Have you ever wanted to know exactly how much power your engine is producing (BHP), or whether some quoted manufacturer spec is actually legitimate? I'll show you in a few steps how to calculate what the engine power (BHP) is from a few dyno numbers. At this point I'd also like to clarify and correct some terminology misuse. BHP stands for brake horse power, but it's actually the power at the crankshaft of the engine
. NOT at the rear wheel.
I just did this out of interest to verify manufacturer or other claims seen online. I'm not actually sure my method is right, but it seems reasonable enough. I won't go into the details of why I do calculations the way I do it, those engineers out there will recognise it, and perhaps even correct me. These formulas will not work with imperial units directly, it's based on the SI unit/metric system, so we have to convert.
I will work with two examples, the 2006 Honda CBR600RR and the 2006 Kawasaki ZX-6R (636).
We can see the CBR produces a maximum of 105.6 RWHP @ 13,250rpm. RWHP is rear-wheel horse power. Which is:
1) 78,746 Watts
To convert horsepower to watts, you have to multiply by 745.7
Now, we need the gear ratio of the sprockets. The stock gearing for 03 - 06 models in the USA is 16/43. Keep in mind that overseas models have different gear ratios, my '06 came stock with 16/42 in Australia. However, since our dyno readout above is from the USA, I will assume it was using 16/43.
We have to divide 43 by 16 and then square it, (43/16)^2
2) Gear ratio squared: 7.223
Now, divide the power by the gear ratio, 78,746/7.223
3) 10,903 Watts
Finally, add this to the power figure from Equation 1. 10,903 + 78,746
4) 89,648 Watts (89.65kW) = 120 BHP
The 'official' Honda power figures for the '03 - '06 model I have seen online is 118 BHP, but I have no idea whether this figure is actually from Honda or not. In reality, there are losses due to heat (friction), noise, so the actual output of the engine will be a bit higher.
Let us now do the Kawasaki.
The power output given is 111.4 RWHP @ 13,000 rpm. Which is:
1) 83,071 Watts
The gear ratio of the sprockets as per the official ZX-6R 2005/2006 manual is 15/43
2) Gear ratio squared: 8.218
Dividing Equation 1 by Equation 2:
3) 10, 109 Watts
Adding this to Equation 1 power figure
4) 93,180 Watts (93.2kW) = 125 BHP
What does the official Kawasaki manual say?
My thoughts are that the maximum output of the engine possible is 95.5kW (128 BHP), but as we know emission standards will have them retuning their ECU or putting additional emissions controls to restrict the power output. Hence why different countries have different power outputs. France in particular has
really restricted levels of output with their ancient laws on a 100HP maximum for any motorcycle, and now with increasing emissions standards in Europe
I think the manufacturers claims for Kawasaki are pretty reasonable, using the formulas, you have to account for the actually losses which I mentioned earlier as well as the losses in the dyno itself. I haven't done the GSX-R600 or R6, but feel free to try it yourself.