by Richard Harley

Created on: February 02, 2009

Deciding how much horsepower you 'need' is really the wrong calculation to do. In this article, I'll outline what you really should be looking at when it comes to choosing a new performance car.

Horsepower alone is a largely irrelevant figure. Early Russian trucks, used for carting about swathes of infantrymen and supplies to the front line, nudged 500bhp. Did it make them speed monsters of the autobahn? Alas no. The only figure you need to examine in any detail is BHP/ton. The aforementioned trucks weighed about the same as a small Eastern European country and hence had a terrible power to weight ratio, resulting in a 0 to 60 of, oh, I don't know - a few days. Torque is important too but that's not the subject of this article.

Let's look at a few examples.

1995 Mazda MX5 1.6ltr - 90bhp

The MX5 is a good example of a lightweight sports car (although I use the term loosely). 90bhp on tap might not seem like a lot compared to an average five door saloon but the power to weight ratio of the Mazda is good. The car itself weighs less than a ton at 960kg, meaning the bhp/ton for this car is actually nearer 95.

Compare this to a 1996 Ford Mondeo 1.8ltr. The Mondeo's power output is nearer 115bhp, over 25bhp more than the MX5. But since the Mondeo weighs over 1300kg, it's bhp/ton crumbles to under 85. Lighter equals faster when it comes to motoring so although you might think your increased power is better in the Mondeo, you aren't going to be winning any drag races against the MX5. Simple so far, right?

Don't forget about baggage and passengers. You can calculate the reduced power of your engine quite easily:

Take a bog standard BMW 525i producing 200bhp with no passengers. The car weighs (for the sake of argument) 1500kg and hence makes 135bhp/ton.

Now add in 5 passengers at 70kg each and luggage at 50kg. That's 400kg. The car now 'weighs' 1900kg but still only produces 200bhp from it's measly 2.5ltr engine. Bhp/ton has now decreased to 106. Using this example, you could expect a significant performance hit when driving this car fully loaded due to the impact the extra weight will have.

Next imagine the Russian truck we talked about earlier - weighing in at 10 tons and producing 500bhp. Thats 50bhp/ton. Not a lot by todays standards. Add in our five passengers and luggage again - the truck now weighs 10400kg but still manages 48.9bhp/ton. Hardly any difference at all. This is why in heavier vehicles like 4x4s, a load on the engine is less noticeable, but in lightweight sports cars, even a full tank of fuel would be a performance hit. Which brings us neatly onto our next point.

How much horsepower should your car have?

The simple answer would be as much bhp/ton as you can afford. However, this might sacrifice things like practicality, so be sensible!

A small hatchback might produce 65bhp and weight just over a ton. This gives a fair 65/bhp, which most people could cope with for everyday driving.

A larger performance saloon might produce 250bhp and weigh 1.7 tons, making 150bhp/ton - again a respectable figure for petrolheads

Finally, a supercar (take a Ferrari F40) would produce around 600bhp and weigh 2 tons, making 300bhp/ton - seriously fast kit.

To summarise:

20-50bhp/ton - Faster to walk instead.

50-100bhp/ton - normal car territory, good fuel economy, fairly standard.

100-250bhp - top end is executive territory, performance saloons and 'hot' hatches, many kit cars fall in the 200bhp/ton area.

250-400bhp/ton - seriously fast cars, normally high end insurance groups, track day or turbo-ed cars might feature here.

400-600bhp/ton - Supercar status i.e Ferrari's, Lamborgini's etc

600bhp/ton - Nice if you can afford it.

I hope that clears a few things up. Happy motoring!

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