question.

[Paul86gt]

I have question about horse power and torque. I was taught that to get hp you multiple the torque rating by the rpm at which peak hp is suppose to be made and then divide by 5252. If that is true then a stock 86 mustang 5.0 is under rated by 17 hp. example. 285 x 4000 rpm divided by 5252 = 217 hp. Now I know that hp is basically a bs term because you can use the same torque rating, increase the rpms and you get a different hp rating. example. 285 x 4100 divided by 5252 = 222 hp. So just out of couriosity what is the real way they come up with hp ratings.
Paul86gt.

[Mike Croke]

First, I think the '86 was rated at 285lb-ft @ 3000rpm (162hp) and 200hp @ 4000rpm (262lb-ft).

Dynomometers measure torque either at the wheels/axles (chassis dyno) or at the crankshaft (engine dyno). Either way, the machinery is measuring the twisting force being exerted. The horsepower number is computed from the torque and rpm.

However, there are lots of tricks to measuring numbers and many pitfalls when comparing numbers. Many moons ago, auto makers measured engine power and published the highest numbers they could by running on an engine dyno with no accessories (sometimes not even a water pump), no exhaust, etc. Over time, SAE standardized the testing in order to get everyone to test the same way. But manufacturers still play games with the ratings for various reasons such as liability, insurance, advertising, etc.

For example, in 1993, Ford re-rated the 5.0 from 225hp down to 205hp. Nothing changed in the engine but the testing procedure was revised. Take any stock '87-'93 5.0 to a dyno and they will put down anywhere from 200-230hp at the rear wheels.

Also, there are differences in how the dyno measures the power. There are step dyno runs where the engine is accelerated in increments at held momentarily at each point to allow the power to stabilize. This method is applicable for engines that may do a lot of towing and will generate the highest numbers. Other dyno runs control how fast the engine is allowed to accelerate so the inertia of the engine is accounted for.