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Lung Transplant

Using OE cats, resonators and mufflers to create a high-flow, quiet and affordable exhaust upgrade for your medium power four-cylinder!

By Michael Knowling

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This article was first published in 2003.

Most atmo four-cylinder cars are fitted with a fairly pathetic exhaust system - typically anything from 1 ¼ to 2.0-inch in diameter, heavily muffled and, of course, modern cars are further stifled by a cat converter or two. All this seems pretty glum, but there is a major upshot for people who want to get creative in their approach to power enhancement; pretty well any more powerful production car caries donor cat converters, resonators and mufflers that can handle the exhaust gas flow of smaller, less powerful engines with a minimum of restriction. You see, exhaust bits that flow acceptably on, say, a 150kW six-cylinder will perform great on an engine with perhaps only half the power output (and, therefore, only around half the exhaust gas flow).

The idea of using cats, resonators and mufflers from other standard vehicles is a dead-set turn-off for many car enthusiasts (it's no good for establishing carpark cred) but, often, it can make a lot of sense financially and in terms of tailpipe noise - as you'll see.

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The exhaust build we'll be covering in this story was fitted to my fiancé's 2.4-litre Nissan Pintara Ti (U12 model). She called for a noticeable increase in performance and fuel economy yet it ABSOLUTELY had to remain quiet; and by that I mean virtually standard. Oh, and - as per usual - the upgrade had to cost the smallest amount of money possible!

Where Is The Most Room For Improvement?

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The standard Pintara exhaust begins with a 4>2 cast iron manifold leading into twin steel secondary pipes that are about 30cm long. At the end of the secondaries, a collector merges exhaust gasses into a single 2-inch pipe; the Pintara's exhaust is 2-inch press bent all the way to the tailpipe. A braided flex joint is fitted about 50cm downstream of the collector, followed by a single 2-inch cat converter and a fairly small 2-inch straight-through resonator.

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Just prior to the rear axle line, the pipe makes a 90-degree turn toward the driver's side of the vehicle, a 90-degree turn back toward the rear and another couple of tight bends over the IRS and into the rear muffler. The rear muffler - the primary muffling device - is internally baffled and is quite large at 45 x 31 x 15cm.

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With fittings inserted at four points along the standard Pintara exhaust system, we hit the road to conduct some backpressure testing. What we found was the total amount of backpressure - from only the cat converter to tailpipe - wasn't too bad compared to the figures we've seen from other vehicles; a total of 40 kPa. This table shows the restriction that occurs through each section of the exhaust.

Rear muffler/tailpipe 14 kPa
Section of pipe between resonator and rear muffler 12 kPa
Centre resonator 2 kPa
Cat converter 12 kPa
Total cat-back backpressure (including cat) 40 kPa

Note. We had already decided not to focus on the standard Pintara exhaust system from the cat converter forward. Changes to the front section of pipe would be a waste of time if we later decide to fit extractors which, we're told, terminate at a slightly different location to the factory exhaust manifold. Obviously, going only cat-back also saves money compared to a full-length system.

Planning the New System

These backpressure measurements revealed some key points that influenced the design of our new exhaust.

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As expected, the factory cat converter and baffled rear muffler were quite restrictive (each posed about a third of the total cat-back restriction), but we were surprised that the length of pipe between the centre resonator and rear muffler was equally as restrictive. From this, we abandoned any thoughts of upgrading just the cat converter and muffler. Furthermore, the labour change involved in swapping a cat and muffler was almost as much as replacing the whole caboodle from the cat-back.

So - having decided on a cat-back system - we set off in search of a suitable OE cat, resonator and muffler upgrade...

Which Mufflers, Resonators and Cats Flow the Best?

Selecting the least restrictive cat converter is fairly easy - you want the one with the largest core cross-section and the fattest inlet and outlet pipes. The density of the honeycomb may also vary across different manufacturers - the denser it is, generally, the higher the restriction.

Inlet and outlet pipe diameter is the deciding factor for selecting a straight-through resonator - the bigger the better from a flow point of view. Perforations punched through to the exhaust gas passage (rather than flush perforations) are also less than ideal.

Selecting an OE high-flow muffler is not so easy, however...

The biggest problem is, since long sections of bent pipe are usually still attached to the donor muffler, it's impossible for you to peer inside. All else being equal, though - again - you want the one with the largest diameter inlet and outlet pipes.

It's also a good idea to take into account the power output of the engine that a given muffler is designed to suit - go for a muffler from an engine with at least 30-40 percent more power than your car's engine.

As mentioned in previous stories, a straight-through type muffler offers maximum gas flow. Note, though, straight-throughs are never used as the primary muffler on a production car; primary mufflers are internally baffled with perforated tubes interconnecting a series of chambers. Certainly, these OE baffled mufflers are very effective at silencing but, unfortunately, don't flow nearly as well as a straight-though muffler; this shouldn't be such too much of a concern for you, however, should you install a baffled muffler with larger inlet and outlet pipes than the rest of your system.

