This article was first published in 2005.
Do you think: ‘WTF?’ when you look at the price of aftermarket exhaust
components like mufflers, cat converters and exhaust tips? After all, there’re
mostly just bent bits of metal... Well, if you’re running a modified car
developing under about 150kW (200hp), take a good look at this story. In it
you’ll see how we sourced a stainless steel braided flex joint, resonator, cat
converter and stainless muffler for a total cost of AUD$135.
The bits are just crap are they, then? Nope – in fact, they’re of a quality
superior to many aftermarket goods. So they must flow really badly? Again – no.
All right then, it was a ‘mate rates’ job – someone just about gave you the
bits? Wrong: they were all bought at normal commercial rates.
In this case we were assembling the exhaust components for a low powered
modified car, but with the exception of the rear muffler, they’d also be good
for power outputs up to the 150kW mentioned above. (And we saw plenty of
mufflers at the same price that would be happy at this higher power figure.)
So what’s the secret? Simple: take advantage of the fact that people love
ripping off near-new exhausts and replacing them. If you’re running a modified
car that has less power than those other cars had as standard, you’ll be
laughing all the way to the bank.
While many think that a modified car with less than 1000kW is for losers,
that just ain’t so. In fact, as I said at
a lightweight, good handling car with a sweet spread
of torque is fantastic on a winding road – even if the kilowatt number is
relatively small. And in modification, aiming for an all-round fun package
without concentrating on just power also gives you a huge advantage – you can
make use of the bits and pieces that everyone else thinks are valueless.
Like those from exhausts.
It simply doesn’t make sense to assume that a 3-inch muffler is the minimum
needed for performance. Or that all 2-inch resonators are suitable only for the
tip. What you need to do is to look at the flow requirements versus cost and
performance. So for example, here in Australia, a Falcon XR6 Turbo cat converter
is suitable for power outputs up to about 250kW – and so with two of them, for
power outputs up to 500kW! On a saner level, cat converters off the local Holden
V8s will be happy flowing well over 200kW, and even base models of the local
late model six cylinder cars have exhaust systems designed for over 150kW.
In short, unless you’re looking right at the top end of the power spectrum,
using single or dual versions of these systems will provide plenty of flow,
plenty of sound suppression - and all at a low cost.
And there’s another advantage: many of these components are now being
produced in stainless steel, so they will probably never need replacing!
In this particular application, an exhaust was wanted that would be 2-inch off the
turbo, running through at least a
2-inch cat converter (typically, the cat is the most restrictive part of a
free-flowing system, so going larger than nominal pipe diameter is better), a
2-inch resonator and then a rear muffler which would be either 2 inch or a
little smaller. (Why would smaller be OK? Almost all factory cars reduce their
exhaust diameters as you travel towards the back of the car. Whether that’s
because the cooling exhaust gases take up less volume – or some other theory –
the bottom line is that the factory engineers don’t have a problem with this
approach, and it doesn’t harm power. Some aftermarket turbo kit suppliers also
The first step was to very carefully measure the available room under the
car. This step cannot be over-stated – you simply must know how much room you have to
play with. For example, in the car we were dealing with, the underfloor volume for the cat converter
was strictly limited in width, being less than 160mm – which is damn’ narrow! On
the other hand, the resonator could be as wide as 160mm and as along as 500mm –
A flex joint was needed because the engine is mounted transversely – as
torque loads are applied, the engine twists on its mounts, which causes the
exhaust pipe to be bent. Without a flex joint, the pipe will end up breaking,
usually at the exhaust flange. Many factory cars use a type of ball-joint with a
special gasket, but it’s easier in the aftermarket to use a stainless steel flex
joint – and plenty of cars use these ex-factory as well.
The next step was to visit a wrecking yard – one of those that allows you to
wander at will. Armed with calipers, a tape measure and a piece of paper with
the max possible dimensions for the muffler, cat and resonator on it, I spent a
solid two hours walking, measuring and comparing. So what did I find?
