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Pure Pipe Perfection - Part 1

More than an 80 per cent reduction in exhaust backpressure, effective noise suppression - and an all-up cost of $630!

By Michael Knowling

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We've seen plenty of magazine articles over the years on the fitting of exhausts. Brand name mufflers, heaps of hours of custom work, loads of polish - and lots and lots and lots of money. This story is different. The reduction in exhaust backpressure is measured; the performance gains on the road are quantified; and the dollars needed to complete the system (including a hi-flo cat!) are relatively tiny. And it gets even better than that. This is Part 1 of a series - in Part 2 we'll introduce a revolutionary new technology that gives the best combination of low idle exhaust noise and superb high performance flow ever encountered. And yes, that's sure a big claim. But sit back this week as we show you how to do an exhaust that's simply unlike everyone else's.

What We Want...

But, to start us off, what are the basics to aim for?

  • Mandrel Bends

In case you don't already know, a mandrel bent pipe is just that - a pipe that's been bent with a mandrel slipped inside it to maintain its (near) full diameter. The biggest benefit is that - unlike press bends - the gas flow through the bend is maintained as much as possible. Notice, though, that not all mandrel bent systems are equally as slick. The best systems can be made when your exhaust fitter has their own in-house mandrel bender - which enables them to make your system in one piece (where possible). Much more common, however, is the practice of welding short sections of pre-formed mandrel bends together wherever a bend is required. This isn't quite as good as having an on-site mandrel bender coz the connecting welds inevitably seep through to the inside of the pipe and cause their own restriction and turbulence. A talented welder and the use of a grinder inside the pipe will help to minimise this in-tube roughness.

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  • Fat Pipe Diameter

Bigger is best - unarguably so on a turbocharged car. Post turbine, these boosted engines need to breath as much as possible and - therefore - the largest diameter tube will be able to carry the gasses with the minimum of restriction. Three inch is presently the largest commonly available diameter you can get - though 3 and 4 inch are now starting to make their debut. There is some argument that a smaller pipe diameter (and therefore more back-pressure) is required for optimal performance on a naturally aspirated engine. This theory, however, is unproven where the fuel and ignition has been optimised to suit the pipe, and where the large pipe is used after the tuned length part of the system.

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  • High Flow Dump Pipe

The dump pipe off the back of a turbo is a great place to pick up some extra horsepower. Gasses exiting the turbine need to have swift flow outward - otherwise, turbulence and flow restriction will rob you of precious power. One method of maintaining this gas flow is to have one pipe for the turbine exit and another one for the wastegate (whether it be an internal or external wastegate). This is sometimes referred to as a "screamer", coz the wastegate pipe is often vented straight to atmosphere - and makes a helluva lotta noise! The other - more common - alternative is to have a smoothly contoured, large diameter dump pipe matched to the flange pattern at the back of the turbo. These work very well also. Note that the factory (usually cast iron) dump pipe off most turbocars is quite restrictive and can be easily improved upon.

  • High Flow Catalytic Converter

An aftermarket high-flow cat converter (on a car running unleaded fuel) really does give minimal restriction. High-flow cats are an often under-rated device - especially considering how beneficial they are. Beneficial, you ask? Yes sir. A cat converter serves to reduce noise, help the environment, get the legal-eagles off your back and - these days - is relatively cheap. In case you're sceptical about their gas flow, bear in mind that a 3-inch cat can out-pace many aftermarket 2-inch mufflers. In other words, a high-flow cat converter's trade-off to outright flow is not usually worth worrying about (not on a streetcar, anyway).

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  • Straight-Through Muffler Design

Muffler design can make or break a good exhaust system. Without a doubt, the best muffler design - in terms of both flow and noise suppression - is a straight-through type. If you don't believe me, just take a look at the AutoSpeed muffler comparison at "Giant Muffler Comparison - Detailed Test Results" Reverse flow, reverb, dogleg mufflers - none of them flow as well as a straight-through. Don't slip-up on this one!

Hungry for an Improvement?

Before running out and whacking a new system onto my Commodore VL turbo (which is already equipped with a high-flow air intake), I first wanted to see how badly the car needed a big exhaust. Was investing a large bundle of cash in a new system gonna be beneficial, or was I just squirting it up against a wall?

