There seems to be lots of enthusiam for the Horsepower vs Speed
information. Steve Laifman has, I think, in the past suggested an
experiment in an effort to find the drag coefficient. I'll voice it again
and maybe we can all participate.
1. Find a level stretch of road sufficiently long to permit accelerating to
60 or 65 mph.
2. At speed, note time and remove foot from accelerator and shift out of
gear. Yeah, thats a lot to do at once and some of you may not make it..
3. Coast until the speed drops to 30 mph. Note time that it took.
4. Repeat step 1 going the other direction. This will remove any grade bias
or wind bias.
5. Repeat steps 2, 3, 4 at least 3 times.
Repeat with soft top up, soft top down.
Now, the tire size, pressure, car weight, and toe in/out also need to be
known. Also any extra weight in the car (eg passenger and your weight).
If we can get enough of you to do the experiment, then we will have a
statistically relevant database.
I will collect the data, and do the math. I will publish the info for all
to criticize. Let's put a time limit on doing the experiments. Lets have a
week for all to input to design of the experiment itself including
equations, methods for data reduction, etc.. So, the experiment design
close on March 20th. Experiments to be done no later than April 20th.
Calculations no later than May 20th.
If you use or postulate data or information, remember it is absolutely
worthless unless you can quote the source of the facts. Opinion, guesses,
and druthers simply carry no weight.
Some one mentioned that the mechanical efficiencies I used were too high.
Well my source has the following to say...
"The power and torque output of a combination of a hypothetical engine,
having a bare engine output of 200 hp and a manual transmission having gear
ratios of 2.55 for first gear, 1.60 for second, and direct drive in high,
indicates the losses of a typical automobile installation. The transmission
efficiencies efficiencies are 94.5, 97, and 99 percent at maximum power in
each gear, respectively."
Since our premise was top speed, the 99% is relevant to the tranny.
The text goes on to say that rear end efficiencies are usually above 95% at
wide open throttle.
My source is "Combustion Engine Processes", McGraw Hill, 1967.
|