Plenty of front-wheel drive cars have stainless steel flex (those that don’t
use the ball-joint described above) but very few are larger in internal diameter
than 2-inch. That size was fine in my application, but in a common wrecking yard
you won’t find too many that are bigger. Note that the flex joints are almost
all welded into place – so to get the flex you’ll need to buy that complete
section of exhaust.
Common in the wrecking yard where I was looking were Magna flex joints – the
front section of the V6 exhaust has multiple, short braided joints. However, I
ended up selecting a longer flex joint which was also on a simpler-shaped piece
of exhaust (and so was likely to be cheaper!). It was from a Daewoo Lanos.
When looking for a cat, initially I was disappointed – in a field of perhaps
500 cars, there didn’t seem to be more than two or three cats still on the cars.
Then I twigged: the cats contain precious metals and so are removed for
recycling. When I finally found the recycling bin, I had a choice of many cats.
As mentioned, the required cat needed to be relatively long and thin - but with
such a wide selection of cats to pick from, it wasn’t hard to find the ideal
candidate. Of course, I cannot tell how many kilometres it has done, but an
internal and external visual inspection showed no apparent problems.
It’s easy to think that a resonator isn’t needed – and in some exhaust
systems, that’s the case. However, if you dislike droning resonances (which can
be particular problem in auto trans cars, especially those with continuously
variable transmissions), a resonator is good insurance. A straight-through
resonator without punched louvres projecting into the pipe will have near zero
flow restriction, weigh little and if buying from a wrecking yard, cost nearly
nothing. So I wanted a resonator!
The chosen one was long and thin, and like plenty of other factory
resonators, the internal tube was not perforated along the full internal length.
In other words, this resonator acts as an expansion chamber, with the smaller number of
holes slowing the rate at which the gas can expand into the cavity. I selected a
long, thin design. It was a floating orphan in the yard, so I am not sure what
model car it came from.
Hmmm, No Rear Muffler
Finding an appropriate rear muffler proved to be a real challenge. The trick
is to fit as large a canister as possible – sure, a muffler much smaller than
standard will also fit in the space, but the chances of excess noise are much
higher than if you use the largest one that you can squeeze under the car. (This
is an oft-overlooked fact.) In this case, the original muffler was fairly small,
and so nearly all 2-inch mufflers were too large to fit. In fact, despite
looking under every car in the wrecking yard, I couldn’t find one.
So at this stage I carted to the front counter of the wrecker the bits of
pipe containing the flex joint, cat converter and resonator. All three
components came with attached sections of exhaust tube, some with flanges and
some with rubber mounts. The total cost? AUD$75.
More Looking for Mufflers
I then went to the other excellent source of cheap secondhand exhaust bits -
a muffler shop. Note that some muffler shops dislike selling secondhand mufflers
– despite making money on what they’ve been given free of charge, they often
seem reluctant to admit to having any good mufflers that they’ve taken off cars!
However, the muffler shop that I went to had perhaps 40 secondhand mufflers – at
I selected a near-new stainless steel muffler off a late model Corolla –
this was ideal as it was exactly the same external size as the muffler it was
replacing, except instead of using the 1.3 inch tube of the original, it uses
1.7 inch tube. (So while it’s not 2-inch, it’s still 70 per cent bigger in
cross-sectional area than the original tiny muffler pipe size.) And at 100kW, the power output of the
Corolla is about double what the modified car will be developing - so the flow
capacity should be fine. (At the muffler shop I also saw at the same price big
mufflers off an HSV V8 Maloo ute – these mufflers would be fine up to
So far, the total price for all the exhaust system components was AUD$135.
Add AUD$5 for a new gasket to suit a flanged connection, and AUD$6 for two nuts
suitable for mounting the oxygen sensors, and the total reached AUD$146.
Despite some of the bits of exhaust bought from the wreckers having flanges,
there wasn’t a matching pair. (Memo to self: next time, make sure you measure
the flanges you’re getting at the wreckers!). Rather than buy a second flange, I
had one cut out by a welder wielding an oxy acetylene cutting torch.
However, this may be false economy in both time and money: by the time I had
ground and filed the flange edges to precisely the right shape, it would have
been easier to buy a new flange (and
much cheaper to have got one at the wrecking yard!).
Next week: we put the exhaust
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