OK, I mentioned in an other thread that I would post my findings about how to calculate whp from your 1/4 mile times and the weight of the vehicle. The following info is based upon what I could remember from physics classes taken 15-20 years ago. I was a bit rusty to say the least, and couldn't figure it out. I was missing a crucial step. Finally I stumbled upon a document from Earle McCaul from the 1970's which not only filled in my missing step, but confirmed my other analysis. That document is attached.
Now, on to the goods.

Short version:
This equation gives the GUARANTEED MINIMUM amount of whp that your car had to lay down in order to get the ET you posted. (Provided the track accurately measured your ET, and you have an accurate measure of the weight of your car - including driver)
The ^3 means "to the power of 3" or cubed. (in other words ET * ET * ET)

This number does NOT factor in the roll resistance of your tires or air resistance.

Long version:
The above mentions a "GUARANTEED MINIMUM" whp measure from ET and weight. How? Well, it is a physical fact that is takes a certain amount of power to move a certain mass a certain distance in a certain amount of time. Well, we know the distance (1/4 mile) and we know the time (your ET) and we should know the mass (mass = weight/32.174) so we should be able to quickly calulate the power required.

There are several advantages to this approach. In this approach we aren't worrying about friction (roll resistance) or air resistance, or even drive train loss. It is simply how much power to move ANY object that far in that time. It doesn't matter if we're talking about a 3240 pound brick or a Crossfire SRT-6 - the physics of motion and power remain the same. We could attempt to calculate the power lost to roll resistance and to air resistance, but the calculations become very complex, and would require a lot more information, some of which is hard to get. Since the power losses due to roll resistance and air resistance are small (not insignificant) we can use the number given from the above equation as a minimum, knowing that you actually laid down more power, which was lost to roll resistance and air resistance.

It is also important to note that this is horsepower as measured at the wheel, NOT crank horsepower. To consider crank horsepower, we would have to know the precise drivetrain loss for your car.

Now back to how we figured this out. We start out with the definition of Power.
Which begs the question what is work?
So what is force?
How does mass relate to weight?
So how do we know what the acceleration of the car was? Well, short answer is that we don't. Are we at a dead end here? No. We can use the tools of physics to replace acceleration in this equation with something else. This uses physics kinematic equations. From kinematics we know that distance equals initial velocity times time plus one half acceleration times time squared.
Well, we know that in this instance that the initial velocity of the car is 0, because all 1/4 races are done from a standing start. So we can eliminate the "Vi * t" portion of the equation, because anything times 0 is 0. That leaves:
We want to solve the equation for a, which is acceleration, so that we can substitute it into our force equation. Solving it for a yeilds:
Since we know the distance (1/4 mile) we can replace distance with 1320 feet (a quarter mile). That gives us:
Now we can replace acceleration in our force equation above:
We can also replace mass in that equation with our weight calculation, giving us:
Which simplifies to:
Now we can substitute our force equation into our work equation.
And we can now substitute our work equation into our power equation.
Now we have to consider units. These equations use standard units of measure, which is why we used 1320 feet for the distance, not .25 miles. Feet are standard measure, not miles - at least as far as physics equations go. The standard English (not metric) power unit in physics is the watt, not horsepower. So we need to include a conversion between watts and horsepower. 1 horsepower = 550 watts. So we need to divide the 108311 by 550 in our equation, weilding:
Putting it in more familiar terms:
Keep in mind that the weight in the equation includes the driver. A Crossfire SRT-6 Coupe weighs 3240 pounds. So if the driver weighs 200 pounds (for a total of 3440), and you ran a 13.550 ET, then you have laid down at least 272.30 whp. Of course, your car would have had more whp than that if you wasted some of it spinning out at the start line. Also you lost some power to air resistance and roll resistance. But you definitely laid down that 272.30 hp to the ground.

 

Attached is a document I found that cleared up some of these calculations for me. The work is from the 1970's and by Earle McCaul, and was published in Hot Rod Magazine in 1973.

 

Try this calculator and see how close it is. It uses weight and trap speed.

http://www.modulardepot.com/hpc.php

 

Also use the drive train loss percentage for wheel hp, crank would be zero loss.

 

I've tried many online calculators, and the problem is that they all give different numbers, and NONE of them tell you where they get the number. That makes the number meaningless, since you don't know what is being considered and what isn't. Most of them don't even tell you if they're calculating whp or crank hp.

That's why I posted the info above, so people could use it and know what they're getting.

The calculator you mentioned above is a good example. What calculations are they using? They don't say. Also, trap speed is not a very good indicator of horsepower. This is because how they measure trap speed. It is an average speed over a specific area of the course - something like the last sixty feet of the 1/4 mile. A final velocity would be much better, but that would have to be instantaneous at the end of the race, and that is hard to do.

 

Exactly. I'm just specifying that crank hp and drive train loss could be calculated, but it is not in the above calculations.

 

I agree that trap speed can be deceiving, but if you have as many timeslips as I do, you should have a pretty good idea of your average. Especially in bracket racing your trap speed will tell if anyone is sandbagging. They are usually pretty close to e.t., and regular track junkies can spot that a 1/4 mile away. Then you factor in weather and it throws everything out the window. Everyday is a whole new set of factors. I'm basically gathering data in 2 month increments because our weather changes gradually. It takes alot more horsepower to run 12.50 at 4000' elevation (DA), than at sea level. It would be cool to show how much horsepower it would take to run let say 12.50 at each elevation. Let's assume with a good driver with no tire spin. Thanks for your hard work with the calculations. It's interesting to see how it's broken down. I'm a calculator guy, who just puts in numbers and believes it must be right. Doh!!

 

I think I'll work on elevation variations next. I know that the NHRA does some king of calculation that evens the playing field between elevations, so I'll start there. That's another nice thing about the above equation, is that we can add on to it safely because we know where it came from and how it works. So we'll see where this evolves to. I get bored sitting in front of the TV, and start puching numbers...

 

Thanks I for one will appreciate the information. The NHRA schedule is a good place to start. The dates for each event are methodical to the best predicatable weather conditions for each stop.