There are some cold hard facts to face up to when you modify a car.
When you modify a car in any way
the probability of being pulled over and issued with a defect notice shoots
through the roof. Equally disturbing is rate at which insurance companies wiggle
out of making good a claim when your vehicle has been tinkered with. Oh, yeah,
and try having your car repaired under warranty when you’ve got a bunch of
go-fast bits bolted on...
Yep, you really are up against it when you decide to modify your ride.
But don’t take your bat and ball and run away just yet!
The chance of being stung by a defect, insurance or a warranty issues depends
heavily on your approach to vehicle
In many instances, there’s no reason why you can’t enjoy the same increased
power output – but without anyone knowing about it. That's not to say that you should set out to deceive - there could be severe legal consequences. But if no-one asks what's been done to your car, well...
Don’t Attract Unwanted
Vehicle modification is largely influenced by fashion and, at the moment,
it’s fashionable to have plenty of ‘bling’ – the more shiny stuff, the
Bling is all you need to attract attention while cruising, but all too often
it attracts the people you want to breeze straight past – the police.
It’s a simple fact - the more overtly modified your vehicle, the higher the
probability of being pulled over.
A list of defect-magnets include... Polished canon mufflers, blow-off valves
and front-mount intercoolers, a too-low ride height, illegal wheel size/offset
and dangerously mounted interior gauges.
And don’t forget the basics – it’s easy to overlook stuff like inoperative
lights and bald tyres!
A Subtle Approach to Mods
To avoid defects and other car-mod related headaches, we suggest you
integrate vehicle mods so they’re barely perceptible to an observer.
Strive to keep your car looking standard – after all, not many people get
pulled over in a stocker (unless they are doing something silly!)
So how do you integrate mods, you ask?
Well, let’s start with the preliminary go-fast bolt-ons – the exhaust and air
Stealth Exhaust Upgrades
First, your upgrade exhaust should not be loud enough to draw attention and
it must meet tailpipe noise regulations. This is essential. Straight-through
mufflers and resonators (which cause almost no flow restriction) should be used
extensively. Another alternative is to use a variable back-pressure valve (see Butterfly Effect.)
The pipe diameter of your upgrade exhaust shouldn’t be excessive. A 4 inch
exhaust is overkill for a mildly modified Subaru WRX – all it does is add noise
and advertise your mods. Note that the layout of the exhaust is also important –
it should follow the factory path to help go unnoticed.
When the bonnet is opened, it’s also wise to have any aftermarket-looking
headers or dump pipes covered with heat shields. If the standard heat shield
won’t fit with the new exhaust, you may need to fabricate a custom shield. This
photo shows a standard dump pipe heat shield being modified to fit an upgrade
Finally, you should resist all temptation to go for a polished canon muffler.
Sure, go for a high-quality looking muffler - but not something that offers
hardly any noise suppression and stands out like you-know-whats.
Remember – you don’t want to be
Our Stealth Pintara Exhaust
When we upgraded the exhaust on our Nissan Pintara Ti 2.4, the main criterion
was to keep noise virtually standard and not to draw any attention.
Here is how we did it...
After assessing various OE mufflers, we selected a rear muffler from a R33
Nissan Skyline Turbo. This muffler is similarly sized to the standard Pintara
muffler and, once its dual outlets were relocated to fit neatly inside the
standard bumper cut-out, you’d never pick anything had been done to the system. It looks
like a factory ‘sports’ exhaust.
And did we achieve any significant performance gains?
We sure did. By fitting an OE Holden cat, new 2 ½ inch pipework and the R33
rear muffler, we slashed peak exhaust backpressure from 40 kPa to 10 kPa!
See Lung Transplant
for full details.
Stealth Air Intake Upgrades
In order to maintain a factory under-bonnet appearance, it’s inevitable that
you’ll need to use an airbox.
And that’s no bad thing.
In most vehicles, the factory airbox can be modified to accept a large
diameter feed pipe which replaces the factory snorkel. The feed pipe you make –
or buy – should be made from black plastic or be painted ‘stealth black’ so it
Once a large diameter feed pipe has been fitted, total airbox restriction
should be only a few inches of water pressure. In addition, you can replace the
factory air filter element with an aftermarket drop-in filter - but don’t expect
to pick up much power.
The induction path from the airbox to the throttle – or the turbo – should
then be examined on a case-by-case basis. Often, heavily convoluted sections of
tube can be replaced with smooth-wall tube which reduces intake turbulence.
Black rubber or plastic pipe can be used for this purpose. Note that the gains
achieved by doing this are often greater than you’ll achieve with an aftermarket
Vehicles equipped with a hotwire airflow meter can benefit from the removal
of the wire screen(s) that protect the sensing element. These screens cause a
considerable flow restriction and their removal is undetectable unless the
intake system is dismantled. Note that changes to mixtures and ignition timing
may occur when the screens are removed.
