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Out the Exhaust - Part One

The complete guide to modifying exhausts.

By Michael Knowling

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A low restriction exhaust system - following the turbine on a turbo car, or following the manifold/extractors on the remainder of cars - is a prerequisite to making good power. In most cases, a high-flow exhaust system should also be amongst your initial group of modifications, since it's a relatively low-cost and bolt-on modification.

How It Works...

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Exhaust backpressure behind any tuned-length manifold/extractor system is detrimental to making power. It should be minimised as much as possible.

Reducing exhaust backpressure serves to improve exhaust gas evacuation from the combustion chamber, enabling more of each new air/fuel charge to be inhaled into the combustion chamber. The more fresh air and fuel burnt, the more power you'll enjoy.

Another benefit of a low restriction exhaust system is a reduction of pumping losses - in other words, not as much power is required for the piston to force the burnt gasses out the open exhaust valve(s).

The on-road benefits associated with a high-flow exhaust system cannot be understated - it gives the potential for improved throttle response, increased torque and top-end power. Fuel consumption can also be improved, depending largely on your driving style.

The magnitude of these gains, however, varies greatly from car to car...

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Turbocharged vehicles usually pick up more power with an exhaust than a naturally aspirated vehicle; this is largely because the turbine wheel is spun faster, resulting in increased intake manifold boost pressure or boost which rises more rapidly. Be careful of companies that claim massive power hikes on turbo cars just by fitting a replacement exhaust - it's important to ask how much extra boost pressure (if any) comes from the fitting of the new system.

Naturally aspirated vehicles are very sensitive to extractor/exhaust manifold design, but - for now - we'll concentrate purely on the pipework behind the final collector. We'll discuss extractor design in an upcoming part of the series.

Fitting a high-flow exhaust system to the manifold or extractors of a naturally aspirated vehicle rarely gives the same percentage gain associated with turbo cars - nevertheless, some impressive power gains can be unlocked. Contrary to popular opinion, backpressure behind the extractor/exhaust manifold is detrimental to power - best power comes when air-fuel mixtures and ignition timing have been altered to suit a restriction-free exhaust system.

Other Factors
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Whenever you increase the volume of air that an engine breathes - like when upgrading the exhaust - there's the potential for air-fuel ratios and ignition timing to alter in a way detrimental to making more power. In other words, you can lose some of the potential gains of a new exhaust if the engine isn't tuned to match.

Many modern cars use a complex engine management system, which can be disrupted by the fitment of a high-flow exhaust. Flat spots and an uneven spread of torque are the most common problems; the only solution is to recalibrate the ECU to maximise the advantages of the new system.

Also, check that the modified exhaust is not causing detonation - particularly on turbocharged cars, which tend to run more boost with the fitment of a free flowing exhaust. If detonation is detected there are a few options - run the engine on higher-octane fuel, retard the ignition timing and/or have the ECU professionally re-mapped. Also look into all measures that reduce intake air temperature - this helps to eliminate detonation.

A Look at the Standard Exhaust...

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It's fair to say most OE (original equipment) exhaust systems are less than ideal from a go-fast point of view.

A factory exhaust is a carefully developed system with broad emphasis on emissions, noise, longevity, simplicity and - of course - manufacturing cost. Unlike us, the factory is not focussed predominantly on minimising backpressure in the search for engine power.

Let's take a close look at the post-turbine exhaust system fitted to the 147kW Nissan S15 200SX/Silvia as an example...

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Inside the S15's cast iron dump pipe you'll find ugly lumps and bumps, and its outlet diameter is quite restrictive at only 57mm ID. Following this is the front section of steel pipe, which measures 54mm ID at the front and tapers to only 45mm ID at the rear. Like every other section of standard system, this pipe is heavily press-bent (the crushed cross-sectional area throughout the bend radius is indicative of a press-formed bend).

The standard catalytic converter bolts to the back of the front pipe, and - interestingly - it has a much larger 66mm ID inlet and outlet. Flowing on from the cat is the so-called intermediate pipe, which - again - closes down to 45mm ID all the way along its length. A centre resonator is integrated into the intermediate pipe.

