desktop dyno!

Fri Aug 3 20:33:16 GMT 2001

   How are you getting the data, specifically the g's? From a G-Tech or
something similar?
   If so, log the test data from a rolling start at 2000 rpm. Second or
third gear is usually best. Log the g forces from 2000 rpm up to red line.
   From that information, along with tire size and vehicle weight, you can
calculate the horsepower and torque at the rear wheels.

For example.
   Suppose my car is logged by a GTech unit as producing 0.5 g of
acceleration at a speed of 60 mph in second gear.
   Acceleration, expressed as "g" force, is measured in feet per second
squared. 1 g = 32.2 ft/sec^2.
   My car weighs 3400 pounds with me in it. But that is **weight**. The
**mass** of the car is needed, so that is done by dividing the weight by the
acceleration of gravity.
      3400 / 32.2 or 105.6 slugs.
   The "slug" is our English unit of mass. Betcha didn't know that!
   Force is defined as mass times acceleration, so to get the force required
to produce that half g of acceleration, you multiply half a g, or 16.1,
times the mass of the car.
      16.1 X 105.6 = 1700.2 pounds of force
   So, the drivetrain is pushing with 1700.2 pounds of force, measured at
the contact patch of the tire. My tires are P245/50VR16's. That's 245
millimeters carcass width, times 50% of that in sidewall height, divided by
25.4 mm to an inch, plus half the wheel diameter, to get a tire radius of--
      245 X 0.50 / 25.4 + 8 = 12.8 inches, or 1.07 feet.
   This matches the tire maker's claim that the tire's rolling diameter is
25.6 inches.
   To calculate the torque being produced at the tire, multiply the force
times the tire diameter in feet--
      1700.2 pounds X 1.07 feet = 1819 ft-lbs.
   We need to know how fast the tire is spinning. The tire is 1.07 feet in
radius, so it's 2.14 feet in diameter. Multiply by 3.14, and it's 6.72 feet
around the circumference. 60 miles per hour is 88 feet per second. So--
      88 feet per second / 6.72 feet = 13.1 rps or 786 rpm.
   Now (FINALLY), horsepower is RPM times torque divided by 5252.
      786 RPM X 1819 ft-lbs / 5252 = 272 horsepower.
   So, a 3400 pound car with driver, accelerating 0.5 g at 60 mph, is
producing 272 horsepower at the rear wheels, PLUS THE POWER REQUIRED TO
OVERCOME FRICTION LOSSES AND WIND RESISTANCE.
   To calculate losses, go out on your test road (I'm assuming it's flat and
smooth), on a dry day with a calm wind. Speed up to the top speed of your
rolling test above and start loggin g forces. Then push in the clutch and
let it coast, or drop an automatic into neutral. For a manual, do not take
it out of gear. You want the gears engaged for a true reading. Use the log
of decelerating g forces in the same formulae above to calculate driveline,
wind resistance, and rolling losses.
   Then ADD the losses to the previous numbers to come up with a very close
approximation of the engine's flywheel power.

   Or, you can use my new online calculator at
http://www.newcovenant.com/speedcrafter/.

   In actuality, flywheel (engine) horsepower is neat to talk about, but
doens't really mean squat when it comes to performance. It's a great
bragging tool. What's important is rear wheel horsepower, because that is
what is really pushing you down the road.
   Or, you can try the calculator at http://www.bgsoflex.com/auto.html.
Choose the "Roadway Vehicle Dynamometer" link. I've never tried it myself,
so I don't know how accurate it is.

> -----Original Message-----
> From: owner-gmecm at diy-efi.org
> [mailto:owner-gmecm at diy-efi.org]On Behalf
> Of Flanagan, Steve
> Sent: Thursday, August 02, 2001 11:30 AM
> To: 'gmecm at diy-efi.org'
> Subject: RE: desktop dyno!
>
>
> If I have data logged info for my car (G's, RPM, Driveshaft)
> can I calculate
> the HP and Torque curves from this info.
>
> The only problem is we launch the car between 5500 and 6700
> and obviously
> the G's are the highest right at launch, so the G's do no show a true
> representation of where max torque occurs.  Maybe the RPM
> acceleration rate
> can be used for HP and then TQ can be backed out from that?
>
> Does anyone know what I can do to determine where I am making
> max Torque and
> max HP and what the curves should be, or is there not enough
> info (on the
> lower RPM scale) to get this info.  RPM range of pass is from
> 5200 to 8600
> +/- 500 RPM.
>
>
> thanks
>
> steve
>
>  -----Original Message-----
> From: russell quenneville [mailto:russellquenneville at msn.com]
> Sent: Thursday, August 02, 2001 3:07 AM
> To: Gmecm at Diy-Efi. Org
> Subject: desktop dyno!
>
>
>
> I am writing a message to see if I can find someone with the
> Desktop Dyno
> 2000 software, and is willing to share it with me.  I am
> trying to dial in
> my engine and I would like to see where roughly the torque
> and horse power
> is so I can tailor the fuel and ignition curves.
>
> Thank-you....
> Russ Quenneville.
>
>
>
>   _____
>
> Get more from the Web. FREE MSN Explorer download :
http://explorer.msn.com
<http://explorer.msn.com>

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