[olddog]

Car manufacturers listed Peak HP at the flywheel with no accessory belts connected and open headers up until 1972 (I think). From 1972 on, the government set a standard that they had to list the Peak HP at the flywheel, with all accessories and mufflers attached as the engine will be operated when installed in the vehicle. It is mistakenly believed that emissions equipment cause a big reduction in power, but it was a new standard in honest advertising.

In addition to this, there are formulas to correct the measure Hp to what the engine would have produced had the test been done at sea level at a certain air temp and humidity. This is because air density is a big factor in the power an engine makes. So to compare different test done at different places on different days (summer or winter) the numbers are corrected to what it would have been at a standard atmospheric condition. It sounds so logical, but notice I said formulas - plural. The formulas are not exact and they keep trying to improve and get it right. There has to be more than a half dozen sanctioned formulas that were used over the years and more than one in use today.

Yep, you can make the numbers come out any where you want and still claim you did it by the book. But which book? I'm not saying there are no honest people trying hard to get an accurate number. I'm just saying there are people who have no honor.