Stollen from a GTR forum:
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The SAE gross power aka brake horse power (bhp) is measured with simulated load by adding belt-driven accessories etc.; however, it left a lot of wiggle room for interpretatkion (if you have time, have a read HERE). That measure is no longer used in America as of 1971-72. One of the newer measures is SAE net power (hp) which is measured at the engine's crankshaft with stock accessories so is a truer measure of engine power itself. Neither measure accounts for transmission loss. (Reference WIKIPEDIA). Theoretically, using same stock accessories, bhp=hp.
Typically only manufacturers take the engine out to measure directly at the flywheel with an engine dyno. That shop is using a Maha chassis dyno to measure the whp by spinning the wheels. Sounds like they calculate upwards from whp to arrive at their estimated engine bhp of 392. The whp is almost always less than hp because of frictional losses between engine and wheels. Some European manufacturers prefer to quote using bhp (over SAE net power) to indicate a more conservative number by using accessories that make it simulate transmission loss as well; that is why some German cars may dyno a whp number at the wheels close to their engine bhp. However, regardless of bhp or hp, whp is whp on same dyno same day.
If your GT-R is making 550 awhp, your flywheel hp would almost certainly be higher than 550 meaning your Audi may need more than 150 engine hp to arrive at 550 whp. Also keep in mind awhp isn't the same as rwhp, depending on the car and how it's measured. Some chassis dynos do measure four wheels (awhp) while others measure two wheels (rwhp) and the equivalent conversion from either awhp-to rwhp or vice versa is calculated (i.e it wasn't actually measured).
As others have mentioned, a 550 whp measure for one car on different dyno and different day is not necessarily an apples to apples comparison. On top of that, even if you manage to get your Audi to measure 550 awhp on same dyno same day as your other car by adding say 180 awhp (550 awhp - say 370 awhp), the different cars deliver the power differently so 1/4 mile ET/MPH may still differ. In the end, track time is still the best overall measure for car performance.
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