The Low-Cost High-Flow Exhaust - Part Two

This article was first published in 2005.

In the first part of this series (see The Low-Cost High-Flow Exhaust - Part One) we looked at designing a cost-effective high-flow exhaust suitable for moderately powerful cars. Well, now it’s time to put our design into practice – come and see the exhaust fabrication, on-road results and the cost rundown!

Turbo Dump Pipe

The dump pipe we chose for our Nissan 180SX is a second-hand stainless steel job with a single ‘big mouth’ entry leading into a 3 inch pipe. This particular dump pipe was fabricated by the same company doing our exhaust installation – Exhaust Technology in Adelaide .

Prior to installation, the mounting flange of our new dump pipe was ground back to remove some minor surface corrosion (this smoothing helps achieve an effective seal against the turbine flange). Oxygen sensor safe sealant was also used to ensure there are no exhaust leaks.

A two-bolt flange can be found at the base of our second-hand dump pipe. This connects to a newly fabricated 3 inch pipe that extends rearward to the area of the cat converter. The newly fabricated pipe comprises a single 90 degree mandrel bend.

Cat Converter

The cat converter we’re using was pulled from a Ford XR6 Turbo. The XR6T cat is relatively unusual in having a single 3 inch inlet and twin 2 ¼ inch outlet pipes.

For fitment to our 180SX, the factory inlet and outlet pipes were cut from the cat converter. This allows the fitter to fabricate new inlet and outlet pipes which are correctly angled to suit the car.

The XR6T cat is mounted in the virtually same position as the original part and is welded to the 3 inch pipe from the dump pipe.

Dual Pipes and Resonators

Our 180SX exhaust uses twin 2 inch pipes from the cat converter leading into twin resonators before merging into a single larger diameter pipe. Twin 2 inch pipes might seem relatively small, but they do offer similar cross-sectional area to a single 3 inch pipe.

From the back of the cat converter, a pair of 2 inch pipes were bent into the required shape using a press bend machine. Care was taken to ensure the cross-sectional area of the pipes was maintained as much as possible. These twin 2 inch pipes terminate at a pair of centrally mounted resonators. 

The resonators we selected are pulled from a Toyota Soarer V8. These are nicely built straight-through stainless resonators with 2 inch inlet and outlet fittings. This photo shows the resonators being cut from the original Soarer pipework.

The 180SX doesn’t offer a lot of under-car space so the twin resonators are mounted closely alongside each other and close to the tailshaft tunnel (to maintain ground clearance). It’s a squeezie fit.

Merge Pipe

Unfortunately, we were unable to extend twin pipes all the way to the end of the system; we couldn’t find a cheap twin inlet muffler that makes use of the available space. Instead, we decided to merge the twin pipes into a single 3 inch pipe (which makes sourcing a suitable muffler much easier).

A search through Exhaust Technology’s used stock pile uncovered this - a Redback high-performance Y-pipe, which we believe is intended for a V8 Holden. The Y-pipe has twin 2 ¼ inch mandrel bent pipes merging into a single 2 ½ inch outlet.

The Redback Y-pipe proved extremely useful in our application. It provided us with a smooth 2 into 1 pipe junction as well as some nice mandrel bends, which we couldn’t resist reworking to suit the 180SX. This photo shows the reworked Redback mandrel bent pipes welded to the back of the resonators and the 2 into 1 junction. Note that the single outlet of the junction was flared from 2 ½ to 3 inch to suit our chosen muffler.

Rear Muffler

The muffler we selected for the 180SX is a second-hand 3 inch Trust unit with straight-through internals. This muffler makes full use of the space available beneath the car and was checked to be in good condition.

As seen here, this particular muffler also came with a mandrel bent entry pipe and mounting flange already attached. These make installation quicker and easier compared to a bare muffler.

The muffler was hoisted into position beneath the car and a 3 inch pipe was fabricated to join it to the outlet of the 2 into 1 junction. A single 3 inch mandrel bend was used for this section.

And that’s it.

Bung on some hangers, whack on a coat of paint and the exhaust is finished.

The Completed Exhaust

This photo shows the main section of our newly constructed 180SX exhaust. With a fresh coat of black paint it doesn’t look much like an aftermarket creation - if anything, it resembles a factory BMW M3 exhaust...

So let’s recap the components that we used.

Starting at the front, you’ll find a second-hand ‘big mouth’ stainless dump pipe, an ex-XR6 Turbo cat converter, a pair of resonators from a V8 Soarer, a merge pipe constructed from a used Redback Y-pipe and a second-hand 3 inch Trust muffler. The only new parts used in the system is a pair of 3 inch mandrel bends, some press bent sections of 2 inch pipe, a couple of flanges and some new hangers.

