Quote:
Originally Posted by low135
They're underrated numbers from factory. Compare them with most other manufacturers and you'll see that. Ford RS makes almost 70hp less than it's advertised bhp. That's a huge number.
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No, they are not “underrated”. Engine ratings are highly regulated, car manufacturers cannot publish any numbers they want.
As I said, one needs to get familiar with the SAE and DIN engine power rating standards and understand the operation fundamentals of modern turbocharged engines to understand what is going on with the numbers.
In short, engines behave differently when in steady state (constant RPM) than when in transient state (while accelerating). Modern turbocharged engines usually tend to make more power in transient than in steady state in the mid to high RPM band while making less in the lower RPM.
Traditionally, power rating standards required the engine to be tested in steady state, which allows for much better control of the test environment for more accurate and repeatable results. The engine is held for a few minutes at a given RPM at WOT until all the parameters stabilize before the measurements are taken. The engine is then accelerated to the next RPM point and left to stabilize, and the process is repeated across the RPM range.
With the avenue of modern turbocharged engines, the SAE has modified the standards to allow manufacturers to take advantage of the engine characteristics and now allows them to test in transient if they so desire. The caveat is that they leave it up to the manufacturers to chose which of the two method they want to obtain their ratings, so it makes it difficult to compare numbers between manufacturers.
BMW and most European manufacturers choose to stick with the traditional steady state method for their ratings likely because it is more accurate and repeatable. This is why the numbers seem “underrated” when compared to numbers obtained on a chassis dyno that operates in transient state.
Excerpt from SAE standard J1349 section 9: