Vlad the short answer is you can't measure it without putting it on a "Dyno" a machine that will measure the power at the wheels. Otherwise you are reliant on what the Re-mapper claims, usually based on either a similar model that was tested or if they have access to a Dyno they will give you a copy of the results. If you need to know exactly what your BHP is there are plenty of places around that will Dynotest for you but it's not cheap.
