The effects of back pressure

A muffler is an ambiguity in the performance world, as it can have both good and bad effects on power output. Back pressure is essential for peak power in almost any application, even 6000hp alcohol-burning Top Fuel dragsters have some built-in back pressure. Before we begin this discussion, take note that 1 atmosphere is the pressure that air is at naturally (uncontained) at sea level.

During the combustion process, when the exhaust valve is open, all of the compressed (and depeleted) air-fuel mixture spills out from the cylinders as fast as it can, through your exhaust manifold, into your catalytic converter(s), the muffler, and finally out the tail pipe. The problem with this is, just before your exhaust valve is about to close again, your intake valve opens up, allowing the fresh air-fuel mixture to rush into the cylinders. This is called overlap, and one of the things you take into consideration when choosing a cam, because it can be used to your advantage.

If there was no valve overlap, it would be 100% impossible to completely irradicate all of the spent gases from the cylinder. This has a two-fold effect on power output, depending on what RPM the motor is running at. At low RPM, this effect actually increases torque, because the least amount of compression is lost during the intake stroke, and the ratio of intake to exhaust gases is high. Unfortunately, as the RPMs increase, there is increasingly less time to evacuate the exhaust gases during the exhaust stroke, and more and more depleted air-fuel remains in the cylinders when the exhaust valve closes. The motor becomes incredibly inefficient near its readline. A motor designed for high-torque applications, such as towing, tends to exhibit less valve overlap then normal. The type of cam used in this application is often called an "RV" cam, because a recreational vehicle doesn't need horsepower as much as it needs low-end torque to get it moving.

Your car would also run terribly if there was too much valve overlap as well. When exhaust gases rush out of the cylinder, they create a low pressure area in the cylinder and the exhaust system, sucking the intake charge right into the cylinder, and right back out into the exhaust system. This is called scavenging. When that air spills out, so does the fuel it was carrying, so the O2 sensor reports a rich condition to the computer, often causing further decreases in the amount of fuel the computer injects. At low RPMs, this effect is most pronounced as there is sufficient time to suck out a significant portion of the intake charge, reducing torque. As the RPMs increase, however, the extra velocity imparted to the intake charge increases the amount that squeezes into the cylinder after the exhaust valve closes, as the valve closes so quickly at high RPMs that barely any intake charge escapes through the exhaust system.

No doubt you see the dilemna posed to designers when they choose a cam for the motor - the right combination of power must be achieved at the intended RPM range of the vehicle - if this is a tiny four cylinder, which must spin high RPM to make any power at all, you have to design in a higher amount of overlap. If the vehicle is mostly intended for low speed towing, you design in a smaller amount of overlap for more low-end grunt. Overlap is one of the reasons why the four-cylinder Acura Integra GS-R makes 170hp at almost 7000 RPM, and your 3.0L makes 171lb-ft of torque at 2000 RPM.

You can't control your overlap without changing the cams, but the effects caused by changing the back pressure are the same. When you reduce back pressure, it is equivalent to increasing valve overlap, and when you increase back pressure, it is the same as decreasing the amount of valve overlap. That's why some people will say, "you need a muffler for torque", or, "you'll have more high-end, but less torque, if you run straight exhaust". They are right, but a muffler's purpose is to reduce sound output, not horsepower! By reducing back pressure in an exhaust system, you increase high-end horsepower at the cost of low-end torque.

You can compensate for this by increasing the velocity of the intake charge. Increasing the intake velocity has the added side effect of increasing back pressure, because there is more air to be evacuated during the exhaust stroke. Note that if you increase intake velocity past the limits of the exhaust system, the gains you achieve are diminished to the point of being non-existent. That power will be there when you do upgrade the exhaust system, which is why something as simple as upgrading the exhaust system can result in huge horsepower gains.

You can decrease back pressure by increasing the size of your exhaust manifolds, or using separate tubes for each cylinder (called headers). Increasing the size of the exhaust pipe and decreasing its length also helps, as well as installing high-flow catalytic converters and mufflers. Of course, you could just cut them off. Although it is illegal in the United States to remove the catalytic converter in a registered vehicle, the muffler is a different story...