Ford rate the new motor at 300bhp SAE Net
SAE = Society of Automotive Engineering
They set standards for how to test the engine, it will be tested at particular temp and humidity levels (or if not correction calculations added to the numbers produced).
Net = this means that it is done on a chassis dyno, so the engine is not actually in the car. But it does have to have all of the ancilary devices fitted, such as water pum, air con, alternator, air filter, full exhaust system, etc.
This figure is often called flywheel or crank bhp. In years past (pre 1973) American manufacures used to measure SAE Gross numbers, these are the same as above but the engine did not have any ancilary devices which could sap power. This is howcome some of the muscle cars of old have 'massive' bhp numbers. However if they where rated today they would produce much lower figures. A Mopar 440 rated at 370BHP SAE Gross probably produces about 290bhp SAE Net. Different cars and ancilaries will affect the amount lost.
Other countries suck as Britain and the rest of Europe generally do not quote SAE figures, either BHP or PS is listed. PS is German for horsepower. This is a Metric and Imperial rating, however they are very very similar. An MG ZR160 produces 160PS or 158BHP (the actual power is the same it just depend on the rating system, just like Centrigrade and Farrenheight for temps)
British BHP ratings will tend to be almost identical to American SAE Net numbers.
Along with the different ratings there are also different methods of producing the numbers. HP stands for horsepower which refers to how fast a horse can lift a particular weight. BHP stands for brake horse power, this uses a braking device to measure the force, ie torque of a motor. Then using a calculation the bhp numbers can be produced
**Confuesed yet?**
Countries such as Australia use yet another rating system kw (kilowatt) this in reality is probably the most accurate measure, but it again depends on how the numbers are produced.
RWHP - this doesn't really exist as it could be claimed as RWBHP if it was measured using a brake device, yet there are also SAE ratinging for it as well.
These are measured on a rolling road as a rule by the wheels rotaing drums in the floor, these devices will then usually measure the rotaional force (torque) and calculate the bhp from these numbers. There are many types of rolling road dyno, in America the Dyno jet and the Mustang dyno seem to be the most common. The Dyno jet just uses a weighted drum which measures the amount of force required to turn it, these dyno's tend to produce higher numbers, a Mustang dyno uses varying loads so is much better for tuning the engine over the entire rev range (eliminating flat spots etc.). But because the load varies the numbers produced will appear lower.
For example if a car makes 220rwhp on a Mustang dyno it may make 230+rwhp on a dyno jet, this does not mean it produced more power on one dyno than the other, just that they are measured and rated differently. In the same manor that 1US dollar has a different value to 1 Australian dollar. They are both dollars but they are not the same REAL value.
Rolling roads only really have one use - and that is tuning, they let you see how the engine is performing. The curves on the graphs are much more important than the peak numbers produced. As they are to often inaccurate, temp, humidity will affect them, even the type and pressure of your tyres will have an affect on the peak numbers produced (however they will not affect the profile of the curves on the graph so the engine will still be performing the same), if everybody added correction factors to their dyno runs then you could compare them, but very few people do. So if you car dyno's is 20rwhp less than someone elses they may infact be producing the same power as the results are flawed. If you and a mate take your cars at the same time to the same dyno and have them run then these figures could be compared to an extent although they can still be out.
The only true way to ascertain true bragging rights it to remove your engine and have benchmarked on a chassis dyno conforming to the same standards as the manufacure.
Chassis dyno's produce higher numbers than rolling roads, because the drivetrain (clutch assy, gearbox, prop % drive shafts, wheels, brakes, bearings, etc.) all produce friction and need power to rotate them, thus giving the lower figures at the wheels.