A Look at the Standard Exhaust Our testing in the first part of this series revealed the factory 180SX exhaust was causing 9.3 psi backpressure. So, out of curiosity, let’s take a look at what causes this restriction... Starting at the front, the standard cast iron dump pipe has a lot of internal lumps and bumps (which cause turbulence) and a relatively small 2 ¼ inch outlet.  The first section of exhaust after the dump pipe connects to the cat converter. This pipe is just over 2 inches in diameter and flares into the 2 ¼ inch cat converter. Interestingly, the cat converter is mounted a long way downstream of the turbine – completely different to, say, a Subaru WRX with its cat converter mounted very close to the turbine. From the cat converter, the next section of pipe is just over 2 inches in diameter and leads into a centre resonator. The entry and exit pipes from the resonator step down to exactly 2 inches. The final section of pipe takes gasses to the rear muffler and is just over 2 inches in diameter. Again, like the resonator, the muffler entry reduces to just 2 inches. At the rear there are twin muffler outlets measuring approximately 1 ½ inches. Overall, the 180SX system is very similar to the S15 200SX exhaust – the biggest flow impediments are its small pipe diameter, heavy press bends, a baffle-type rear muffler and a cat converter with a relatively small core cross-section. In short, there’s plenty of room for improvement – as indicated by our initial backpressure measurement.

Before and After Results

So what improvement came from our custom 180SX exhaust?

Well, in standard form, we measured 9.3 psi exhaust backpressure at full power and the car (with an automatic transmission) managed to accelerate from 0 -100 km/h in around 8.4 seconds. Tailpipe noise was 60db(A) at idle and around 65db(A) when free-revved to 4000 rpm. These tailpipe noise figures were taken with the SPL meter held at 45 degrees from the tailpipe at a distance of 50cm.

Now let’s look at the new exhaust’s credentials.

With the custom exhaust bolted to the back of the turbo we saw a maximum backpressure of just 2.5 psi – a reduction of 73 percent! Not surprisingly, the car now has much improved response and torque in normal driving conditions and is tremendously quicker at wide-open throttle. This is reflected in a monumental 0 – 100 km/h improvement – this time has tumbled from around 8.4 to 7.6 seconds!

Note that this incredible performance gain was achieved through a combination of reduced backpressure and a resulting increase in boost pressure. Turbo boost pressure now builds faster, reaches a higher peak and holds a higher value to the reline. Peak boost has increased from 10 psi to 12.5 psi and at the redline (7000 rpm) there’s 7 psi instead of 6 psi as previously. Certainly, this across-the-board boost increase is a major factor in the recorded performance improvement.

However, tailpipe noise has increased. The system is quieter than the majority of turbo-back performance exhausts but is still obviously an aftermarket development. Fortunately, there are no annoying resonances (which can be a problem in auto trans cars) and the note is nice and deep. In terms of SPL, we recorded 63dB(A) at the tailpipe with the engine idling and 88dB(A) when revved to 4000 rpm. In caparison, a full-length 3 inch exhaust we fitted to a Subaru WRX achieved the same noise level at idle but was considerably louder when revved.

And you may be wondering if the new ‘split pipe’ exhaust is heavier than the standard system.

Interestingly, it isn’t. The factory exhaust (from dump pipe to rear muffler) tips the scales at 24.5kg and the new system manages to undercut it by 1kg. We were expecting the new system to be slightly heavier due to its bigger cat converter and more intricate pipe arrangement. However, this isn’t enough to (literally) outweigh the factory cast iron dump pipe and extensive heat shielding.

Cost

So how much did this low-cost high-flow exhaust set us back?

Well, we shelled out AUD$480 in total – AUD$100 for the cat converter and AUD$380 for the rest of the components and labour. However, we would have saved about AUD$80 if we had sourced the XR6T cat converter from Exhaust Technology – they typically have a couple lying around which can be picked up for much less than we paid.

We should again point out that the second-hand XR6T cat converter is brilliant from a cost and flow point of view – but its twin outlet pipes requires a relatively complex (and costly) pipe system. A full-length twin pipe arrangement is relatively time consuming to build and can be very difficult to fit in cars with limited undercarriage space. Keep this in mind if you decide to follow in our footsteps.

Our 180SX’s exhaust has achieved a 73 percent backpressure reduction with a tremendous on-road performance improvement, a mild noise increase and offers the scope to generate around 180kW while maintaining very low backpressure (and, realistically, it would be fine for applications around 200kW).

At a price of AUD$400 to AUD$480, we reckon this system comfortably lives up to our “low-cost high-flow exhaust” label. Sure beats spending AUD$1000+...

Contact: Exhaust Technology +61 8 8272 7500