You could always try do a few tests yourself, and get a few full rev range maps and work out the energy, and therefore the power required to accelerate the vehicle over the test, using Excel. I tried this with a few 3rd gear starts in a 150bhp diesel, my estimate wasnt too bad, I worked out that the car had about 136bhp, assuming the gearbox/drivetrain is 85% efficient. If I tried it a few more times, with torque set to log every .5 of a sec I would be confident of getting a better estimate.

Edit: Coming to think of it, if your ecu is not capable of giving out more then 4 parameters per second, and you have a good few dials displayed, or are record any other obd parameters this will definitely effect the vehicle speed recorded through the obd, and will make the estimate wayy off. So if you try remove as many dials on the opened page, and delete any other pids that are in the log, it could improve your efforts.

By a rev range map, I simply mean go out and do your own test, and using excel go get your own torque and bhp map, just like a dyno will print out! What you do is find a nice long flat straight road with no traffic, and depending on how quick your car is, a high speed limit, and starting in 3rd gear at very low revs plant the trottle and accelerate the whole way to the max rpm the car will do, but stay in 3rd gear the whole test.

Then you need to find the tire diameter, final drive ratio, 3rd gear ratio for your car, and in excel you can work out the force that the car was being propelled at using F=MA, and from the force you can work out the torque (T = FD), and then the engine power (P = T*RPM*0.85), where 0.85 is the amount of power lost between the drivetrain/gearbox etc.

If I’m wrong on any of the above formulas, let me know!

It seems that the idea of having minimal readings working at the same time might be the answer.
I had a single graph on display today for BHP when i joined the M4 from a slip road, and although i was in 4th and didn’t push it too hard, i did see a reading of 193bhp, which is much closer to the figure i would have expected.
When i get more time, and more fuel i will try the run on a quiet road in 3rd, 4th, 5th and 6th and see what results i get.

Thanks for the feedback, i’ll bear all of those tips in mind the next time i try it. Just need some dry weather now so i can get some traction.

You can calculate it also by just looking at the change in kinetic energy (1/2mv2), and subtract the losses due to aerodynamics and rolling resistance. Power is defined as the rate of change of energy.