Interesting Formula

[nsxtasy]

The new Road & Track has an interesting formula in the Technical Correspondence column, which can be used to estimate either the horsepower of a car or its trap speed in the quarter mile, when one of those numbers is known but not the other. It's:

BHP = weight * (mph/234)**3

where weight is the actual weight of the vehicle including fuel and driver.

 

[Michigan NSX]

Originally posted by nsxtasy:
...BHP = weight * (mph/234)**3...

What is (mph/234)**3?

I know (mph/234)*3 is mph divided by 234 times 3. What does the second * signify? Was it just a typo or am I not reading it correctly?

------------------
'91 Black/Black

 

[milz50]

It seems that '**' is a symbol for an exponent. This is the same as the '^' character used in formulas.

For example, my NSX has a trap speed of about 101 mph. It weighs about 3250 pounds (??) with me in it.

The calculation with these numbers would yield:

BHP = 3250 * (101/234)^3 or equivalently,
BHP = 3250 * (101/234) * (101/234) * (101/234)

(make sure that you follow the order of operations - in this case divide before multiplying)

Which equals ~261 horsepower. Seems somewhat optimistic.

Obviously, this is just a quick and dirty method. Handy, nonetheless.

 

[naaman]

what is bhp? I see this all the time, but I can never find any explanation as to what the "b" is for. As far as I've seen, bhp=hp, which is why I'm confused as to why there are two terms to represent the same number.

My best guess is that it the term attempts to specify engine output instead of horsepower to the wheels...?

 

[nsxtasy]

Originally posted by naaman:
what is bhp?

BHP is "brake horsepower". It's a specific method for measuring horsepower, and is the most common way of doing so.

The article didn't specify whether it's horsepower at the wheels or at the crank.


[This message has been edited by nsxtasy (edited 06 July 2002).]

 

[Lud]

hp = bhp = net hp at the crank as measured by an engine brake dynamometer (hence "brake hp"). This is what fancy race shops and manufacturers use. They hook an engine and accessories up to it directly.

A chassis dyno is what people like us typically use. It is the machine you strap a car to and measure the force turning the wheels. This measures torque and RPM and calculates rear wheel hp (rwhp).


So, since we are estimating peak engine HP and not rear wheel HP with this forumla, 261 is probably a pretty reasonable estimate.