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After scouring the wreckers armed with measurements of what would fit beneath the U12 Pintara undercarriage, one of the most attractive upgrade cat converters that popped up belongs to nothing more exotic than the VL-onward Holden Commodore. The Holden Commodore cat converter (the same for all six-cylinder and V8 models) has a 2 ¼-inch OD pipe size but, more importantly, a relatively large core cross-section; the area of its 2 ¼-inch pipe size is 15 percent greater than the Pintara's cat, while its core area is around 30 percent greater.

We would've preferred a larger-again 2 ½-inch OE cat converter, but these proved very difficult to find and relatively expensive. We favoured the Holden cat over other 2 ¼-inch units because they are very common, cheap (anywhere from $10 to $45 for one in top condition from a wrecker) and, using 2-bolt flanges at each end, they are easily replaceable.

The size of the cat converter is relatively inconsequential to the car's final noise output, but - in order to keep tailpipe noise to a near-factory level - you must pick your mufflers and resonators very carefully.

After some stuffing around (which we'll come to) we've come to the conclusion that, where possible, it's a good idea to pinch the cat, resonator and muffler(s) from one particular car that's more powerful than yours. As soon as you start mixing mufflers and resonators from assorted vehicles the chance of ending up with a relatively loud system increases dramatically.

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In the case of the Pintara, though, there wasn't enough space for a large body muffler anywhere except under the boot. This meant that V6 and V8 Commodore exhaust systems were not suitable since they use (as we later learnt!) a large under-floor baffled muffler as well as a medium size baffled muffler at the rear. The Pintara simply didn't have space for both.

Facing this, we went looking for vehicles that rely on a very large primary rear muffler for silencing - cars such as the WRX, 200SX and R33 GTS25T (factory rated at 155-160, 147 and 186kW respectively). We soon dropped the idea of using any exhaust parts from the 200SX since its pipe diameter is smaller than that on the ol' 96kW Pintara, but the GC8 WRX and R33 systems looked promising...

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The WRX muffler features a 2 3/8-inch inlet pipe, while the R33 unit's is slightly larger at 2 ½-inch; certainly, both units look to offer considerably better flow than the standard Pintara 2-inch muffler. Extrapolating figures from previous flow tests it seems that either of these baffled mufflers should flow about as well as a straight-through 2-inch muffler - and that's ideal in this case since, as you may recall, the restriction through our existing 2-inch straight-through centre resonator was barely measurable.

And what about noise suppression? Well, as we said in our "Rex Blows" article, the body volume of a muffler is a very important consideration in relation to silencing performance. In this case, the standard Pintara rear muffler body measures 20,925 cubic centimetres, the GC8 WRX muffler comes in at 19,320 cubic centimetres and the R33 GTS25T muffler is a whopping 25,850 cubic centimetres.

Obviously - based on its body volume and pipe size advantage - our preference was the R33 GTS25T muffler; it was also better suited to the Pintara's floorpan.

And then there was the issue of a centre resonator...

Since the Pintara, WRX, 200SX and R33 GTS25T use only a small centre resonator we thought we'd fit the new exhaust with just its cat and big rear muffler and - if necessary - add the resonator later. The fewer components we needed for the job the lower the final cost. If a resonator was required, we were considering those from the TR and TE-onward Mitsubishi Magna V6s; these have 54mm and 60mm OD pipes respectively.

How We Learnt the Hard Way...

Before the R33 muffler entered the picture we equipped our Pintara with a 2 ¼-inch baffled rear muffler from a VP V6 Commodore. At that stage, you see, we were under the (false) impression that the V6 Commodore had only a small centre resonator and relied on the baffled rear muffler for 'silencing power'... The result was a disaster. At idle and at light cruise the car was reasonably quiet but, as soon as power was applied, the exhaust R-O-A-R-E-D.

So - aside from thinking it would keep the car quiet - what was it that made us chose the Commodore V6's 2 ¼-inch rear muffler?

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Well, we'd revisited the article "Giant Muffler Comparison - The Mufflers!" where we had flow tested a bunch of aftermarket 2-inch mufflers as well as - luckily enough - a standard Pintara muffler and V6 Commodore rear muffler. The test showed that the 2-inch Pintara muffler has appalling flow (188 cfm at 28-inches of water) whereas the 2 ¼-inch Commodore unit offers a handy 50 percent more flow (281 cfm at 28-inches of water).

In comparison, the best aftermarket 2-inch straight-through and reverse flow jobs managed around 360 cfm and 250 cfm respectively.

So the Holden rear muffler flowed reasonably well - it just didn't do enough to keep things quiet.

Pipework and Fitment

The first thing to sort out before the installation of our new exhaust was the diameter of the mild steel pipe we'd be using - would it be 2, 2 ¼ or 2 ½-inch, or would it be a combination?

Well, given we'd measured barely any restriction through the standard 2-inch centre resonator, we figured the next pipe size up - 2 ¼-inch - would be ample for the straight section between the cat and the first bend. A larger 2 ½-inch pipe would be simply overkill and more likely to be boomy.

