# re :HP formula

```Dan Bocek <dan@di.com> asked:
>Anyone know the formula for calculating HP from vehicle weight and
>instantaneous acceleration?

Here we go.  A few basic Physics relationships are: 1) P = f*v, and f=m*a;
thus P = m*a*v.  You need acceleration, weight of the car (m), and speed
(v).  If you use units of horsepower, weight in lbs., speed in mph, and
acceleration in g's (1 g = 32.2 ft/sec^2), you get:

HP = m*a*v/375.

As an example (one I'm currently working on), take a '95 BMW M3.
w=3180+220=3400 lbs.; a=.34 at 60 MPH.  You obtain approx. 185 HP.  This is
rear wheel HP; to convert to std. flywheel HP, divide by .75 (the
approximate efficiency of the drivetrain), and you get 245 HP, which is in
fact, close the the actual peak horsepower at the flywheel.  For the past 5
years, I have been using a g-Analyst and this basic relationship, along
with a number of SAE correction factors to calculate absolute torque and
horsepower numbers for tuning/modifying my cars.

Terry Donohue
tdonohue@xnet.com
'71 BMW 2002Ti (170 HP)
'90 Audi 200TQ (200 HP)
'95 BMW M3 (250 HP)

```