Automotive Horsepower Guide: Mufflers to Tail Pipes

In 1980, I built a 400-ft-lb, 404-hp 350 replacement for the anemic 158- hp 305 in my California Pontiac Trans Am. Back then, it took real effort to exceed 400 ft-lbs and similar horsepower. Imagine my disappointment when, no matter what I used for mufflers, the output dropped by some 20 ft-lbs and 25 hp. But my experience as a co-designer of a no-loss system for the original Mini Coopers encouraged me to try the same stunt for V- 8-application mufflers. Aided by an acoustics expert, the result was the Sonic Turbo manufactured by Cyclone (a division of Walker/ Dynomax). A big Hot Rod magazine muffler shootout at Gale Bank’s facility demonstrated that my 21 ⁄4- inch Sonic Turbo’s muffler outpaced everybody else’s 21 ⁄2-inch mufflers. This sparked a sales run of hundreds of thousands, which seemed to inspire the industry into more aggressive muffler design efforts.

 


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Building an exhaust system along the guidelines shown here proved an effective method of muffling a very high output engine down to acceptable street levels without losing any power.

Building an exhaust system along the guidelines shown here proved an effective method of muffling a very high output engine down to acceptable street levels without losing any power.

Now, thirty years later, you, as a hot rodder, inherit that legacy. These days, all the parts to build a no-loss system are at hand and affordable. What seems so often to be lacking is system tech know-how; but here, that’s about to be set right.

 

Simple Steps to Success

We dealt with the exhaust manifold (headers) and collector system’s role in power production in Chapter 13. From what was learned there, it should only be a short step to appreciating how parts selection and positioning of resonators, routing pipes, crossovers, mufflers, and the like will be a winning factor. This will be especially so if catalytic converters are involved in the equation.

 

In this chapter, we deal with catalytic converters (green-tipped arrows), X-pipes and crossover pipes (black-tipped arrow), entry and exit pipes (red-tipped arrows), and mufflers (blue-tipped arrows).

In this chapter, we deal with catalytic converters (green-tipped arrows), X-pipes and crossover pipes (black-tipped arrow), entry and exit pipes (red-tipped arrows), and mufflers (blue-tipped arrows).

 

For an emissions-legal street exhaust system, catalytic converter flow is the biggest problem. Be sure to diligently research the flow capability of whatever you buy.

For an emissions-legal street exhaust system, catalytic converter flow is the biggest problem. Be sure to diligently research the flow capability of whatever you buy.

 

I initially wrote on no-loss exhaust systems as early as 1980, so I am surprised that, as of 2010, it is still commonly believed that high output and minimal noise are mutually exclusive. As detailed here, a quiet system that allows within 1 percent of open exhaust power is practical, and can easily deliver a 40- plus-hp advantage over your lessinformed competition.

 
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Cats and Mufflers

If you are attempting to build an exhaust system that has to be equipped with cats, then you are in for a rough ride. But it is doable, so let’s start at square one. Essentially, there are two key factors to avoiding exhaust-system-induced power loss.

Inappropriate catalytic converter and/or muffler selection and installation effectively cancels any advantages system length/diameter tuning could have delivered. The questions most often asked are: What does it take to get it right? How much power are we likely to lose if the system is optimal? The simple answers are: “Not much” and “Zero.”

To build a no-loss muffled, highperformance/race system, it is vital we work with the two key exhaust system factors in total isolation from each other. The first of these factors is the pressure-wave tuning from our length/diameter selection, and the second is minimizing backpressure by selecting cats and mufflers with suitable flow capacity for the power concerned. For the most part, this involves nothing more than knowledgeable component selection and installation.

Cat and Muffler Flow

Carbs (at least in the United States) are selected based on flow capacity rather than size, simply because the engine does not know size-only flow. This begs the question as to why the same should not apply to muffler selection. The answer (muffler manufacturers: please note) is that it should, as the engine’s output is influenced minimally by size but dramatically by flow capability. A muffler purchase based on pipe diameter has no performance merit. The only reason for knowing the muffler pipe size is for fitment reasons.

