I recently had to buy a petrol filler flap for a 2003 Opel Corsa bakkie. I was amazed to learn from the agents that this simple little flap costs R1154.

It comes painted in undercoat only, so the cost of spraying it to match the bakkie's colour has to be added. Is there any reason why it should be so expensive?

I declined, as politely as possible, to buy the flap and will be scouring the scrapyards for a replacement. - NE

No reason that I can think of, especially since the very similar petrol flap for one of the Corsa's main competitors (the Ford Bantam bakkie) of the same age sells for R125.

When I asked at a local GM dealer about the price of the Corsa's flap, the woman at the spares counter confirmed the price quoted to you, with barely concealed astonishment.

The most charitable explanation I can give is that it's just an error that has crept into GM's computer system. Perhaps GM can enlighten us. Good luck in the meantime with the scrapyards.

In last week's Clinic you mentioned a petrol engine with breakthrough technology by Mazda. Please tell us more. - Just Curious

The breakthrough by Mazda engineers came as they tried to find a way of overcoming the problems which have traditionally prevented the compression ratio of a petrol engine from being raised much beyond about 11:1. The efficiency of a petrol engine can be improved by raising its compression ratio, but if you carry this too far, you run into problems.

The problems stem from the fact that you cannot prevent a gas from heating up when it's being compressed. If the compression ratio is too high, the air/fuel gas mixture will get so hot that irregular combustion will result.

This is what causes "knocking". On modern fuel-injected engines, this is controlled by the electronic management system.

But this sacrifices some engine performance, and the result was that the compression ratio was still pegged at around 10:1. Mazda's engineers found they could limit the heat build-up during compression by:

•Using a carefully designed exhaust manifold to ensure that as little as possible of the hot exhaust gas re-enters the cylinder.

•Using the cooling effect of the petrol sprayed into the cylinders in direct petrol injection systems to best advantage.

•Giving the engine a relatively narrow bore so that the flame front quickly reaches the edge of the cylinder (before detonation).

This allowed them to raise the compression ratio to 13:1, or even 14:1, without adverse side-effects. Thus Mazda has chosen to follow a different route from the small-engine-plus-turbocharger path trodden by other manufacturers.

This has certain advantages, such as lower cost and no turbo lag.

Mazda are confident that their Skyactiv-G engines can achieve a 20% fuel saving over existing ones. This represents a major leap.