. That is air density (AD). AD is basically the amount of air that is in a cubic foot of air. The Earth’s air has a certain amount of weight to it. As a level of reference we will use psi. The weight of the Earth’s air is 1 BAR or 14.7psi. = 1 ATM (atmosphere). 14.7psi is a standard starting point. It is the right relative humidity, air temperature and baro to achieve 0 feet of elevation or sea level. Using those three measurements you can come up with the DA. DA is basically a term used to describe the correct elevation of the air that day.

    In theory if you double the amount of oxygen that goes into a engine (along with fuel) you will double the output. This is true. But to get a true double amount of power from a power adder stand point you usually need to run more than 14.7psi of boost. This is because of air temperature and parasitic loss. When you compress air in heats it. Air that is hot has LESS DENSITY. Hot air expands and so the air molecules are not as close together. This makes the air lighter or less dense. Less air means less oxygen means less power. This is why we run heat exchangers (intercoolers). Cooling the air back down will increase air density and power with in the same cfm of air. The bigger the intercooler the better. One of the reasons is air restrictions and pressure drop. You will have some drop no matter what. Some will always come from the fact that by cooling the air it becomes denser and shrinks, reducing pressure slightly. The intercooler being to small in cfm flow also causes pressure drop. This is a serious horsepower killer. The turbo is working extra hard (using engine power) to move a specific amount of air to make our 10psi of boost. The problem is you might be seeing 15psi of boost at the intercooler inlet and 10psi at the intake valve. Your turbo has to move more air than necessary taking power away from the crank in order to do it. The same goes for the down pipe and exhaust system. If it is too restrictive the engines power is used to push the exhaust out. Now don’t be running out and putting a six inch down pipe on your car. The turbine will only flow so much exhaust. You will not benefit going to anything larger that a four inch down pipe on a turbo as big as a T4-76GTS.

    I hope this squashes some of the myths of boost.

    Any questions class?

    (Written by BIG JAY of TurboMustangs.com)
  2. damn you were right about the good reading.......