 

Evaluating muffler flow, and modifying as required to optimize it, pays dividends in output. As unlikely as it may seem, not all muffler companies do this.

Evaluating muffler flow, and modifying as required to optimize it, pays dividends in output. As unlikely as it may seem, not all muffler companies do this.

 

X-pipes and crossover tubes allow a measure of muffler flowcapacity sharing. This helps reduce backpressure and improves noise reduction.

X-pipes and crossover tubes allow a measure of muffler flowcapacity sharing. This helps reduce backpressure and improves noise reduction.

 

The engine cares nothing about muffler pipe size but it really cares what the muffler flows, and that is dictated by its internal design. From this, it follows that the informed hot rodder/engine builder should select cats and mufflers based on flow, not pipe size. In other words, positive results are based on flow, not on pipe diameter. There are many ways to enhance the accomplishment of sufficient flow to get the job done.

We can firmly establish that its flow, not its size, dictates the success of the exhaust system. In Illustration 14-1 we see a clear demonstration of how an ineffective component, such as the cat or the muffler, can influence the apparent pipe size seen by the engine.

Let’s start by viewing a muffler installation as three distinct parts. In the top left drawing are the in-going pipe, muffler core, and exit pipe. Assembled, they appear as in the top right drawing. In the middle left drawing is a typical muffler. Due to a design process apparently unaided by a flow bench, the muffler’s core flow is significantly less than an equivalent length of pipe the size of the entry and exit pipe. Because the core flow is less than the entry and exit pipe, the engine sees the muffler as if it were a smaller (and consequently more restrictive) pipe like the reduced-diameter tube at the middle right.

The “core” flow of this muffler is the prime restriction and has little, if anything, to do with the inlet and outlet diameter of the muffler itself. What we need for best performance is a muffler (or cat, for that matter) that simulates the lower pipe where the core flow is actually greater than the inlet and exit pipe. Result: The muffler is seen by the engine as a near-zero restriction.

A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (11 ⁄2 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we see about 560 cfm from a 21 ⁄2-inch pipe. If we have a 21 ⁄2–inch muffler that flows 400 cfm, the engine reacts to this just the same as it does to a piece of straight pipe flowing 400 cfm. At 115 cfm per square inch, that’s the equivalent of a pipe only 2.1 inches in diameter.

This apparent pipe size concept is important to appreciate. The reason is that so many racers fixate on having as large a pipe into and out of the muffler as possible. As you should now see, this concern is totally misplaced; in all but a few cases, the muffler is the point of restriction, not the pipe. The fact that muffler core flow is normally lower than the connecting pipe can be offset by installing something higher flowing, such as a 4-inch muffler, into an otherwise 23 ⁄4-inch system.

 

Here is a dyno test of the Magnaflow C6 Corvette system. Basically, we have a 7 to 11 ft-lbs increase in output occurring over the entire RPM range. Peak power climbed by 11 hp.

Here is a dyno test of the Magnaflow C6 Corvette system. Basically, we have a 7 to 11 ft-lbs increase in output occurring over the entire RPM range. Peak power climbed by 11 hp.

 

14-1. For a clear explanation of what is being shown here, refer to the text.

14-1. For a clear explanation of what is being shown here, refer to the text.

 

Even as inexpensive as it is, this Walker Dynomax muffler is very effective. It provides good flow and output while suppressing noise. The design is a prime example of the extensive use of flow-bench results.

Even as inexpensive as it is, this Walker Dynomax muffler is very effective. It provides good flow and output while suppressing noise. The design is a prime example of the extensive use of flow-bench results.

 

This stainless-steel-construction Walker straight-through muffler may not look the most glamorous you’ve ever seen, but high flow and good noise suppression results in relatively quiet power.

This stainless-steel-construction Walker straight-through muffler may not look the most glamorous you’ve ever seen, but high flow and good noise suppression results in relatively quiet power.

 

Muffler Flow: How Much is Needed?