Once past the axle, gasses flow into a bend and - finally - the Calsonic rear muffler. The right-angle bent pipe leading into the muffler is 45mm ID and the box has twin 42mm ID outlets.

Not surprisingly, the S15's standard exhaust system is extremely restrictive in its role of flowing the exhaust gasses of a 147kW engine. The system's flow capacity is impaired by small diameters, press-formed bends, poor contours inside the dump pipe, and - we guess - a restrictive catalytic converter, resonator and muffler.

It's a shocker.

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Looking toward naturally aspirated vehicles, the TH-early TJ series Mitsubishi Magna 3.5 suffers massive backpressure from its rear muffler. Recognising this, Mitsubishi's TJ Sports and VR-X models are equipped with a higher flowing - though noisier - replacement muffler. This muffler alone is responsible for much of the Sport/VR-X's 13kW (9 percent) power hike.

Unfortunately, however, vehicles such as the Magna TJ Sport/VR-X leave little scope for future exhaust improvements - the factory has already been there to search for power.

Another example where the manufacturer has already wrung the maximum out of the exhaust system is the little Suzuki Swift GTi. In standard form, its high compression 1.3-litre DOHC, 16-valve mill puts out 74.3kW - that's a substantial 57kW per litre (naturally aspirated). With so much effort put in to ensure efficient breathing, you can bet there's not much room for effective bolt-on exhaust mods - and that's been proven time after time on dynamometers around the world.

Here's the upshot - on the majority of vehicles, it is possible to achieve decent power gains through exhaust mods. On other cars - those that are already highly tuned - there isn't much potential to pick up easy kilowatts.

It all comes down the amount of backpressure through the standard system - how much room for improvement does it allow?

Making Measurements
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Total exhaust backpressure can easily be measured with a pressure gauge connected into the front section of piping.

The easiest approach is to unscrew the oxygen sensor (if the car has multiple oxygen sensors, then unscrew the one closest to the engine) and temporarily replace it with a pressure-sensing fitting. Run a hose from there to a boost gauge and you're able to measure exhaust backpressure values. Peak backpressure will be seen on the gauge when the engine is running at maximum power in lower gears - the higher the indicated pressure, the worse the exhaust flow.

Note - if the oxygen sensor fitting is integrated into the exhaust manifold, your total backpressure measurement will include part of the restriction through the manifold.

Alternatively, if you want to measure the restriction across one particular section of the system - let's use the catalytic converter as an example - you'll need to drill small diameter holes through the exhaust immediately before and after the body of the converter. Next, weld one end of a short length of metal brake line into each hole - these create the pressure sensing fittings for your boost gauge.

Connect the pressure gauge to the fitting closest to the engine and temporarily plug the second fitting. Run the engine to its maximum power and take a measurement off the boost gauge. Next, reverse the fittings and repeat the power run.

The pressure restriction across the cat converter is calculated by subtracting the post-cat pressure from the pre-cat pressure. For example, if 60 kPa backpressure is measured prior to the converter and 50 kPa is measured behind it, we've got a 10 kPa flow restriction across the core.

You then know there's the potential to make up to a 10 kPa improvement in the catalytic converter area alone.

Different Approaches to Modification...

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There are a number of approaches to improve the post-turbine or post-manifold exhaust flow of your vehicle:

  1. Remove/modify/replace existing mufflers or resonators
  2. Remove/modify/replace the existing catalytic converter(s)
  3. Replace sections of pipe
  4. Replace the entire exhaust system

Let's take a look at the pros and cons of each approach...

1. Remove/Modify/Replace the Existing Mufflers or Resonators

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The practice of removing, modifying or replacing OE mufflers/resonators can be either very effective or a total waste of time - it depends how restrictive the existing components are.

As mentioned, those vehicles that come highly tuned from factory generally won't respond to this sort of minor tinkering. Replacing the rear muffler on our example Suzuki Swift GTi will give no worthwhile power gain. Don't bother with it - unless you're only chasing a different exhaust note...

On the other hand, mufflers and resonators absolutely strangle the power output of some vehicles.

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The Porsche 993 twin-turbo, for example, is equipped with a pair of extremely restrictive back mufflers - modifying their internal passages to a 'straight-through' layout can pick up an impressive 7 percent peak power (as measured on an AWD chassis dyno). Tailpipe noise is increased, but not by a huge amount.

