Using an old copy of an automobile magazine, these are the cars with hp and lb ft I am trying to work out the RPM for (only power in hp and pulling power in lb ft are quoted) :
2007 Honda Accord 2.4 sedan, 2.4-liter 187hp 4-cylinder, 164 lb ft pulling power
The math I did to work out RPM:
187 x 5252/164 = 5,988.56 (rounded up to 6000rpm, how far should i round up in significant figures or decimal places)?
2007 Peugeot 207 1.6 HDi 110 GT, 1.6-liter 110hp 4-cylinder, 177 lb ft pulling power
The math:
110 x 5252/177 = 3,263.95 (rounded up to 3,264 rpm, or should it be 3000 rpm?)
I used these formulas:
Torque and horsepower relations:
T = HP * 5252 / RPM
HP = T * RPM / 5252
RPM = HP * 5252 / T
In my two examples, have I got the math correct for car engines? With RPM, when do I need to round up correctly and what is the correct amount of significant figures?
You are making the assumption that the peak horsepower and peak torque occur at the same engine speed. That is almost certainly not the case. As your equations show, horsepower is torque times engine speed times a constant to get the units right. For many engines torque is about constant over a long range of speed so the horsepower is rising linearly with engine speed. Eventually something causes the torque to fall off. Horsepower keeps rising for a while, then peaks and starts to fall.
As your input data has three digits I would round the output to three digits as well.