The first point to appreciate here is that optimally sized collectors/secondary pipes are not sized so as to meet the engine’s flow requirement, but more by the need to produce the desired pressure-wave characteristics. For instance, a 700-hp V-8 engine may have a dyno-optimized 33 ⁄4-inch-diameter collector. This diameter, in conjunction with the length, resulted in the system tuning-in at the desired RPM. But from the standpoint of flow, a 3-inch pipe from each bank is capable of handling all of such an engine’s flow requirements.

Without data to the contrary, it seems safe to assume that the more a muffler flows, the better. This, fortunately, is not so and here’s why: Increasing muffler flow unlocks potential engine power. Once all the potential power is unlocked, further increases in exhaust system flow do not produce any further benefits in terms of power. But what may be good for power may not be good for noise; any excess flow capability can lead to a noisier system. From this, we can conclude that too much muffler flow serves no useful purpose and possibly costs more money than necessary.

The trick here is to use just the right amount of muffler—no more and certainly no less. This allows the full power potential of the engine to be realized at the lowest cost, without undue compromises in terms of noise. Now the question is: How much flow is enough?

 
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In about 1985, many race-series officials anticipated that mufflers would be mandated for race vehicles, so I embarked on a series of tests to determine a race engine’s minimumflow threshold. Performing such tests might look easy, but to get meaningful results, it is very important to separate, as far as possible, the effects of flow and backpressure from the effects of pressure-wave tuning. It is entirely practical to do this by means of a pressure-wave termination chamber, which for all practical purposes is a very-oversized resonator box. Knowing when and how to use a big resonator box can be a great aid in building a high-performance system.

 

14-2. This chart shows the effect that flow and backpressure have on the power output when their effects are separated from the pressure-wave tuning due to header and collector sizing. These numbers were developed from several engines with cams ranging from about 290- to about 298-degrees duration. As can be seen, after the flow exceeded about 2.2 cfm, all the power from increased flow had been unlocked; so added flow produced no further increases.

14-2. This chart shows the effect that flow and backpressure have on the power output when their effects are separated from the pressure-wave tuning due to header and collector sizing. These numbers were developed from several engines with cams ranging from about 290- to about 298-degrees duration. As can be seen, after the flow exceeded about 2.2 cfm, all the power from increased flow had been unlocked; so added flow produced no further increases.

 

This 3-inch Borla muffler flowed the same as a straight piece of pipe of equal length because its core flow was higher than the inlet- and exitpipe flow.

This 3-inch Borla muffler flowed the same as a straight piece of pipe of equal length because its core flow was higher than the inlet- and exitpipe flow.

 

To get an idea of how lack of sufficient flow, and consequently backpressure, can affect output, check out the results of Chart 14-2. The curves shown are the results of tests run on a number of engines of various types. The only common element of significance between these engines was the use of a cam of 290 degrees or more of seat (advertised) duration and CRs of 10.5:1 or more. From the curves on this graph, you can see that the trend is: As flow is increased (and consequently backpressure reduced) on an initially flow-restricted engine, power increases rapidly at first and then gains diminish. Once the available flow exceeds about 2.2 cfm per hp, increasing muffler capacity drops the possible gains to less than 1 percent.

Having established, within reason, from the graph that 2.2 cfm per open-pipe horsepower means zero loss from backpressure, we can determine how much muffler flow a particular engine needs based solely on its open-pipe horsepower. Just make a reasonable estimate of the engine’s open-pipe-exhaust-power potential and multiply by 2.2. For example, a V-8 making 600 hp on open exhaust will require 600 x 2.2 = 1,320 cfm. Two 660-cfm mufflers will get the job done and contain the backpressureinduced power loss to 5 hp or less. From the foregoing, with mufflers rated in CFM, you can see how easy making an appropriate choice gets.

Before leaving the subject of muffler flow requirements, I advise against letting anyone convince you that a muffler system has to have some backpressure for the best output. To put it bluntly, that’s just a bunch of baloney.