If, however, cutting the original mufflers open, modifying the internals and re-packing it all sounds too complicated, you can usually buy a new aftermarket muffler to suit. There are a huge number of aftermarket mufflers to choose from. Alternatively, you may be able to replace an existing muffler with a simple length of exhaust pipe - this offers minimum restriction, but the downside is a lot more noise.

Certainly - if there's a worthwhile power gain to be found - tinkering with the factory mufflers and resonators is a great step for anyone on a budget. A new straight-through muffler can cost as little as $50 and fitment will typically be less than $100. One of those shiny brand name mufflers might be your preference, but - often - the extra cost doesn't make a lot of sense.

(Note - technical details on selecting mufflers will be given later in this series.)

As a final bonus, your car will likely shed a couple of kilograms with the removal of the factory muffler(s) - OE mufflers are often quite large and heavy.

2. Remove/Modify/Replace the Existing Catalytic Converter(s)

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Since their introduction, catalytic converters have been perceived as the cause of all evil. In reality, however, they have tremendous worth in all vehicles except for dedicated competition machines.

In the past few years, aftermarket catalytic converters have become available in a large range of sizes - you can get them in all diameters up to 4-inch. Furthermore, the price of a big aftermarket cat has come down considerably - a brand new 3-inch cat can now be bought for less than AUS$200.

Inevitably, a cat converter will cause some exhaust restriction, but - assuming it's the appropriate diameter - it shouldn't be enough to worry about. This small flow penalty is more than compensated by vastly improved emissions and reduced noise - few people stop to consider that a cat converter doubles as a muffling device.

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Anyone found ripping the inside out of their catalytic converter or removing it altogether is being a tight-arse and irresponsible.

Removing a cat converter's honeycomb internals will undoubtedly improve exhaust flow - but not enough to offset the associated downsides. In addition to pumping out more toxins, your car is at major risk of being defected - and so it should be.

Note that a catalytic converter typically has an optimal lifespan of up to around 60,000 kilometres - after this stage its efficiency declines and there's the possibility that the internal honeycomb will collapse. We're told if your car's converter is older than about 10 years, it is worthwhile replacing it.

3. Replace Sections of Pipe

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On certain vehicles you can find some appalling sections of exhaust pipe. Most often, factory exhaust bodges can be found when a new range of engines has been introduced to an existing body shell or the suspension has been changed - you'll often find areas where the exhaust pipe has been partially crushed in order to squeeze it through a tight gap.

It's unlikely you'll achieve the same sort of power gains possible with cat converter, resonator and muffler mods, but - still - replacing sections of pipe is a very cost-effective approach.

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In the case of this Holden Commodore VL turbo, for example, you can very easily and cheaply replace the front section of exhaust between the dump pipe and the cat converter. Using - say - 2¼-inch mandrel pipe to match the diameter of the rest of the system (in order to maintain smooth gas flow) it's possible to pick up maybe a kilowatt or two as well as improved throttle response.

There are no downsides to replacing select sections of exhaust - noise will remain virtually unaffected and with one exception, there are no legal issues. (The exception? Changing the pipe in front of the cat converter will alter its warm-up time, so affecting starting emissions.)

4. Replace the Entire Exhaust System

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Replacing the entire exhaust system is obviously the all-out approach. It has the potential to give a massive power increase, however, it is by far the most expensive route.

When replacing the entire exhaust system there are a number of things to consider - pipe diameter, types of bends, number of mufflers, types of mufflers and type of cat converter (where appropriate). Then you've got to think about system cost, durability, noise and weight - not to mention overall restriction...

All of these considerations will be discussed later in this series.

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To give you an idea of what's possible, bolting a full turbo-back aftermarket exhaust to a Nissan S15 200SX/Silvia can give up to 16 percent extra power (partly thanks to a simultaneous boost increase of up to 1.8 psi).

On the other hand, fitting a similar system to a MY01 Subaru Impreza WRX gives no extra top-end power. Down low there's a big improvement, but - without any engine management system mods - that's about it.

In Part Two of Out the Exhaust we'll look into muffler designs, cat converters and how to select/design a complete new system...

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