What equation is used to calculate Horsepower? I found one that said: T * RPM / 5252 = HP, but then that equates to the 420A having 111 horses at 4500 RPM! Then the torque starts to steadily drop, yet the horsepower goes up to 140 at 6000 RPM.

I have quite a few equations jotted down in a notebook, but for some reason, this one doesn't make sense. All the others I can confirm with the available information.

well, a 420 has roughly 140 HP so it seems right. but all cars are different. it may be just a general equation.

I think it may work for N/A cars but I don't know for turbo cars. there are too many equations out there

Well, if you say so.

Actually, I found another equation that feels awkward. HP / 16 = IS (HP = Horsepower, IS = Injector Size in lbs/hr) and that equates to 8.75 lbs/hr. Now, to change lbs/hr to cc/min, the equation says to multiply the 8.75 by 10.5092. That gives us a cc/min of 91. 9555. So our stock injectors on our eclipse are 92 cc/min?
I thought that maybe if I multiply that by 4 (no. of injectors) it would seem right, but that number comes out to 367.822! But I don't think the stock injectors are 368 cc/min.

And some equations I just can't check because I dont' know where to find the information for them (volume of piston compression, volume of piston bumps, head gasket compressed volume, and deck to piston distance) I am assuming I would have to take those measurements myself...

All cars are differant and design etc. An engine will not just start out making per say in this case 140 HP , it has to work up to that point. Engines HP and TQ are all due to design , air velocity , path , induction . On the lower end of the rpm TQ is built by air travel, path and velocity of the travel , as the RPM builds and nears its upper end ranges the TQ will fade cause the air is traveling at a higher rate into the engine and HP starts to build and caps off when maximum air velocity and effective efficiency have stopped. This is the reason for adding aftermarket air intake systems to take out restrictive air boxes. factory air intake systems and air boxes have "dynamic air chambers" which are restrictive , and for the most part all these boxes and piping do are design to make reverse air resonance of the intake system as it draws air at differant rates thru it , semi cancelling the intake air growl , which yu can hear when an aftermarket intake is installed .

A freer flow of air helps achieve more HP at the same RPM and broader range of TQ.

Lots of cars now use variable intake runners , or varied intake length , to make the path that the air travels at lower to mid rpm, longer to help the engine produce more bottom end TQ. The longer the air path the better and broader range of TQ. And mid range to upper RPM the length of the air travel is shortened so that the air can get into the engine alot quicker helping produce more HP. Mitsubishi has been using a Variable Geometry intake plenum since 1990 on selected V6 models.

Cam design also helps in making HP and TQ. Also variable timing . As with MIVEC which is near identical to operation as VVT,VVT-i,VVT-iL , it is seemless and is working through near the whole range of RPM kicking in as early as 1500 RPM like VVT,VVT-iL , unlike VTEC which is stepped and kick end onlee at the upper end of the RPM bracket to aid in HP while Mitsubishi's MIVEC and Toyotas various VVT variations aid in TQ and HP . Hondas i-VTEC however comes closer to MIVEC and VVT-i .

Calculating engine size , injectors etc wont give yu magical numbers in wut HP yur spossed to have. knowing the static compression , air velocity range , displacement -bore and stroke and fuel injection type (GDI,SFI,MPFI) and velocity and pph per min are all wuts needed to give a range of where the HP should be at.

http://www.rceng.com/technical.aspx