There are two worlds, everyday and racing. While the performance principles are exactly the same for both, the way they are applied is different. Pipe diameter is one such area of difference. A 360 ci V8 engine that turns 6000 rpms regularly needs dual 3-inch pipes to carry the flow. The same displacement engine that putts around at around 3500 revs does not. The point here is that what you see on a racer, again I wail on you about pipe diameter, does not apply to your street rig unless you want to pay money to lose torque, mileage and a bit of everyday driveability. Maybe even some of your hearing.
Maximizing the Scavenger: Mufflers
Mufflers, most specifically stock mufflers, are usually the biggest impediment to exhaust flow. Their main redeeming quality is that they are quiet. Power and a quiet exhaust system are not mutually exclusive but there are usually extra costs involved in making that compromise. The aftermarket can devote that time and effort, charging you the extra dinero it takes to design and build it.
There are two common types of aftermarket performance mufflers, passive and reactive. The passive types operate on the principle of sound absorption. They are usually straight thru and packed with fiberglass, stainless steel wool or even material made of lava rock. On the upside, they are cost effective and typically have little or no restriction. On the downside, they tend to be noisy and if the packing isn't top quality stuff, it blows out and the muffler gets even louder. Reactive mufflers use tuned sound chambers, flow directors and baffles to control sound. The better-designed ones have little or no restriction and can be whisper quiet.
Muffler generalities include that you want approximately 2 - 2.5 CFM (cubic feet per minute) of flow per anticipated or actual flywheel horsepower. Finding the flow rate of a particular muffler can be difficult. The hot rod magazines periodically test them and some muffler manufacturers list this information. The size of the inlet and outlets are an indicator of flow rate, but even a big hole can be fitted to a restrictive muffler.
Because the muffler is the single most restrictive part of any stock exhaust, a simple muffler swap is an inexpensive first modification. With the small stock pipes, however, you may not be able to find a performance muffler with small inlets, or you may not be satisfied with the smaller inlet unit's flow performance. No problemo! The time-tested trick is to use a pair of bell-mouthed adapters and use a muffler with a larger inlet and outlet.
Maximizing the Scavenger: Pipes
Maximizing the scavenging effect starts with careful selection of pipes. The right diameter pipes (both at the header and in the system) maintains the velocity of the exhaust flow. The wrong size pipe slows it down. Bigger isn't necessarily better. You need the correct size for the power output. See the nearby chart for some rules of thumb on that. You kinda have to pick your poison. A system that works well for a 4.3L V6 making 200 hp may be a little small for that same engine pumped up to 280 hp and would restrict upper end power. The other side of that logic is that the bigger system installed on the lower power engine will tend to decrease low-end torque and fuel economy. Bottom line, you'll get better results for a daily driver with conservatively sized pipes than with sewer-sized pipes. Smaller pipes also equate to a quieter system. Unless your engine is really built, and built for high revs, a 1/4 to 1/2 inch increase in pipe diameter over stock is all you need to help unleash a noticeable improvement.
Maximizing the Scavenger: Headers
The OE has made definite improvements in the way they design exhaust manifolds. Rather than the simple and inefficient log type manifolds of years past, many newer ones have individual runners and are sometimes even tubular in construction. Thanks, guys! We'll take it... but there's still room for improvement.
Headers consist of multiple tubes from the exhaust ports that enter into a collector, or collectors. There are two basic styles, X-into-one (a V8 or four cylinder would be four-into-one, a V6 would be three-into-one, a straight six, six-into one, etc) and Tri-Y. The X-into-one has a division between the shorty and long tube styles. All headers work by enhancing the scavenging process. This is done by precisely selecting the tube diameters and tube lengths. Like any other part of the system, the bends should be minimized and the tubes mandrel bent.
Let's start with X-into-one headers, the most common type. The most efficient ones for street and trail rigs are long tube headers and tube length is important to tuning for peak torque at a particular rpm. Longer tubes tend to move the torque band lower and shorter ones do the opposite. On most V8s, equal length tubes are not vital because of the firing order. Pipe diameter and length are most important. Non-race headers use small diameter tubes (the exact size chosen according to the flow rate at the exhaust valve) and are best at between 24 and 36 inches of length, according to what will fit. Equal length tubes are important on most V6s, inline sixes and four cylinders. Fours are also more sensitive to the primary header tube length.
Shorty headers are often used with modern vehicles that have a shortage of space. The same benefits arrive as with long-tube headers but there are fewer of them. Their benefits fit somewhere between a long tube header and the stock manifold. Where a long tube header might give you a big bump in low and midrange torque, the shorty will be less pronounced in that area but offer a noticeable bump in the midrange and upper end. In many cases, a shorty header is your only legal alternative to the stock manifold. Long tube headers can be an adventure to install in some of the newest rigs.
Finally, there are Tri-Y headers, which branches the cylinders three times according to the firing order. The cylinders are paired with the next nearest one in the firing order. Cylinder one on a V8 is paired to number three, and five and seven are paired. Those two pipes are paired yet again at a collector mated to the exhaust pipe. One fires and the outgoing flow crates a partial vacuum in the branch to number three. When that exhaust valve opens, the scavenging effect is more pronounced, "sucking more exhaust out and more clean air and fuel in. The third pairing achieves the same effect.
Few Tri-Y headers are available, though for a limited number of applications, are generally more expensive than four-into-one. For a torquer, the Tri-Y header is generally more beneficial than a four-into-one because it scavenges better at lower rpms and over a longer period in the rpm range. When at it's peak, however, a long tube four-into-one offers better scavenging but at a higher rpm and for a shorter period in the rpm range
With all headers, those that are ceramic coated have been demonstrated to deliver a skosh more power because they hold more heat. Heat loss of the exhaust gasses at the header tends to slow the flow a little. In fact, heat loss is one plus to keeping a cast iron manifold, especially if it's particularly well designed. Ceramic coatings also help the header last longer. Ditto for the stainless steel headers on the market.
Catalytic Converters
As a final element of the exhaust equation, let's look at catalytic converters. The modern honeycomb designs flow well. They do break up the pulses somewhat, but in most cases, they are a "gotta have" item. Modern cats are probably no more restrictive than a good free flow muffler. On a mildly built engine, a replacement cat isn't a vital necessity, assuming it's in good shape. Consider adding a performance cat to the budget if your rig has a lot of miles on it since cats tend to get more restrictive with time. If your engine produces lots more than stock hp, the increased exhaust flow may outstrip a stock converter and a larger one needs to be fitted and that's another case for a performance cat.
For more info check out:
BD Diesel Performance
www.dieselperformance.com
Dynomax
www.dynomax.com
Edelbrock
www.edelbrock.com
Gale Banks Engineering
www.bankspower.com
Gibson
www.gibsonperformance.com
JBA Headers
www.jbaheaders.com
Magnaflow
www.magnaflow.com
MBRP
www.mbrp.com