 

Pressure Waves

Now, we can move on to methods critical to utilizing suitable-capacity mufflers in the system without disrupting the length-induced pressurewave tuning. Probably the best way to ease into this somewhat complex subject is to consider some of the published muffler test results done between about 1990 and 2008. At face value, these tests indicated that sometimes lower flow mufflers induced at least some backpressure, and that resulted in the test engine making its best power. In all such tests that I studied, the conclusion (as opposed to the tests) were incorrect. There are a number of reasons for this and all are relevant to building a no-loss exhaust system.

 

14-3. From this illustration, it can be seen that simply adding a straightthrough glass-pack muffler to a length-tuned system disrupts the optimized secondary length. This being the case, no matter how well the glass pack may flow, power is reduced. Conversely, the open internal design of a Flowmasterstyle muffler allows the effect of the tuned length to remain unchanged.

14-3. From this illustration, it can be seen that simply adding a straightthrough glass-pack muffler to a length-tuned system disrupts the optimized secondary length. This being the case, no matter how well the glass pack may flow, power is reduced. Conversely, the open internal design of a Flowmasterstyle muffler allows the effect of the tuned length to remain unchanged.

 

The first point cancels the supposed validity of thses back-to-back test results because of (see Illustration 14-3) the various internal muffler designs involved. Many mufflers consist of a number of inter-connected chambers, each presenting the exhaust a different ease of access. Others are the glass-pack variety. These types represent opposite ends of a spectrum and have a substantially differing response to arriving pressure waves.

When we discussed collector lengths in Chapter 13, I emphasized that, in most cases, it was more critical than the primary pipe lengths. Adding even a zero-backpressure muffler to a system with alreadyoptimized lengths can alter the pressure-wave response. The added length can simply tune the exhaust out of phase with your requirements and result in a drop in power.

The technique to use here is to install mufflers that don’t alter the tuned lengths of the system. Let us assume the mufflers being tested are attached directly to the end of the collector. A pressure wave is reflected either at the end of the exhaust pipe or when a sizable increase in crosssectional area occurs. Open-chamber mufflers, such as Flowmasters, often appear to the pressure wave much the same as the end of the pipe. The result is that the pressure waves see no change in length, and reflection occurs much as it did prior to installing the muffler.

 

This Magnaflow muffler is a straightthrough type. It has extremely good flow characteristics but you need to preserve the collector-tuned lengths.

This Magnaflow muffler is a straightthrough type. It has extremely good flow characteristics but you need to preserve the collector-tuned lengths.

 

Because of its intrinsically differing design, a glass-pack muffler acts significantly different. As the exhaust gases enter, they do not see what appears to be the end of a pipe so much as an extension of the collector. The result is a reduction of power, even though there is no measurable backpressure involved. From this, we can safely conclude that most comparative muffler tests were, in fact, “pseudo pipe length” tests.

Although many invalid conclusions were drawn, these tests still demonstrated some important facts. The most important is that the engine’s needs, in terms of flow and pressure-wave-length tuning, must be isolated, one from the other. This is easy to do by means of a pressure wave termination box (PWTB, or resonator box).

Incorporating a big resonator box into a system produces a layout along the lines seen in Illustration 14-4. Given adequate volume, the PTWB makes everything downstream appear invisible to the header’s primary and secondary tuned lengths. With a flow capability of 2.2 cfm or more, the muffler also appears virtually invisible from the flow standpoint. As a result, we have a muffled system that produces virtually the same power as an open exhaust.

The point of all the muffler selection and installation tech discussed so far is to produce an acceptably quiet system. If it does not do that, the point of the exercise is lost. By using no more muffler flow than needed, we are giving whatever mufflers are selected the best chance of doing the job.

 

Ideally, the PWTB needs to be shaped as in the upper drawing. If it is being used for a V-8 engine configuration, then it is advantageous to use a balance tube, preferably at position No. 1. Failing that, at No. 2, but is not usually quite as effective. An empty muffler (No. 3) can be used, but the exit pipe must have a suitable lead in, or flow losses result.

Ideally, the PWTB needs to be shaped as in the upper drawing. If it is being used for a V-8 engine configuration, then it is advantageous to use a balance tube, preferably at position No. 1. Failing that, at No. 2, but is not usually quite as effective. An empty muffler (No. 3) can be used, but the exit pipe must have a suitable lead in, or flow losses result.

