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Old April 7th, 2004, 11:25   #15
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Join Date: May 1999
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Default The absurdity of fixed percentage drivetrain loss

No, that's not what I'm saying at all. Horsepower measurements by ALL car manufacturers follow standardized test methods according to either the SAE, DIN or JIS (Japanese), depending on the country. These standards define test parameters, ambient conditions, etc., for engine dynamometers, NOT chassis.

However, car manufacturer's will tend to design the engines a bit on the generous side with respect to published horsepower ratings. The primary reason is that if a company claims 90HP, they better make damn sure there isn't even the possibility of one less or they will have some very irate customers (recent example is the Mazda RX-8). Manufacturing tolerances and other variabilities (fuel, ambient conditions, engine condition, state of tune, etc.) mean that, in a large sample population, there is a normal distribution in the power output of the engines, and OEMs would want to quote the lower spec limit rather than the mean. How this lower spec limit is defined by the OEM and the inherent variability of engine design, manufacturing and the control systems combined determines how power ratings quoted by the OEMs differ from what is actually measured in an independent dyno.

There are other reasons why this may be done as well (regulations, taxation and insurance ratings); it is documented that some cars are intentionally rated extremely conservatively for these reasons (e.g., unwritten 280HP limit for Japanese market cars among the Japanese automakers).

Summary: VW quotes 90HP in literature, but it's intended to be at the crank, even though rolling-road tests here have shown that completely stock A4s can make AT LEAST 90HP at the wheels. VW is just being generous [img]/images/graemlins/wink.gif[/img] -- actually they're just hedging that the variabilities mentioned above won't result in an uproar if some engines are measured to be below the claimed output. Also, it has already been mentioned that 3rd party dyno tests are not done in particularly well-controlled conditions from a scientific standpoint (too many variables unaccounted for). GoFaster pointed out above that there can be significant variation from one unit to another of the same type, and even greater variation among the different brands out there (e.g. Dynojet, Mustang, etc.).

For A3/B4 TDIs that have dynoed far less WHP... hmmm I wonder why: How many miles are on those engines? [img]/images/graemlins/rolleyes.gif[/img]
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