Does your car have the power you think it does?

Car buyers are faced with numerous facts and stats when choosing their next car. Many have to be taken with a pinch of salt.

Who ever obtains the fuel consumption figures some car brands boast of? Which drivers are able to reproduce the quoted 0-60mph time that was recorded by an expert race driver on a test track in perfect weather?

But there's one figure everyone seems to trust. Bhp or brake horsepower, which measures the amount of power your car's engine produces. Surely that figure is reliable? Or is it?

When I got together with a bunch of my mates with our wide selection of cars for a track day, I suggested we test their bhp first. For between 20 and 60 GBP, anyone can take their car to a 'rolling road' test facility - there are hundreds across the UK. Visit rollingroadregister.com for your nearest.

We took our cars to Shark Performance, Mansfield, who have the latest equipment. The test is simple: a technician drives your car up onto rollers, straps it down, then 'drives' the car on the spot while sensors measure everything. It's quite exciting to watch, as the engines noisily give their all and the graph slowly climbs. How many horses have bolted the stable since the car was built?

First was everyone's favourite budget sportscar, the Mazda MX-5. On paper, it should have 115bhp, yet when the wheels stopped rolling the computer print-out told a different story: just 97bhp. That's a loss of 15%.

Next was a 30-year-old rusty Vauxhall Cavalier. This had 14% of its quoted power. Then we tried a shiny BMW 850i with a whopping 5-litre V12, which was sold with a claimed 300bhp. This engine should be capable of scorching a hole in the ozone the size of Bavaria - but the biggest engine of the day provided the biggest surprise; it was 22% down at 233bhp.

A modest BMW 316i shouldn't have more than 100bhp but the computer measured a startling 184bhp. The owner eventually lifted the bonnet to reveal a hefty 3.5-litre Alpina engine transplant. I hope he remembered to inform his insurers.

Of the others, a Mercedes SL had lost 15% of its original power, a Porsche 944 was down 11%, a Cosworth down 10%. The 'closest' result of the day was a brand new Caterham SuperSports that was just 7% down on its quoted power.

Finally it was the turn of the Maserati BiTurbo, widely regarded as one of the most unreliable cars ever made. It somehow managed to break down on the 100-yard journey between car park and rolling road. We'll never know how many Italian stallions were under the bonnet.

So what did it all prove? That you can't trust manufacturers' bhp figures, especially when a car is second-hand. Mechanical wear and tear and irregular servicing can really reduce the power of an engine, in the case of that 850i, by more than a fifth. That means a corresponding decline in performance and economy. It's not a cause for alarm - it just another thing to be wary about. Don't be seduced by manufacturers' statistics and check any potential purchase has a good service history. And be VERY careful with old Italian supercars!

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