Calculating HP
 

Hello,

Is there a way to figure out the WHP of a car by knowing the following parameters:
time elapsed when accelerating from 1 speed to another say 30-100 mph using 1 gear
tire/wheel size
gear ratio
final drive ratio
weight of the car.

Thank you,

I would not imagine so. Horsepower is just work done and getting an accurate number requires a torque and an rpm essentially.

Just out of memory though I remember seeing this: http://www.060calculator.com/ . You could use whatever system they use and change a variable to try to get your horsepower figure. Hope that helps.

quarter mile requires driving skill for launching the car, also is grip dependent.
I figured probably logging the RPM from certain speed like say 30mph to 120 mph or so within certain amount of time, and knowing the gear ratios and all other variables, should provide a good HP / torque figure.

Found this formula on a physics forum, rpm data was sampled in milliseconds...

Wheel speed= Engine RPM/(FD*Gear Ratio)
Velocity= WS*Tire Dia*0.0523598
Mass*(Dia/2)*1000*((v2-v1)/(t2-t1))=Nm
Nm*0.001341022/5.3618= Ft.lbs
Ft.lbs*RPM/5252=hp

Simple trap speed in the 1/4 and weight of car will get you ballpark, not sure how accurate you are looking to get.

If you have a way to datalog WOT pulls, there is software out there that essentially dynos your car. I use it all the time.

http://www.bradbarnhill.com/projects/VirtualDyno/

I have an AEM EMS that is able to data log runs, so I just use a nice flat smooth stretch of road that I found, start in the same lane at the same place in 3rd gear from 2k and run it up to 9k. Pin point the pull on the data log, convert it to a .txt file and upload it to the software. The gear ratios are weird for s2ks, so I had to fiddle with some of the parameters to get 'sounds about right' numbers, but the results are very repeatable.

You can get average wheel horsepower, but it's going to be a poor estimate, and max wheel horsepower will be impossible. You're ignoring rolling resistance, drag, human error, and lots of other things.

Formula is as follows


(gear ratio * tire size/vehicle weight) + $75 for two dyno pulls = wheel horsepower

As already mentioned, using something that interfaces with the computer will be greatly more accurate, or the better suggestion of spending $75 (less than two tanks of gas) for dyno pulls makes a lot more sense. However, if you wanted to do something like this, I'm sure that you could come up with a formula and estimates and get a decent number.

Some quick thoughts. Going from 30 mph to 100 mph is a bad idea. You don't such a wide RPM range. Doing that would only decrease the accuracy. Tire diameter, gear ratio, final drive, etc don't matter. You only need RPM and vehicle speed. That would include the effects from all of those.
At higher speeds, you need to better determine how much drag the car has to overcome.
At lower speeds, the car will accelerate more quickly so speedo lag and timing accuracy is more of a problem.
Make sure the road is level.

The easiest way to approximate this would be to have a stock car do a similar pull in similar conditions and compare the elapsed times. From there you can start estimating how much more HP you have. A non-conservative estimate for rolling resistance and aerodynamic drag would be to coast down from the upper speed down to the starting speed.

This site, using a simple equation, does a pretty good job. All you need is vehicle weight and 1/4 mile trap speed. It is a rough estimate, but unless you are driving the proverbial barn door or something super slick, the estimate is very close. Based on the known parameters for my car, this came within 2% of RWHP.

http://www.ajdesigner.com/fl_horsepo...trap_speed.php