BHP - What is it?

[Pugguy]

BHP

BHP is a term used to measure a engines power output, it stands for ‘Brake Horse Power’, with the ‘Brake’ being the force used to measure it- A brake force is applied to the rotating engine to measure how much the engine’s own power can resist this brake force.

Horse power is the term used for comparing this power to other ‘like’ means of power. In the day when it was invented the only other form of power for doing work was the horse, so the inventor of the first engines (steam) used ‘HP’ for a example of how much his engines could pull in comparison to standard horses.

These days, it is not possible to measure an engines BHP, this can only be calculated from measuring your cars torque and using an equation (Horsepower = Torque x rpm / 5252) to calculate the power.
As a general guide for common cars where max power is reached at 6000rpm with good tractability you need roughly the following engine capacities to achieve the BHP figures:
125 Bhp : 2 Litres
250 Bhp : 4 Litres
500 Bhp : 8 Litres & so on.

These figures are only 'general' but are a good guide to start from. The 16 cylinder ‘Bugatti Veyron’ production car has a power output of 1001BHP. This uses the 8 liters to develop 500bhp then they effectively doubled the capacity by fitting a turbocharger with a boost pressure of 18psi.

Of course you can also lower the capacity of the engine & improve the flow efficiency of the engine( Bigger valves, improved cylinder head ports & induction system), thus improving the flow potential of the engine & allowing it to rev higher, therefore processing more air per minute without the need for bigger capacity. (This is the basic reason that a full race car can develop so much power in relation to its capacity.)

So you want more power without increasing the capacity & keeping the drivability of the vehicle?
Of course you do! Well you must first process more fuel. Typically getting more fuel into the cylinders at high rpm is not a problem (at this stage at least!) so we must cram more air into the cylinders, as the fuel will not burn efficiently without the correct amount of oxygen.
A turbo with a boost pressure of 1 bar (around 15 PSI) will effectively double the volume of the cylinders due to the fact a normally aspirated engine will rely on the atmospheric pressure of 14.7psi (at sea level) to ‘push’ the air into the cylinders on the induction stroke- so if the turbo is pushing a further 14.7 PSI then the total pressure in the manifold is 29.5psi effectively doubling the amount of available air, This will double the engines power output( well at least in theory, in practice it hard to do this as the extra pressure creates restriction to exhaust flow & extra heat thus reducing the density of the charge etc) . There are of course other factors that make this difficult, Inc flow efficiency of the engine cylinder head-induction system, ignition timing & resisting ‘detonation’. All of which make engine development more difficult & typically more costly!

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