When it came to the torturous pipe route near the back of the vehicle, however, we opted for the freer flowing 2 ½-inch pipe. Note that we discounted using 2 ¼-inch mandrel bends because 2 ½-inch press bends flow at least as well and are much cheaper.

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With our desired exhaust specs sorted out, Chris - from Chris Hardwood Custom Exhaust and Mechanical Repairs - got to work ripping out the old Pintara system. He commented that the only way to remove the standard middle/rear section is to cut the pipe - there are no flange plates.

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Up front, fitment of the Holden 2 ¼-inch cat involved some sneaky work on the original flange plate. Chris simply slotted the flange's bolt holes to match the slightly wider spaced holes of the Holden cat. Note that the Holden cat is exactly the same design as the original Pintara part - it is simply upscaled.

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Flowing on from a new flange on the back of the Holden cat, Chris welded a straight length of 2 ¼-inch pipe making sure there was enough space left to - if necessary - add a centre resonator. At the end of the 2 ¼-inch straight section, the pipe diameter flares slightly to 2 ½-inch in preparation for the jostle through the rear pipe route.

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Chris used the factory rear section of exhaust as a template for press bending the new 2 ½-inch rear pipe; note that the new pipe is one piece from the 2 ¼ to 2 ½-inch transition all the way to a new 2-bolt flange located just before the rear muffler. A flange plate makes things much easier if we ever have to dismantle the exhaust later down the track.

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With the big Nissan R33 Skyline GTS25T muffler picked for the job, Chris cut off its existing inlet and outlet pipes and fitted it to the end of the new system using a short length of bent pipe off the back of the rear flange plate.

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At 40 x 34 x 19cm the R33 muffler was a relatively tight fit and, while fabricating the new heavy-duty metal hangers, Chris commented that it is quite weighty. He laughs, "I've replaced the exhaust on a R33 GTS25T with a 3-inch straight-through system, and I reckon half the performance gain came from the reduced weight..."

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To make things a little more difficult for us, the R33 muffler's outlet pipes are spaced a fair distance apart and cannot be accommodated by the Pintara's relatively narrow bumper cut-out - not without looking a bit strange, anyhow... The solution was to cut the outlet pipes off, flare the ends that had been cut and re-weld them in an offset position closer to the centreline of the muffler. This made things look a bit more attractive; the finished look is very 'factory sports'.


First let's quantify the reduced backpressure and improved performance - this was, after all, the main reason for the exercise.

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With our trusty pressure gauge tapped in just upstream of the new cat-back system, we took to the street again and were very pleased to see peak exhaust restriction was now 10 kPa - just a quarter of the restriction of the standard system!

And did this equate to better acceleration? You bet. Throttle response is improved and there's a noticeable torque gain that increases as revs rise - the top-end is now very much smoother. Note that we are not saying torque is lost at the bottom-end; the gain is simply more focussed at higher rpm.

In stopwatch timed 0 - 100 km/h sprints the car showed a strong improvement - a best of 9.3-seconds versus 9.7 in standard form. Of course, there's no reason for a HSV to quiver at the traffic lights but we are talking about a good gain nonetheless. Fuel economy, too, has been slightly improved but this is difficult to quantify at this stage; we keep 'trying out' the performance gain...

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Maintaining a factory tailpipe noise level was very important and - after our stuff up with the Holden muffler - we are extremely happy with the end result. We couldn't have hoped for better. At idle, tailpipe noise was just 61dB(a) while free-revs up to 4000 went to 75dB(a) - only 2 and 3dB(a) louder than with the standard exhaust! Out in traffic and on the freeway the exhaust is also resonance-free, which means there's no need to add a centre resonator.

And the cost factor? Well, again, we're pretty proud that we've managed to keep a lid on the total price. Our low-kilometre second-hand Holden cat cost $45 from a wrecker, the R33 GTS25T rear muffler set us back only $20 (most performance exhaust shops have no real value for them) and Chris Harwood's pipework cost just $150; total cost is just $215!

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So - in a nutshell - we've used a readily available second-hand cat and muffler to quarter the Pintara's cat-back exhaust restriction while simultaneously maintaining a virtually factory noise output a-n-d the final cost is just over 200 bucks. If you can create a cheaper high-flow exhaust system that maintains factory noise levels and emissions compliance, please let us know!


We've recently found out that 1999/2000 Australian-delivered Subaru Impreza WRXs are fitted with a centre section of exhaust that could be put to good use by the owners of Pintaras and similar vehicles. The centre section incorporates a 2 ¼-inch cat (which serves as a secondary cat on the MY99/00 WRX) and small bodied 2 ¼-inch straight-through resonator, which might come in handy in some instances. These centre sections are available second-hand through Exhaust Technology in Adelaide for around $35 - a bargain!


Chris Harwood Custom Exhaust and Mechanical Repairs
+61 8 8391 0954

Exhaust Technology
+61 8 8272 7500

All Panels and Parts (Holden and Ford wrecking specialists)
+61 8 384 8418

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