[TSM330i]

There is a big problem with chassis dyno's. It depends on what type of dyno they're using. The only way to truely test hp figures is to pull the engine out and put it on a bench dyno. Chassis dyno are usually used to measure a gain for aftermarket parts to gauge the increase of the stock output.
I had a long discussion with Katech Engineering at Corvettes @ Carlisle a few years back. My stock Z06 dynoed at 445 rwhp on the dyno, with a 15% powertrain loss that is 524 crank hp. When I told him this he said that all the engines from GM that they've dynoed on a bench dyno come in right about 505hp. He said the difference is the way the chassis dyno measure hp.
Don't put too much credit to these numbers. Dyno a 335 back to back with the S4 and see the difference.