AS SIMPLE and humble as your exhaust system seems, it can be an easy thing to get wrong. There are many important factors at play that will determine its performance and longevity, from selecting the correct header design, primary and collector sizes, system diameter, muffler choice and type of steel, to the style of bends, joins, welding techniques and coatings.
This article was first published in the May 2011 issue of Street Machine
A quality exhaust can unlock additional horsepower from an existing engine combination and vastly improve the way your streeter sounds. Upgrading your exhaust system may also be necessary when engine modifications make the existing system restrictive.
All the pieces necessary for assembling a performance exhaust of any size are widely available at exhaust shops all over the country, and online, so grab your mandrel bends, headers and mufflers and follow along as we watch Darryl and Tark from Liverpool Exhaust fabricate a trick system for a customer's sweet-looking Brougham that's had a healthier engine installed.
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1a. After removing the old extractors, the heads must be cleaned. Use a paint scraper to clear old gasket material or sealant - make sure no harmful gunk fulls into the exhaust ports - before wiping down with Prepsol.
1b. You can see the difference between the Tri-Y and the new Hurricane four-into-one, painted in blue heat-proof. Larger in every way, the Hurricanes feature 1 3/4 in primary tubes and 3in collectors.
2a. Fitting headers from below eases navigation around obstacles. With 12 years of experience, Darryl has seen just about every type of obstruction and clearance issue; older vehicles with steering boxes close to the firewall give the most headaches.
2b. Here the obstacle was the starter motor - the motor was rotated out of the way and the mounting block gently trimmed to provide sufficient clearance.
3. Liverpool Exhaust prefers carbon-graphite gaskets with a steel middle layer. These single-use gaskets require no sealant and bond to both surfaces once exhaust heat is applied. Lack of liquid sealant is important for EFI as the oxygen sensors can be damaged by sealant falling into the exhaust. Check for clearance; ideally you want 10-15mm for things bolted to the engine and a bit more to the body/chassis.
4. Starting from the middle and working outwards, Darryl gradually tightens the 3/8in header bolts, spreading load onto the gasket evenly. Tighten the bolts only as much as is possible with a spanner. "Excessive tightening is not necessary," Darryl says. Over-tightening can result in a snapped bolt, which is a pain. You also need to be particularly careful with alloy heads as you run the risk of stripping the soft metal.
5. With the headers inched up and clearance issues resolved, it's time to put together the second section. Liverpool Exhaust fabricates systems in three main parts - the headers, the middle with the X-pipe and mufflers, and the tail pipes - all joined by flanges and gaskets that allow for easy removal of either rear section if you want to dump at the diff or transmission. The first step is to bolt flanges to the collectors.
6a. As the Brougham was being fitted with a twin 2 1/2in mild steel system, the first pieces of tubing after the headers had to be flared to match the three-inch collectors. To flare the 2 1/2in pipe, Darryl uses a hydraulic spreader.
6b. The aim is to get as tight as fit as possible while still allowing the tube to slide inside the flange. It's MIG-welded once flared correctly. This type of fitment provides a better seal and better flow.
7. The X-pipe is set in the preferred location and the rest of the centre section is built around it. The location of the X-pipe has more to do with convenience and available real estate than performance. It has to be mounted forward far enough for the mufflers to be installed before the diff, while allowing as gentle a path for the exhaust flow as possible. Darryl only track-welds each piece into place at this stage.
8. Muffler choice is vital to the final note and volume of the exhaust system - get it wrong and you'll be disappointed and possibly defected. The Brougham runs a pair of free-flowing S-bend Hurricane mufflers that have enough of a curve in the flow path to reduce noise without impeding performance. With mufflers like these, you can have your cake and eat it - good power without being insanely noisy.
9. The tubing is solid approximately 15mm into the inlet of the mufflers to prevent exhaust leaks and disruptions to flow. Once Darryl is satisfied they're positioned symmetrically and in line, he joins them to the rest of the system. At this stage each join in the second section is welded as far as possible on the car before being moved to the work bench for completion. This prevents the section from warping.
10a. To fasten the exhaust, Darryl bent up a pair of metal hangers (A), which slot into HQ mounts (B) already welded to the Brougham's floor, though he says mounting to the chassis is preferable. The mounts feature rubber blocks that absorb vibrations and allow some give.
10b. Darryl welds the hangers to the folded sections of the muffler housing or to the outlet pipe as these are much stronger than the main body.
11. Tailpipe construction begins with flanges welded to the rear of the mufflers and another pair bolted to these using V-nuts that won't shake loose. This section can be tricky to construct as it must run up and over the diff while maintaining maximum clearance for moving suspension parts as well as fuel and brake lines. Even so, Darryl uses gradual mandrel bends pieced together that won't impede gas flow.
12. The quest for symmetry was one of Darryl's aims throughout the installation but by the time he got to the last piece it became vital. A pair of crooked exhaust tips poking out the back of a car can ruin the whole look so he grabbed an extra pair of hands to tack-weld the tube while he held it exactly in position. Once both sides are finished, they will have round-bar hooks welded to them for hanging the tailpipes.
13. Eyeballing the positions of the tips isn't enough for some people; if you demand perfection, grab the tape measure and take your time. When it comes to exhaust tips, you could spend days selecting a design but there are really aren't any performance gains or restrictions between them. More often, the choice comes down to what suits the style of the car, and some owners simply leave them off all together.
14a. You can see the pipes weaving up and over the diff, retaining maximum clearance. While removed for the final welding, Liverpool Exhaust coated each section in heatproof paint that protects the mild steel from corrosion.
14b. There are also carbon graphite gaskets between the modular sections. The system sounds throaty, without droning or excessive barking - just what you want in a tough street exhaust.
CUT 'N' PASTE
X-PIPES are the smoother-flowing version of the old H-pipe. They help balance the exhaust gas pulses coming from the engine and free up a few additional horses. Vehicles not fitted with an X-pipe tend to sound more like speed boats at higher revs, which isn't so cool. Like the rest of the system, the X-pipe is fabricated from mandrel bends. Mandrel bends as they maintain their internal diameter, resulting in minimal restriction and maximum power. Darryl starts the X-pipe by eyeballing the centre of each bend and tracing the outline (above).
Darryl then slices it out with an angle grinder.
Once the edges are de-burred and the mating surfaces are as close to flat as possible.
The two pieces are then welded together.
WHILE it's true a stainless system would have a longer life, there's really no performance gain from using stainless rather than mild steel, which when painted will last a very long time. Some believe stainless can sound a little more tinny, while the highly polished finishes available on some expensive stainless systems tend to dull off within hours, according to Tark.
It took an experienced professional a full day to complete the job - with all the right tools and materials at his fingertips. However, building an exhaust is a job that can be attempted at home, especially if you opt for off-the-shelf tailpipes and extractors. You can buy the mufflers along with a box of mandrel bends, cut them in the right places and weld them up.
"Don't rush things," Tark advises. "Take your time and do it right."
The Brougham's system cost $2600 and Liverpool Exhaust considered it to be pretty much an entry-level set-up. From this base, common upgrades include TIG-welding (whether in stainless or mild steel) and ceramic coating, which is an increasingly popular option that Tark says can reduce radiated heat from the header pipes by up to 60 per cent. The coating also offers a modest performance gain.
CONTACT Liverpool Exhaust via www.liverpoolexhaust.com.au or on (02) 9602 3111 if you're after a killer exhaust system for your street or race car.