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The Low-Cost High-Flow Exhaust - Part One

Designing a low-cost high-flow exhaust.

By Michael Knowling

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At a glance...

  • First of two-part series
  • Exhaust backpressure measurement
  • Designing a low-cost high-flow exhaust
  • A close-up look at pipe diameter, cat converters, mufflers and dump pipes
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This article was first published in 2005.

Fitting a high-flow exhaust to a moderately powerful car can be an expensive exercise. Large diameter mandrel bends, exotic cat converters, elaborate dump pipes, numerous polished mufflers and many hours of labour can add up to a bill of AUD$1000 - and sometimes considerably more.

But when we decided to fit a high-flow exhaust to our Nissan 180SX we did it for a total cost well under AUD$500. And does the new system perform? You bloody betcha!

In the first of this two-part series, we’ll take you through the process of designing a low-cost high-flow exhaust...

Check the Existing Backpressure

The first step before modification should be to measure the backpressure of the existing exhaust system. That’s a point worth repeating. The first step before modification should be to measure the backpressure of the existing exhaust system.

So why is this so important, you ask?

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Well, if you don’t have any idea how restrictive the factory exhaust is, it’s impossible to size the new system so that you don’t unnecessarily spend cash chasing gains that aren’t there. Let’s face it – if the factory exhaust isn’t that restrictive, it’s not worth spending a large wad of cash for what can only be a small gain.

Don’t be scared off by the idea of measuring backpressure. It’s e-a-s-y.

The most convenient way to measure backpressure is to temporarily remove the oxygen sensor near the beginning of the exhaust system. The sensor can be unscrewed from the exhaust and left to dangle in the engine bay. Now you’ll need to find a bolt with the same thread as the oxygen sensor. Drill a small diameter hole through this bolt (from the head through the bottom) and weld a short length of metal pipe to the head. If you don’t have any welding equipment, any exhaust shop should be able to do this for you in a couple of minutes.

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This is what your backpressure measurement fitting should look like. The threaded end screws into the provision for the oxygen sensor and the pipe end connects to a hose leading to a positive pressure gauge (any gauge that goes up to 15 psi should be fine).

In the case of our 1.8 litre turbo 180SX, the backpressure measurement fitting was installed immediately downstream of the turbine so that total exhaust system backpressure was being measured. At full power (around 6000 rpm in second gear) our testing showed a peak exhaust backpressure of 9.3 psi.

Standard Backpressure Measurements

Let’s put your exhaust backpressure measurement into context against some other cars.

The highest backpressure measurement we’ve seen came from a bog-stock Holden VL Turbo that we used to own. Peak backpressure (measured immediately behind the turbine) was an astonishing 13.2 psi. We can’t be sure if the exhaust on our particular car had a blocked muffler but, if it is representative of other examples, we can only suggest upgrading your VL Turbo exhaust NOW! See Pure Pipe Perfection - Part 1 for our VL Turbo exhaust upgrade article.

In comparison, a 1994 Subaru Impreza WRX (fitted with a more powerful Japanese-spec engine) saw up to 8.8 psi exhaust backpressure. Fitment of a high-flow exhaust then achieved a power increase of ten percent. Details of our WRX exhaust upgrade can be found at Rex Blows

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Meanwhile, the standard exhaust on a 2.4 litre Nissan Pintara causes a relatively low 5.9 psi backpressure – though this was measured before the cat converter, so it’s not representative of total exhaust backpressure. Total exhaust backpressure (after the exhaust manifold) would likely be in the vicinity of 7 – 7.5 psi. Our Pintara exhaust upgrade can be found Lung Transplant

These figures should give you a guide to how good or bad your car’s existing exhaust system is. Be honest. Do you really need to spend lots on an upgrade?

Designing the New Exhaust

Pipe Diameter and Bends

Now that we know the backpressure caused by the existing exhaust system we can make some informed decisions.

Let’s start off by determining the appropriate pipe diameter for the new exhaust system.

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Based on some previous AutoSpeed backpressure measurements, we can make some generalisations on what pipe diameter is required to give next-to-zero backpressure at a specific engine output (assuming similar engine efficiency and thus airflow).

Testing on the 2.4 litre Pintara showed that a length of straight 2 inch pipe causes barely any backpressure on an engine making around 100kW. A 2 ¼ inch pipe can be considered plenty big enough for this power output. Testing on the VL Turbo and ’94 WRX showed that a straight length of 3 inch pipe is ample for engines making up to around 200kW. On both of these vehicles, fitment of a full-length 3 inch system (including cat converter, muffler and bends) caused just 2.9 psi backpressure.

