I know that my 305 TBI is said to have gotten 170 hp from the factory floor. What I've searched for and can't find out is where this hp rating comes from. Is that the motor by itself, or is that the car on a dyno measuring hp at the wheels? Basically, where did GM get this number?

Also, I know that the formula for hp in cars has been updated now and then, creating differences. Has that happened since 1991?

Basically I'm trying to compare the hp the engine is putting to the pavement to replacement engines. I've seen on the boards that the 290 hp 350 crate motor would likely make around 210 to 220 hp at the rear wheels. So does this mean this particular 350 has 120 hp on the factory motor, or only 40 hp on the factory motor? (Lets just compare it to a fresh 305 TBI for this example)

 

Factory HP ratings are always flywheel HP. Years ago the HP ratings were a stripped down engine. Now the HP ratings are with all the accessories.

Read this
http://www.hardtail.com/techtips/hpexplained.html

 

Ok, cool, so by the equations that the article claims, my car would be making around 136 hp at the rear wheels, and that 350 crate motor would be making roughly 232 at the rear wheels, resulting in a roughly 96 hp increase from the fresh stock engine.

Obviously these numbers are totally in theory, and with 170,000 miles on my motor, I bet I'd be lucky to be seeing 100 hp at the rear wheels. I'm just interested in these numbers as I'm trying to decide on an engine replacement.

 

That's pretty much how it goes.
The only thing I don't agree with is your thinking that your 170k mile engine has <100rwhp. Unless there's something seriously wrong with your engine (SES codes or mechanical problems), you shouldn't lose more then maybe 10hp from the original numbers. I'd be willing to bet that you're running about 130rwhp in your current condition.

 

Well, to a point... crate engines are actually still tested with the pre-1972 method, simply because there are so many different applications and confugurations, they just cannot give a number for each, as each would vary.

For example... GMs generic replacement "290hp 350ci" crate that comes with a lame cam, smog heads, low 8:1 compression, etc... is basically the same as what everything with a whopping 180hp or 200hp back in the '70s came with. But since it was rated with the old method, with no accessories and open exhaust, the numbers are somewhat skewed.

Just a heads up for you