October 1999

IN SEARCH OF:

LOST HORSEPOWER - PART 3

By Steve Grosekemper

In my previous two articles, we discussed how to maximize   the amount of air/fuel charge into the cylinder and when to ignite it. This month we are going to see how to get these spent gasses out as quickly as possible.

An exhaust system has a seemingly simple job. Get the spent exhaust gases from the engine to the rear of the car without waking up the entire neighborhood. However, most of us are looking for a little more out of our exhaust system. We want it to create the maximum flow and output for our cars. This is not always an easy task to accomplish.

The difference between a good exhaust system and a poor one is like evacuating an auditorium row by row in orderly single file, or yelling A FIRE! @ and closing all but one small door! What is important in a street exhaust system is getting the gases out efficiently while making the car run in the manner and volume level it was designed to. The two extremes of the spectrum might be a car that is whisper quiet but has poor performance, to a car that won = t run below 5000 RPM but goes like it was shot out of a cannon above 6000 RPM.

The ingredients of a good exhaust are tuning (size, shape and length of the tubing), restriction (the resistance put on the exiting gases) and acoustics (the effects of sound vs. airflow).

When we say a car has tuned exhaust we mean all the runners are the same length. Think of each exhaust pulse as a ball going through a tube with a common junction. If the runners are the same length and the engine fires each cylinder in an evenly spaced pattern, the pulses will exit as if they were in single file because no two pulses will try to enter the collector (common junction) at the same time. As the impulses exit the collector, they tend to act like a syphon and pull the rest of the gases through. An example is > 65- = 74 911 = s (illustration #1) or after-market header systems (illustration #1a). Two examples of systems which are not tuned are > 75- = 77 911 = s (illustration #2) and > 78- = 94 911 = s (illustration #3). The reason for this was one of necessity. The > 75- = 77 system uses thermal reactors which have very poor flow and uneven tuning. For the > 78- = 94 911 = s, a system was needed that could utilize a common path for a catalytic converter, then a final muffler, and keep the package in the same small area! Porsche succeeded in all of these demands. However, performance was sacrificed. This is evident to anyone who has installed an early exhaust system on a later car and felt the 20 plus horsepower increase.

An exhaust system can be further tuned by the size and the length of the runners. A larger engine needs a larger tube. Runner length   can determine what RPM range will make best power. A good example is 911R headers. Designed for a 2.0L 911 racing engine developing over 210 horsepower at 8000 RPM. However, it made less than half that horsepower below 5000 RPM. If this exhaust was installed on a 2.0L 911T (125 hp at 5800 RPM) it would barely run, let alone increase the horsepower. That is why an exhaust should be matched to the engine size, induction type and camshaft profile.

Restriction is something we should be aware of. It's the only exhaust factor we can change without doing anything to a car. A problem indication might be a slight decrease in power or a hissing under severe acceleration. How can this factor change by itself? The most common failure that can cause excessive back pressure is catalytic converter failure. This usually happens when you experience an ignition misfire or an overly rich fuel condition. Raw fuel is dumped into the hot converter, the converter bed melts down and blocks the flow of exhaust gases. This melted mass can break loose and become clogged in the muffler. If you suspect this, the necessary test is not too difficult. You measure the exhaust pressure in pounds per square inch (psi) in the exhaust system under load. This is done by connecting a vacuum/pressure gauge to a piece of steel tubing which has been tapped into the exhaust system upstream of the converter and muffler. The steel tubing is used to dissipate the heat. The exhaust is very hot and will melt the gauges vacuum hose. Run the hose into the car and check the pressure as you accelerate to redline in second gear at full throttle. If the reading is below 5 psi for a stock exhaust, that = s great. If it is above 6-7 psi you may have a problem. For a high pressure reading, test the back pressure again between the converter and muffler. If the reading drops, the restriction is in the converter. If it stays the same, it may be in the muffler or in both components.