Ahhh, but what happens to our pipe diameter hypothesis when pipe bends are thrown into the equation?

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Well, not that much changes if you use mandrel bends that maintain the pipe diameter. Some exhaust shops have an on-site mandrel bending machine which means the entire system can be bent from a single length of pipe. This is an ideal situation. But, more than likely, your exhaust shop will use a number of pre-formed 30, 45 and 90 degree mandrel bends which are welded together to form the system. This isn’t quite as good because some of the welding will penetrate the pipe and cause turbulence.

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There’s nothing wrong with using cheaper press bent pipe – so long as you have designed the system to take them into account. In situations where a tight radius bend must be made, it’s advisable to use the next larger diameter pipe for that section. For example, you should step up from 2 to 2 ¼ inch pipe where the system takes some sharp turns. This approach will achieve similar gas flow to a mandrel bend that’s the same diameter as the rest of the system. Note that gentle radius press bends offer very nearly the same flow as mandrel bends – it’s only the tight radius bends (which involve heavy pipe crushing) where press bent pipes cause a considerable flow restriction..

So let’s apply some of this info to a couple of real-world examples.

If you are lightly tweaking your Suzuki Swift GTi (which makes 74kW in standard form) you can use the above information to see there’s no need for anything bigger than a 2 inch mandrel system or a 2 inch system with 2 ¼ inch press bends. A 2 ½ inch system is overkill and 3 inch (which we have seen used!) is the stuff of madness.

Now let’s look at our demo Nissan 180SX.

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In standard form, the 1.8 litre turbo 180SX generates 130kW. If we were leaving the car near-stock we would probably use a 2 ½ inch mandrel exhaust but, since we plan to take it to around 180kW, we will use a pipe diameter approximately the same as a 3 inch pipe.

Now that we know the diameter of our pipework we can source the appropriate cat converter, resonator and muffler.

Sourcing a Cat Converter

The cat converter is generally the most restrictive component of an exhaust system. It’s for this reason you should make a special effort to buy the freest-flowing cat converter available.

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Flowbench information is tremendously important in cat converter selection but, given this type of information is limited, you’ll usually need to rely on other criteria. Buy the cat converter with the largest diameter inlet and outlet pipes, the largest core cross-section and the widest spaced internal honeycomb.

Unfortunately, these large diameter high-flow cat converters are quite expensive - you’ll often pay several hundred dollars for a newie. But there’s no reason why we can’t make use of near-new OE cat converters that are commonly thrown away.

And, yes, there are some very good OE cats to be found.

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One of the best cat converters on a bang-for-buck basis is from the Holden 3.8-litre V6/5-litre V8. Independent flowbench testing has shown the Holden AC cat offers gas flow similar to a poor performing 3 inch cat. You can easily pick up one of these cats in good condition from AUD$10 – and they aren’t hard to find...

But, for our 180SX, we wanted a cat that offers really good flow.

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The part chosen for the job is a cat converter from a Ford BA Falcon XR6 Turbo. The XR6T cat (which has a single 3 inch inlet pipe and twin 2 ¼ inch outlets) is factory fitted to an engine making 240kW and it is widely acknowledged that you need a very special cat converter to improve flow on the Falcon Turbo. The standard XR6T cat is regularly used in power-up applications close to 300kW and is more than free-flowing enough for our Nissan. We paid AUD$100 for this particular example.

Using the XR6T Cat

The twin outlet of the XR6 Turbo cat converter makes exhaust design more complex than using a single outlet cat.

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If there’s enough space beneath the car, you can run twin pipes from the cat all the way to the tips. Keep in mind that a twin 2 inch system has almost the same cross-sectional area as a single 3 inch pipe, and a twin 2 ¼ inch system is considerably bigger.

Unfortunately, the available selection of twin inlet/twin outlet mufflers is relatively limited and you may be forced to merge the twin pipes into a single larger diameter pipe before the muffler. This is what we’ve decided to do.

The cheapest and most readily available 2 into 1 pipe junction can be found in various late-model Holden Commodores. The Commodore pipe junction uses twin 2 inch inlets and a single 2 ¼ inch outlet.

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Unfortunately, the inside of the Holden 2 into 1 junction is also pretty daggy. As seen here, the leading edge of the single outlet pipe protrudes into the gas flow and is likely to cause considerable restriction. It might be tempting, but the Holden junction is not the most attractive way to merge a pair of pipes.

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A better option is to use a 2 into 1 collector from a set of extractors or an aftermarket Y-pipe (which is designed for the front section of a V8 exhaust). This Redback Y-pipe gave us a nice 2 into 1 junction that’s designed to accept twin 2 ¼ inch pipes and a single 2 ½ inch pipe (which is easily flared to 3 inch).

