This article was first published in 2007.
It doesn’t suit those people who like black and
white answers, but many aspects of car modification simply aren’t able to be
dogmatically categorised in either the affirmative or negative. Let’s take a
look at some of these questions with plenty of shades of grey in the answer.
A cam will always improve power but knock
Simply not so!
Firstly, a modified cam (or pair or quartet) of
cams will only improve power if the rest of the engine can breathe sufficiently
well that the original cam specs were a major limitation on power. So it’s more
likely that a cam will ‘work’ if the intake and exhaust are free-flowing, and
the valve sizes in the head are sufficient that longer duration and/or more lift
promotes better gas flow.
To put this another way, in terms of mechanical
mods, changes to cams should be amongst the last on the list to be done - along
with a compression ratio increase. So a cam is likely to improve power if done
as the last step in a sequence of modifications. If the modification is done
early in the sequence, even a good grind can result in only a trivial increase
And will a high performance cam always ruin fuel
economy? Nope! It depends very much on the cam specs and the engine management
modifications done in conjunction with the cam change. For example, a cam that
develops better than standard bottom-end torque (not very common but possible)
can be tuned to run leaner cruise mixtures than a cam that gives away lots of
torque at the bottom end. Furthermore, if the cam gives more efficient
breathing, pumping losses will be reduced and so fuel economy can improve.
However, overall, you’re much more likely to
experience a decrease in fuel economy with a modified cam than you are to get a
Stiff anti-roll bars will always improve
As their name suggests, anti-roll bars resist body
roll. They do this by trying to lift the inner wheel, in turn dragging the body
back to being closer to horizontal. Ignoring the very important aspect of
modified anti-roll bars whereby the understeer/oversteer balance can be tuned,
stiff anti-roll bars will almost always make the driver feel as if
the car is handling better.
The car will turn-in more quickly, react to
throttle and steering inputs more rapidly, and (obviously!) have less body roll.
But a car with overly stiff sway bars will also
often have less outright cornering speed than one with softer ‘bars, and –
critically – in the wet will be much more skatey. The latter is an extension of
the ideas that a car stiff in body roll will respond very quickly to driver
inputs – in wet conditions, that response may be so fast that consequent
instability occurs. So, yank on steering lock and the front tyres turn so hard
that the car slides into understeer; lift-off fast and the back slides into
oversteer – and so on. The response will not be as soft – and sometimes you want
a soft response.
But, having said all that – there are those shades
of grey again. In most cases, road cars are set up so soft in roll that bigger
anti-roll bars will make an almost universal improvement to handling.
It’s worth chasing everything for
Some people will insist that their exhaust uses
only mandrel bends. They’ll replace the standard filter with a high-flow oiled
cotton design; they’ll change the spark plugs to high performance units and run
And they’ll rightly be able to show dyno
improvements for each of these steps.
Trouble is, there are those shades of grey again.
Most modifications of the type described here make very little difference to
total power. So, if you’re racing in a formula that allows you to change these
aspects but prohibits others, they might be worthwhile following. But the 1, 2,
3 or 5 per cent gains (total, not cumulative!) that come from things like plugs
and oils are costs that would be better put towards buying a high-flow cat
converter (the cat is the biggest restriction in pretty well any high
performance exhaust – it’s not the bends!) or simply replacing the factory
airfilter more often.
Rather than chasing everything, pick out those
aspects that are worst – and change those first. But again, you can’t say that
mandrel bends or quality oils or aftermarket filters are no good – that’s much
Plastic aftermarket suspension bushes are a
This is one of the classics – because in some
application plastic (eg polyurethane) suspension bushes can make a major
difference and in other cases they can show little benefit – and even worse,
wear out very quickly.
It’s fair to say that plastic bushes should not be
used in cars where they are subjected to large angular rotations under high
loads. That’s because a plastic bush has a crush-tube within it that actually
rotates. (In comparison, the rotation of a rubber bush causes the rubber to
torsionally deflect internally – there are no sliding surfaces.) So the
crush-tube rotates within the bearing that is formed by the plastic bush –
that’s why you need to apply lubricant when they’re installed. But, of course,
there’s no way of keeping that bearing greased over the long term...
Plastic bushes can squeak and groan, they can be
chewed-out within a year, and they can make only an imperceptible difference to
handling. Or they can revolutionise the precision and poise of a car and last
for a decade.
The shades of grey inherent in this question are
best answered by a mechanic who sees a lot of your model of car – he or she may
not be able to comment authoritatively on the handling benefit but at least
they’ll be able to tell you about durability. Some will want to bend your ear
about them for quite a few minutes – such is their dislike of the bushes...
An off the shelf chip will make my car go
This is one I have been writing about for as long
as I have been writing about modified cars.
Many off the shelf modified chips are quite poor
in performance gain for the money you spend. However, if you have a turbo car
and the chip tuns up the boost, an off the shelf chip can give a good
performance increase at a price that is quite reasonable.
But a chip for a naturally aspirated car that
makes major claims regarding power – but you don’t need any mechanical mods and
you can run your car on the same octane fuel – should be regarded as suspect.
Very bloody suspect. But – and here are those shades of grey again! – in some
cars you can get a good gain in just the situation described above. For
example, the car may have been massively detuned to suit local fuel – and they
went too far. (Or, didn’t go far enough and it’s always running into ignition
retard caused by knock sensor activity.)
The best engine management retune approach will
always be a custom tune on a dyno in real time but if that’s not viable, get a
money-back deal on the chip – if you’re unhappy or the chip doesn’t match its
percentage power gains on a dyno, you get your money returned.
This is been just a short series of examples of
something that applies to nearly every area of car modification. Very few things
are unambiguously good – or unambiguously bad. The trick is to realise that
there will always be good aspects and bad aspects and the decision is in
weighing those up.
If you’re not in a position to make those
judgements for yourself, ask your mechanic: what are the downsides of these
changes, or what are the trade-offs? (Everyone will be only too happy to tell
you the good aspects!) Any workshop that claims there are no negatives simply
doesn’t know what they are talking about or are liars. Good workshops will tell
you both sides of the story – and in doing so will move away from black and
white and into those shades of grey...