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The only way to tell is by measuring the engine's displeasure, in the form of exhaust back-pressure. Back-pressure can be measured with any accurate boost pressure gauge connected through the wall of your exhaust pipe. Under high engine load, pressure builds in the exhaust and you'll soon see how choked up the engine gets. In my VL turbo's case, I decided to take measurements at two different points in the factory system - one at the first bend in the exhaust pipe, and another right up behind the turbocharger (in the cast elbow). My aim here was to identify how bad the factory dump pipe/elbow was. The pressure fitting in the exhaust pipe was easy to tap (I had a short length of metal brake line welded flush into the side of the pipe), but the one behind the turbo needed a little more ingenuity. I opted to make up a fitting to screw into the location of the factory EGO sensor. This was constructed from a threaded bolt with a hole drilled through the middle and a short length of pipe welded onto the end. Rubber hose was used to connect each fitting to the in-cabin pressure gauge.

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Now, I had already suspected that the standard (2 inch) Holden AC cat converter was collapsing - and this was confirmed when I saw how far the needle on the pressure gauge swung around. On the road - at full power - there was an 80 kPa (11.6 psi) build-up in the exhaust bend and a full 90 kPa (13.2 psi) at the back of the turbo. Ouch - that's a helluva lot! Note that the closer to the engine you take your measurement, the higher the cumulative back-pressure reading will be. Naturally.

Curious to see how much of that monumental amount of back-pressure was caused by the obviously clogged cat, I dropped the sucker from the car and took to its inners with nothing less subtle than a crow bar. After a couple of minutes of battering, the entire inside of the cat converter was removed and its hollow shell then re-fitted to the car. Back on the road (with the boost pressure gauge fitted to the first exhaust bend) the exhaust back-pressure was still through the roof. Try 70 kpa (10.3 psi) at full noise... Indeed, a noticeable improvement in response and torque was felt with the collapsed cat guts removed - but, regardless, this level of back-pressure was still way too high for any high performance application.

Placing an Order...

After reading the opening section of this story, you'll no doubt be up to speed with what's good in a high performance exhaust system. But the problem is, they are a very expensive taste... When we rang up some of the high-profile exhaust shops, we were amazed at their ridiculous prices. As high as $1300! Yep, thirteen hundred bucks for a lousy bit of pipe?!

Now - as ever - you can really save yourself a bundle if you ring around and collect quotes. After trying probably five or six different exhaust shops, I finally tracked down someone who couldn't believe some of the prices I was hearing. His name was Chris Harwood - of Chris Harwood Custom Exhaust and Mechanical Repairs. Note that Chris's quote to do my job (with the cat supplied by me) was only $410 - less than one-third the price of some other rip-offs! He also seemed genuinely interested in taking on the task and had previously done some other impressive custom work.

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From the outset, Chris knew that he had to install what I wanted. First of all, the pipe diameter had to be 3-inch - and there were a few reasons for this:

There was little point in upgrading from the factory 2 diameter to only a little larger.

There was a heap of room available to fit a 3 incher.

Going whole-hog to a 3 inch also gives the scope for more power further down the track.

Chris advised that a pre-formed 3-inch mandrel section of pipe was also readily available off-the-shelf to go over the back axle of the Commodore. This was a welcome suggestion, as this would minimise restriction (as there would be no welding between short lengths of mandrel bends) and keep labour time (not to mention cost!) down. Note that the use of mild steel tube was fine by me, coz these big sucker pipes last a lot longer than smaller diameter ones. To me, the much more expensive stainless steel option seems unnecessary.

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My cat converter of choice was a 3-inch Exhaust Technology item - proven to flow quite well, relatively cheap (at around $220) and now tried-and-tested on countless vehicles. Then (with this cat converter to be mounted up front), I only wanted a single muffler - and, of course, it had to be of a straight-through design. Note that using just a single muffler also helps to keep a lid on costs. Initially, I was thinking about having this muffler box located prior to the pipe going up and over the axle - but this soon changed when I saw the daggy internals of the necessary centre-offset muffler that was in our budget. The junctions where the two end nipples butted up against the internal (diagonal) perforated tube had terrible protrusions. No thanks Chris - I'll take a hot-dog style centre-centre straight-though muffler mounted at the tailpipe! At least, that way, the coppers can plainly see that the car is muffled.

Note that not all centre-offset mufflers will have the same flow deficiency as mentioned. More expensive units have a curved pipe leading into the perforated diagonal section - but we were trying to keep cost down, remember?!

Unfortunately - in the VL-T's case - part of the turbo wastegate bypass is integrated into the top of the cast dump pipe. This would make constructing a suitable new dump to match the turbo difficult (and expensive). Realistically, it simply wasn't worth it - not with the factory dump pipe causing only 11% of the overall restriction.

