Here at Motorsport and Performance, we like to know that we are doing our job properly and to do that we need accuracy and repeatability.
It's because of that you will see us using WHP numbers, which are naturally not as big as BHP numbers. Which we agree might seem counterproductive when we are in the business of making big numbers!
Before you can understand why - you need to know what each measurement is.
- Break Horse Power aka BHP is the calculated Horsepower measured at the flywheel of an engine
- Wheel Horse Power aka WHP is the calculated horsepower measured at the wheels of a car
How can the two be different? Basically, all of the moving parts between the engine flywheel and the road surface require energy to move.
I'm sure you are aware of the first law of thermodynamics.
The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another but can be neither created nor destroyed.
In short, any energy used to turn the gears in the gearbox with the added resistance of gearbox oil, the prop shaft, the differential, the driveshafts, wheel bearings, brake rotors and even the rubber resistance in the tyre which can fluctuate based on tread depth, tyre pressure, heat and so on is 'lost' as heat and sound before it can be used to create forward momentum or be measured.
As such torque measured at the wheels will always be less than torque measured at the flywheel and as Horsepower is a calculation based on Torque and RPM
(Revolutions per minute x Torque) / 5252 = Horse Power
the same applies to Horsepower.
Now that you get that, why do we only hand out WHP figures?
The short answer to that is 'because that is what we are measuring'.
Firstly, rather than a rolling road, we opted for an AXLE-HUB dyno. Benefits of this are many, but the primary and most obvious is the lack of squishy tyre and lack of traction through them, particularly on big power cars.
Having a mechanical connection to the car gives us repeatability and the ability to measure over 1000 wheel horsepower without shredding a brand new set of tyres.
Which is important, it gives us mechanical accuracy that cannot be improved upon.
To give a BHP figure, we would have to take this accurate, measured number and multiply it by an estimated transmission loss value.
As it is impossible to know precisely how much energy was lost turning the driveline because the figure will change from car to car (yes, even two 'identical' brand new cars) this 'transmission loss value' can only be estimated. Unless you were to remove the engine from the car, test its power on an engine dyno before refitting it back into the car for a power test on a chassis dyno you DO NOT know how much power was lost through the drivetrain - as you might imagine, removing engines for a simple power test is somewhat impractical in the real world.