The math doesn't add up. Something sure looks bogus
Dynamometers come in two basic flavors, "Chassis" and "Engine".
An Engine dyno usually measures the HP of the bare motor, no accessories or driveline. You have to take the engine out of the car to put it on an Engine dyno. Engine dynamometers measure the raw HP the motor produces at the Flywheel.
A Chassis dyno measures the HP the car puts down at the rear wheels. This HP is the engine HP minus the losses for the driveline and engine accessories (alternator, water pump, PS pump, etc.).
The losses associated with the driveline and accessories are considered to be "constant", they don't vary with engine RPM. This isn't quite true, but it makes the math easier if you treat it this way, so everybody does.
If you know how much power is required to turn the accessories and driveline, you know what the losses are, and you can compute the raw power that the motor is making by measuring the HP at the rear wheels and "compensating" for those losses.
While the accessories on all Corvettes are the same, the drivelines are not. The Automatic transmission wastes more power than the Manual because (real technical jargon here) it's got more "stuff" that has to be turned. That extra stuff creates more drag, and requires more power to overcome.
In addition, the power consumed by the accessories on an LS6 is the same as the power consumed by those accessories on an LS1. Since the LS1 makes less HP, the power lost to the running gear is a larger percentage of the overall power produced by the motor. Example: if it takes 40HP to turn all the running gear, that's about 10% of the LS6's 405HP, but it's 11% of the LS1's 350.
No one seems to know what the actual accessory and driveline losses are for a Corvette. I've seen numbers ranging from as low as 12% to as high as 15%. They're different for the LS1 and LS6, but here's the key... when you increase the Engine HP with aftermarket Mods, the running gear power loss stays the same (it's 40HP whether the engine makes 200HP or 500HP). This means that as the engine HP increases, the percentage loss given up to the driveline and accessories decreases.
In the Ad you quote, the modified LS1 made 324HP, which they "interpolated" to 416HP at the Flywheel. Simple math says they're assuming a 22% loss! (1-324/416=0.22). That's kind of generous if you ask me. Consider... If their "Stock" 304RWHP rating is accurate, and we believe GM's numbers and the LS1 makes 350HP at the Flywheel, the actual loss in this "stock" LS1 Corvette is only 13%. (1-304/350=0.13%). Accordingly, their actual "interpolated" Flywheel rating should have been 324/(1-0.13), or a much lower (but still respectable) 372HP.
Assuming you copied their Ad correctly, by their own admission their Mods are good for 20HP. If the stock LS1 is 350HP Gross, that's a 370HP Gross when you add 20HP of Mods.
Their estimated 416RWHP is clearly in error.
Finally, the Z06, which made 350HP at the rear wheels, is "interpolated" to 412HP at the Flywheel. This says the assumed losses are about 15% for the Z06. The Z's losses actually *should* be a smaller percentage. But curiously, if you assume the stock LS1 made 304HP, and that's actually a Gross of 350HP, the LS1 has about a 13% loss. If the Z06 makes 350HP, and that's actually a Gross of 405HP, the LS6 has about a 14% loss.
Since the LS1 had an Automatic transmission, and it has lower base HP than the LS6, the LS1 has to have *greater* losses than the LS6, not less.
So, even their "stock" Dyno numbers don't add up.
That makes their assumed 22% loss figure, and 416HP Gross estimate *MIGHTY* fishy.