Then - after doing a little research - I later learnt that there's a restrictive lip hiding inside the factory elbow. I lowered the front section of pipe for a close look, and - sure enough - there was a raised lip about 10mm back from the 3-bolt exhaust flange. This ugly stuck out about 5mm all the way around and was probably responsible for much of the overall restriction across the elbow. Removing this restriction would surely be the next best thing to having an all-new dump pipe off the turbo. Being in an awkward position, though, I figured it was easiest for Chris to grind away this lip when he had the car up on the hoist.

Chowing Down...

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Starting at around 9 in the morning, Chris ripped out the standard Holden system and chucked it all in the bin. Take a look at these close-up shots - it's no wonder the VL-T's exhaust restriction was so high! Rippled pipes, bad press bends and crappy mufflers. Get the hell outa here you lot!

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Prior to fabricating the new 3-inch exhaust system, I asked Chris to take to that raised lip on the inside of the cast elbow with his air grinder. Note that a thick rag was stuffed further up the cast pipe to prevent flying debris from entering the turbine. This job took only about 5-10 minutes and resulted in a baby-bum smooth inner surface.

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Chris could now lay his hands on some fat new 3-inch pipe. At the front end of the system, the VL needed to have a couple of mandrel bends cut up and tack welded in-line. Wherever possible, Chris ground back the inside of these pipes to minimise in-tube roughness. About a metre back from the beginning flange, the ET 3-inch cat was located under the factory heat shield - high enough so that ground clearance won't be a problem.

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After doing a bit more of a dart around with some cut up mandrel bends (the underside of the Commodore simply does not allow for a straight length of pipe) the tube then arrived at the back axle. Here, Chris welded into place his pre-formed mandrel section to make this otherwise time-consuming area a breeze.

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Often - when you're getting a custom system fitted - this part of the exhaust takes a while to get right, and several sections of mandrel bends are needed. In other words, the installer has to focus a lot of time and expensive mandrel bends in this area. And guess who pays for it... But with the use of the pre-made section of exhaust, that time, cost and flow restriction were vastly reduced! Should be more of it. Notice that Chris paid particular attention to pipe clearance to the Panhard rod.

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The next - and final - section of the system lay beneath the boot floor. This is where I elected to stick the one-and-only muffler - a straight-through, centre-centre Redback measuring 130mm round x 360mm long (part number N726GP). Now, while Redback might not have the same ring to it as A'PEXi, you simply cannot go wrong with a muffler like this. Not for an incredible cost price of $63.95 (through Chris)! Chris fitted the muffler to the vehicle, paying particular attention to its direction of flow. Note that I didn't fit a chrome tip, coz I didn't want to attract too much unwanted attention to the size of the exhaust system. Especially not from those cars with flashing blue lights on the top!

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Once the system had been routed and securely tack-welded together, Chris removed the whole caboodle and welded 360 degrees around each pipe join - again, careful not to let the weld penetrate into the pipe too much. With the pipe completed, it was then mounted under the Holden's body using the factory exhaust hangers. These were simply heated up and re-bent to suit the 3-inch pipe diameter. Using the factory hangers helps to ensure there's no pipe movement (which can otherwise cause the pipe to knock against the floor).

At around 1 o'clock that afternoon, Chris was washing his hands and the job was done. Notice that this speedy installation (largely due to the pre-formed over-axle pipe) was a key factor in determining his low fee. I've seen custom systems take more than an entire day at some exhaust shops. And, again, it's the customer who has to pay for that time.

Ahh, the taste...

After choosing the right vital indigents, the VL's new exhaust has made it a different car - and for a grand total of only $630 (including the $220 cat). Throttle response is vastly improved, boost builds much quicker (accidental wheel spin off the line is now very common!) and top-end urge is markedly up. And peak exhaust back-pressure? Well, our initial reading at the front of the system (just behind the turbo) has fallen from a huge 90 kPa to a mere 20 kPa - a stonking 82% reduction in back-pressure! And restriction across the factory cast dump pipe was not even measurable! Of course, this reduction in back-pressure has also yielded a significant increase in 0-100 km/h performance, with times falling from 7.6 to 7.1 seconds (measured during 30-odd degree C ambient temperatures).

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Now, you'd expect a minimally-muffled 3-inch system like this to be loud - or, at least, a little on the boomy side. Not so; the VL system is much quieter than we had expected. The combination of the 3-inch ET cat and the rear-mounted Redback muffler have worked brilliantly - but, no doubt, we'll still be able to make the system significantly quieter. How? With our never-tested-before secret weapon, which we covertly slipped into the system just before the pipe went over the rear axle.

Secret weapon, eh?

You'll have to wait till next issue to check that one out!


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

Exhaust Technology
+61 8 8272 7500

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