Mitsu_man, here it comes!
Well, I like to share stuff that someone might find useful, that's all. Now to the BHP measuring thing. Mathematically, power can be defined as:
P = m x a x V + Cd x A x Pa x V x V / (2 x R x Ta) + m x 9.81 x 0.013 x V
True Wheel Power = Power for acceleration + Power for overcoming aerodynamic drag + Power for overcoming rolling drag
(In Watts)
where:
m = Total mass of vehicle in kg
a = Acceleration in metres per second per second
v = Speed in meters per second (km/h / 3.6)
Cd = Drag coefficient
A = Frontal area of car in sq. metres
Pa = Atmospheric pressure in Pascals (101325)
R = Molar constant of air (287)
Ta = Ambient temperature in Kelvin (°C + 273.15)
The tricky thing is to be able to measure vehicle speed over an acceleration run. What you need to do is to stick the car in a gear where you can accelerate from idle to redline at full throttle without running into trouble with the local boys in blue.
While doing that, you need to get a signal for the engine speed. When I said MP3 player, I actually meant one that could record sounds as well, or better yet, a mobile phone with a microphone connection (for handsfree). You need to take a signal from one of the spark leads (or the alternator / rev meter) to retrieve a kind of pulsing sound. A bit like something you get on short wave AM when a distributor-type petrol car goes by.
This varying frequency "noise" is then downloaded to a computer and then plotted using a computer program and then the peaks of noise are referenced with the recording frequency. Actual calculation of the frequency is not necessary. You can reference it to a fixed reference speed. Say 60 Hz at 50 km/h in 3rd gear. So 30 Hz signal would mean 25 km/h in 3rd gear.
In all fairness, it sounds quite complicated, but I'll let you know for a fact that this is probably the most realistic power measurement you'll ever do.
If measuring RPM is not your thing, you can use a video camera. Fix your camera phone on the speedometer and do an acceleration run. Then download the video and forward the video frame by frame to measure intervals between each 5 km/h increment. Assuming you counted 18 frames for an increase of 5 km/h, it took 18/24 or 0.75 second to do the 5 km/h transition. Plot each of these transitions and you can calculate the acceleration between each 5 km/h transition. If you want, you can send me a clear video and I'll show you how to do it.
The good thing about a video is that if you have a boost gauge in the video frame as well, you can see the effect of gear ratios on how the engine comes on to boost.
Or...
Save yourself the DIY headache and go for this thing:
Dynamic Dyno, The Do-It-Yourself Dyno Solution!!! - Generate Horsepower And Torque Curves Easilty!
The only snag is that it works only for petrol engines as it picks up the ignition signal spikes from the lighter socket. Diesels will not have this, so it will not work for diesels.
You can also get the Dynolicious app for iPhones, but I'm not a huge fan of that since it measures horsepower from acceleration, and you can have errors in the accelerometer readings. Hence my preference for using the speed signal.
Cheers!
Sami