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This photo shows the 2 into 1 Redback junction after the outlet pipe has been flared to 3 inch. As you can see, its internal surfaces are very smooth – a stark contrast to the ugly Holden junction.

Sourcing Mufflers/Resonators

This is where personal preference comes into play.

If you don’t mind a relatively loud exhaust you can get away with just a single rear muffler. This keeps cost, exhaust backpressure and weight to a minimum. But if you want a system that’s quiet you will need to add more mufflers/resonators – and you’ll need to choose them wisely.

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If you want to keep noise to a minimum, you should pick a rear muffler that makes full use of the space available beneath the car. A large-body muffler typically contains more sound absorption material than a smaller muffler.

In addition, it’s important to pick a straight-through type muffler because they cause only a 10 percent flow loss compared to an equivalent straight length of pipe (as tested on a flow bench). In comparison, reverse-flow type mufflers can cause up to 40 percent flow loss... See Giant Muffler Comparison - The Mufflers! for further details on muffler types and how they compare in terms of flow.

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We chose a second-hand 3 inch straight-through Trust muffler which uses all of the space beneath the 180SX. However, after previously purchasing a second-hand muffler which turned out to be stuffed, we gave this muffler a very close inspection. We thoroughly examined the internal perforations, banged it on the ground to see if anything except carbon came out and checked that the case was in good condition.

Fortunately, this muffler looked in good condition.

OE Mufflers?

In many instances it can be cost-effective to use a second-hand OE muffler.

Most OE mufflers are built with internal baffles to reduce noise. These baffle-type mufflers are quite restrictive compared to a straight-through muffler but if you can find one that has a larger than necessary pipe diameter you can get away with it. For instance, if you’re designing a 2 inch exhaust you can realistically use a 2 ¼ inch baffled muffler from, say, a Holden Commodore or Ford Falcon. These mufflers generally have good sound absorption and – when you pick one that’s suitably upsized – they can perform very well.

For articles that discuss using OE mufflers in a performance application see Lung Transplant The Cheapest Exhaust You've Ever Heard Of, Part 1 and Edward the Elephant's New Exhaust - Part 1

When it comes to centre resonators, you’re pretty safe with whatever you buy – just make sure you’ve got the suitable pipe diameter and it’s a straight-through design.

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While new aftermarket resonators are available quite cheaply, a second-hand OE resonator can be picked up for next to nothing. For maximum noise suppression, we suggest using the largest bodied resonator available (for the same reason we go for the biggest possible muffler).

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Late-model Subaru WRXs, Mitsubishi Lancers, Honda Accords and Nissan Pintaras all have nice straight-through resonators that range from 1 ¾ to 2 ¼ inch diameter. We used a pair of 2 inch stainless steel resonators from a Toyota Soarer V8.

Another Noise Suppression Trick

Interestingly, it seems that using a relatively small tailpipe or rear-most section of exhaust is an effective method of further reducing noise.

A tapering exhaust diameter is common in many production cars as well as the highly developed APS (Air Power Systems) aftermarket exhausts. For example, the APS ‘210kW’ kit for late-model WRXs uses a 3 inch pipe for the majority of the exhaust but the rear section tapers to just under 2 ½ inch. This particular exhaust is claimed to reduce backpressure from 9.6 to 3.5 psi and we can vouch that it has a very refined and hushed note.

It’s an idea that’s worth investigating if you want the quietest exhaust possible.

Sourcing a Turbo Dump Pipe

If you own a turbo car, it’s likely that the factory dump pipe (the front section that connects to the turbine) should be replaced as part of your system upgrade.

Aftermarket dump pipes can be extremely expensive to buy new. However, most performance exhaust shops (such as Exhaust Technology in Adelaide) usually have some second-hand examples lying around.

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A twin-pipe dump (such as seen here) comprises a large diameter pipe adjacent to the turbine wheel and a smaller pipe adjacent the wastegate bypass. The idea of a twin-pipe dump is that turbine and wastegate gases are kept separate and, as a result, there’s reduced turbulence and backpressure.

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A twin-pipe dump wasn’t available to suit our CA18DET-powered 180SX so we used a second-hand single entry ‘big mouth’ dump pipe with a 3 inch pipe. As you can see, this dump pipe has a large diameter bend which is shaped to match the shape of the turbine flange. The single pipe dump mightn’t have the theoretical advantage of a twin-pipe dump – but it’s a vast improvement over the cast iron factory part (which measures just 2 ¼ inch diameter).

Stick around - in the second, and final, part of this series we’ll install our low-cost high-flow exhaust and assess the results!


Exhaust Technology +61 8 8272 7500

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