I had my stock Cayman S dyno'ed on sunday, it turned out 278 HP at the wheels and 339NM (250,77 ft/lb) of torque. That's more than okay I think, from results I've found in previous threads. It had 19" Carrera Classics on at the time - could anyone speculate on how big the difference would be with 18" standard wheels?

Is there any difference making a dyno test with a soft or a hard rev limiter - I'm thinking about "SPORT" mode engaged or not?

Besides that, how do you measure power at the flywheel?

To properly measure flywheel horsepower you need to remove the engine from the car and use an engine dyno. The calculations usually done to estimate flywheel horsepower from rear wheel horsepower, typically expressed as the reciprocal of a percentage of loss, can be misleading. (For one thing, the power consumed to spin the drivetrain at a given rpm doesn't change in relation to the power of the driving engine, so we know percentages are misleading.) Perhaps it's just me, but I've never been interested in flywheel horsepower, only power measured at the tires, because you don't drive on flywheels.

Good one

But that also means that my car probably has well in excess of 300 HP at the flywheel? Most threads I've read state about a 10-15% loss in the drivetrain, which would be somewhere between 305-320?

The wheel size has no effect on the power produced by your car. Changing the wheel size can cause an improperly calibrated dyno to give different (erroneous) readings. See Wheels & HP, a loss of power with larger or heavier wheels for a comprehensive discussion.

You cannot know the drivetrain loss or the flywheel HP unless you remove your engine from your car and measure the flywheel HP on an engine dyno. Try not to even think about it.

Thanks! I'm not doing that - for sure.

Is it correct to say that the torque measurement is "more" correct than the RWHP measurement? Meaning that torque isn't as dependant on drivetrain loss and other factors as RWHP?

Most dyno's actually only measure torque. Horsepower is figured off of that.
horsepower equals torque times rpm divided by 5252. The dyno will do the math and show a horsepower figure.

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[stock Cayman S dyno'ed on sunday, it turned out 278 HP at the wheels ]

Your claim of 278HP for a stock Cayman S seems quite high....almost all others posted here are in the 260HP range...

....I think your dyno machine is very generous....

are you sure the car is stock?

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Yes. But I do have the red taillights, which add a secret number of HP
(what kind of a question is that btw?! )

I don't know about the generosity of the dyno, but some of the other cars (a lot of BMW's) also scored higher than expected, so maybe it was a little too kind. Here's one that had 276, though: Stock Dyno 276 at the wheels?

Having the dyno done now, I'm wondering what I got from it. Since it's far from an exact science and it's not really comparable to other dynos, what can we read from a dyno chart?

Not much. The best use of a dyno is for back-to-back comparisons of a modification. Like this:

1. Dyno the baseline configuration.
2. Make the modification - preferably with the car still on the dyno.
3. Dyno again.

The difference you measure will be fairly reliable. The greater the time between steps 1 & 3, and particularly if it's a different dyno setup on a different day, then the less reliable the results.

That curve is very untypical, in fact its the smoothest and most rounded one I have ever seen...ever!
Maybe useful if you want to compare your forthcoming mods, aside from that, err...err. Maybe you want to get a second "opinion" from another Dyno? (not Dynojet brand). Just a suggestion for what its worth.