Oh, and let’s look at one of the less popular induction mods – a larger
A larger throttle body serves to provide greater response and torque at
part-throttle positions and causes less restriction at full power.
In many instances, a larger diameter throttle can be bolted on from other
vehicles – for example, the entire Nissan/Holden RB30 throttle body bolts on to
the Nissan E15 turbo engine. Where a factory upgrade is unavailable, your
existing throttle body can be bored out a few millimetres and fitted with a
larger diameter butterfly.
Nobody would have any idea.
Our Stealth VL Turbo Intake
The Holden VL Commodore Turbo air intake system responds very well to the
Fitting a 100mm feed pipe to the bottom of the airbox, replacing the pipe to
the turbocharger and removing the airflow meter screens dropped peak intake
restriction from 29.5 to 13 inches of water – a 56 percent reduction in restriction!
What’s more, these mods went completely unnoticed by everyone who ever looked
under the bonnet!
See Modifying the VL Turbo Intake
for full details.
Stealth Boost Upgrades
If you own a turbo car, there are many ways to increase boost pressure.
One of the least obtrusive is to insert a small restrictor (like seen here)
in the hose from the turbo compressor outlet. The pill serves to control the
amount of air bled by the factory boost control solenoid. This mod is completely
invisible even during a detailed inspection – your car will just have a few psi
extra boost for no apparent reason!
Alternatively, boost pressure can be increased in a late-model turbo car
using electronic methods. And this brings us on to our next topic...
Stealth Engine Management
The most difficult to find engine management upgrade is a re-write of the
factory ECU (ie a chip). A chip allows you to make changes to mixtures, timing,
boost pressure and various other parameters – and none of the changes can be
detected unless somebody accesses the ECU.
An interceptor module is generally easy to detect - any aftermarket labelled
computer and the splicing of wires is a dead give-away. Note that a plug-in
adapter loom can be used to install an interceptor to some popular vehicles –
this eliminates the need to cut any of the factory loom, bringing the mod one
step closer to stealth.
Aftermarket ECUs – which, if you read closely are suited for off-road use
only – are easily identifiable. Their aftermarket enclosure is the biggest
telltale. Most systems also require a MAP signal hose from the engine bay to the
computer - we are led to believe that such a hose is illegal.
Stealth Intercooler Upgrades
If you’ve got a polished front-mount intercooler that required hacking half the bumper away, you’re asking for trouble. Accept it.
A better approach is to conceal the upgrade intercooler core behind the
factory bodywork. Paint the core black so that it matches the car’s radiator and
air conditioning cores and keep it tucked in behind the bumper and you should be
Intercooler plumbing can also be made to look factory. All you need is
black-painted steel pipe and black rubber hose joiners. Make sure you keep the
pipe route neatly arranged.
This photo shows a very neat aftermarket turbo kit – note the black rubber
intercooler pipes, which look entirely factory.
Oh and, since we’re in the area of intercooler piping, we should mention
If you absolutely must have one, you’re best bet is to use a recirculating
valve (one that vents charge-air back to the turbo inlet). Note that
atmospheric-venting blow-off valves are illegal in most areas. The black
blow-off valves fitted to various Saabs and Japanese import engines are
relatively easy to integrate to look factory. A pair of OE blow-off valves can
be used in extreme applications.
But we must stress, if you’ve got the ‘pssht’ thing happening, you really are
asking for a defect.
Stealth Modifications at Their
One of our favourite aftermarket power-up kits is the APS ‘Phase’ upgrade for
the Ford XR6 Turbo. It’s worthwhile looking closely at these kits to appreciate
how nicely the mods are integrated...
The Phase 2 kit comprises a UniChip interceptor, free-flow exhaust, air
intake, bigger injectors and an additional fuel pump. These mods give the XR6T
280kW at the back wheels (about 100kW more than a stocker!) with barely a hint
of any aftermarket tinkering. The exhaust is easily within legal noise
requirements and the only other visible mod is a replacement snorkel into the
You’d never pick it.
The Phase 3 kit is even more impressive. We’ve seen a Phase 3 kitted XR6T
crank a massive 327kW at the wheels (that’s almost double factory power!) and it wouldn’t
get a second glance from the boys in blue. That 100 percent power gain is
evident only by the upgraded exhaust and intake snorkel from Phase 2 plus a
replacement wastegate actuator and front-mount intercooler kit.
The wastegate actuator upgrade could be identified by only the keenest-eyed
XR6T aficionado and – more impressively – the entire front-mount intercooler kit
looks entirely factory. The black painted bar-and-plate core is completely
stealth, as is the associated pipework. Even the hose clamps match the
You could just imagine how the conversation would go if the car ever did get
“Yes, officer, it’s all standard under the bonnet.
“What’s that? Well, um, that’s my friend’s 11 second time slip...”