Formula needed for estimating horsepower from RPM, gear ratio and accel time

[babydriver]

Recently, I decided to purchase a custom tune for my FR-S using the OFT device. As part of the process, I made a datalog of a 2000-7500 rpm pull in third gear. This turned out pretty good for starters, with a quite linear gain in RPM through that range. There also is a time index graduated in tenths of a second. I thought about developing a formula to estimate the average WHP based on the more or less linear relationship between the RPM "curve" and the time index.

I can think of the following factors:

Engine RPM change vs. elapsed time index (obviously)
Overall gear ratio (transmission plus rear end)
Weight of vehicle
Tire diameter

If I can derive the rate of acceleration, then use the weight of the car, the number of turns made by the rear wheels at any given RPM, vs. the time needed to accomplish this, then average torque and horsepower ought to be able to be derived.

Before I go to the trouble of developing a formula, however, I wondered if there is such a pre-existing formula available. Has anyone worked this out already?

Thanks!

[FRS Justin]

It doesn't quite work that way, as suspension efficiency will play a factor. I have all sorts of formulas that work off ET weight mph which will give you a VERY close hp estimate but torque is a whole other animal. I never found away to figure that as gear changes affect the formula. Here is a link to some very good calculators

http://www.wallaceracing.com/Calculators.htm

Look at this one.

http://www.wallaceracing.com/horsepower2.htm

[justatroll]

Just calculate the change in kinetic energy over time.
change_in_energy = 0.5*M*V2^2 - 0.5*M*V1^2. (where M is the mass of the vehicle in kg, and V is velocity in m/s with V1 being the starting velocity and V2 being final Velocity)
This will give you an answer in units of energy or Joules.
The amount of time it took to increase this kinetic energy is in seconds.
So the energy used is Joules per second or J/s (1 Joule per second is the definition of a Watt)
This is now a measure of Power (units of energy per second).


From there you can convert from J/s to hp with.
Horespower = E(J/s) * 0.001341


(Or 1 hp = 745 J/s)
You don't need the engine RPM, or the wheel speed, etc.

[Kodename47]

Virtual Dyno?

[babydriver]

Originally Posted by Kodename47

Virtual Dyno?

Something like that.

If I calculate the result only in third gear, then there is no gear change to deal with. One less variable.

[Ultramaroon]

Originally Posted by babydriver

Something like that.

If I calculate the result only in third gear, then there is no gear change to deal with. One less variable.

+1 to @justatroll. Don't forget to consider drag, or use second gear to minimize its contribution.

[cjd]

Originally Posted by Ultramaroon

+1 to @justatroll. Don't forget to consider drag, or use second gear to minimize its contribution.

Why would his clothes make a difference? Wait, wrong thread?

More seriously - sample rate will play a role here since the lower the sample rate, the longer you need to get good data. 3rd is probably the highest gear most people can wind out on public roads without too much risk...

[mrk1]

Virtual Dyno does what your looking for, however like anything including a real dyno its very easy to manipulate setting to skew the results.

[Kodename47]

Originally Posted by babydriver

Something like that.

If I calculate the result only in third gear, then there is no gear change to deal with. One less variable.

No, I meant Virtual Dyno: http://download.cnet.com/Virtual-Dyn...-75870487.html