 

14-4. Here is a move I found worked when I had the wrong-size Flowmaster for the tail pipes involved, but had some adaptors on hand to do the install anyway. The result was a small but measurable increase in low-speed torque with no downside at the top end. I presumed this was from the antireversionary effect of the smaller collector pipe entering an intrinsically larger muffler. Power seen at top end was still the same as that on open pipes.

14-4. Here is a move I found worked when I had the wrong-size Flowmaster for the tail pipes involved, but had some adaptors on hand to do the install anyway. The result was a small but measurable increase in low-speed torque with no downside at the top end. I presumed this was from the antireversionary effect of the smaller collector pipe entering an intrinsically larger muffler. Power seen at top end was still the same as that on open pipes.

 

Building a system with these guidelines proved to be an effective method of muffling a very-high-output engine down to acceptable street levels without losing any power.

Building a system with these guidelines proved to be an effective method of muffling a very-high-output engine down to acceptable street levels without losing any power.

 

Unfortunately, from one engine type to another, mufflers can be a little inconsistent and unpredictable in terms of noise suppression. Engines with high compression ratios and long-duration cams are usually more demanding in terms of noise reduction. Big cubes, shorter cams, and lower CRs are easier to muffle. There will inevitably be a system built to all the right specs that fails to reduce noise to the levels hoped for. A good start here, in order to avoid this as much as possible, is to peruse some of the bigger muffler companies’ web sites. On these sites you can see and hear chassis dyno tests of a wide variety of mufflers (including stock) on an extensive range of vehicles.

Also be aware that system installation can also affect the sound level experienced, especially in the vehicle’s interior. Tail pipes ending under the car’s bodywork use it as a sound box in much the same way as the strings cause a guitar body to resonate. Your best option is to have the tail pipe(s) go all the way to the rear or have a side exit. If the rear exit is used, further noise reduction, however small, can be had by downturned exit pipes, angled slightly in toward each other.

As far as output goes, the postmuffler tail pipe length has no measurable effect on the power if the large change in cross section discussed previously is present upstream (toward the motor) of the tail pipe. An open-type muffler or a resonator box provides this cross-sectional change. The tail pipe length exiting most glass-pack installations is also of little consequence if a resonator box is used, but can be of significant influence if not.

 

Crossover and Balance Pipes

The addition of a balance or X pipe can refine nearly all V-8 exhaust systems. These have two potential attributes: reduced noise and increased power. Extensive dyno testing of both these factors has indicated balance and X pipes are 100- percent successful at reducing noise. The reductions measure from a minimum of 1 dB to a maximum of 3 dB, with 2 dB being common.

As far as power is concerned, things are a little less certain. With engines between about 325 and 550 hp, experience indicates that, in about 60 percent of the cases (mostly with balance pipes), the engine can deliver as much as 12 additional hp with 5 to 8 being the most common. The other remaining 40 percent tested showed virtually no change in output, either up or down. Based on such results, we can conclude that a balance or X pipe is always a positive asset and never a negative.

Balance-pipe sizing seems to be not overly critical. The only really influential dimension is the pipe diameter. This needs to have an area at least equal to that of a 21 ⁄4- inch-diameter pipe (4 in 2) with 21 ⁄2 to 23 ⁄4 being preferable. Though limited to tests on engines up to a little less than 600 hp, there seems to be no measurable benefit to using a crossover pipe bigger than 23 ⁄4 inches in diameter. As for the crossover length, dyno results indicate that 18 inches long responds in virtually the same manner as 72 inches long.

 

The Ultimate System?

The system above is one I designed for a 700-hp, naturally aspirated street/strip small-block Chevy installed in a 1986 Corvette. It produced acceptable street-noise levels without any measurable drop in power. Although you may have to adopt some slightly different steps toward getting an acceptable installation, keeping sight of the principles involved delivers similar results. Step outside the guidelines, and you are on your own.

 

Written by David Vizard and Posted with Permission of CarTechBooks

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