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Not Black and White

You just can't be categorical

by Julian Edgar

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At a glance...

  • Cams and power and economy
  • Stiff anti-roll bars
  • Filters and oils and plugs for more power
  • Plastic suspension bushes
  • Off the shelf engine management chips
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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 economy...

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.

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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 in power.

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 gain.

Stiff anti-roll bars will always improve handling....

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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 power...

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 special oil.

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 too definite...

Plastic aftermarket suspension bushes are a good thing...

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...

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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 hard...

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...

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