Jazz
12-07-2009, 10:57 AM
http://www.gtechpro.com/rr.html
:stripes:
Over the weekend I picked up a G-Tech Pro RR. Was a steal for me at $100 (new price is $299). It downloads to a computer and gives accel/decal/G’s etc and will record for 2 hours. Measures horsepower and torque and plots a graph for them (based on vehicle weight). It even calculates RPM from the voltage drop when each spark plug fires. The theory behind that technology.
Accelerometers are used to calculate 0-60 etc. This is also sufficient to compute the instantaneous power to weight ratio (P = F v = m a v).
For other calculations that require knowledge of engine speed, this is taken from the cigarette lighter. While the signal there is nominally a constant ~14 V or so, it's rarely regulated. Each time a spark plug fires (assuming you're not driving a diesel), the current draw causes a slight dip in voltage at the lighter. Knowing the number of cylinders in the car, the frequency of these little dips can be used to calculate rpm. This requires a fair bit of amplification and low frequency filtering to get right, but should be pretty reliable in most cars.
:stripes:
Over the weekend I picked up a G-Tech Pro RR. Was a steal for me at $100 (new price is $299). It downloads to a computer and gives accel/decal/G’s etc and will record for 2 hours. Measures horsepower and torque and plots a graph for them (based on vehicle weight). It even calculates RPM from the voltage drop when each spark plug fires. The theory behind that technology.
Accelerometers are used to calculate 0-60 etc. This is also sufficient to compute the instantaneous power to weight ratio (P = F v = m a v).
For other calculations that require knowledge of engine speed, this is taken from the cigarette lighter. While the signal there is nominally a constant ~14 V or so, it's rarely regulated. Each time a spark plug fires (assuming you're not driving a diesel), the current draw causes a slight dip in voltage at the lighter. Knowing the number of cylinders in the car, the frequency of these little dips can be used to calculate rpm. This requires a fair bit of amplification and low frequency filtering to get right, but should be pretty reliable in most cars.