Date:         Sat, 30 Oct 1999 11:33:44 -0300
Reply-To:     "T. Thompson" <tonyt_eye@NS.SYMPATICO.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "T. Thompson" <tonyt_eye@NS.SYMPATICO.CA>
Subject:      Power Measurement and Comparison of Modifications
Content-Type: text/plain; charset=us-ascii

While I agree that a dyno test is a comprehensive measure
of an engines performance and that the proposed baseline
test and then retesting with the mods is a great idea, I'm
often surprised at how many people will take a manufactureres
claims as gospel without challenging them.

A very simple but effective measure of power can be obtained
by measuring your 0-60 times. Without sounding like a physics
professor the concept is as follows:

1.              Power=Force*Velocity
2.              Force=Mass * Acceleration
3.              Acceleration=2*distance/time^2
4.              Distance=0.50*Velocity*time (Since Initial Speed=0)

Substituting all the above equations gives you

Power=Mass* Velocity^2/Time

Now if you are going 0 to 60 I can make Velocity a constant
And if I stick in a couple of constants so you can work in lbs
and HP (rather than kg, Newtons and Watts)
The formula actually gets pretty simple:

                Horsepower=0.437*Weight/t

Where Weight is in lbs
and t is the time to go from 0 to 60 MPH

Road and Track listed a 4 speed, 1.9litre Vanagon at a test weight
of 3635lbs with a 0-60 time of 18.30seconds.
The power as calculated above is 0.437*3635/18.30=86.80HP
Actual Power was 82 HP.

My 85 Westy with an automatic weighs 1980kg (4366lbs) with 2 persons
aboard and takes 23 seconds to get to 60 which results in 82.95Hp

Now before anyone gets upset remember this is not the same as dyno test
since a whole bunch of simplification and averaging out is used to
calculate this power, the important point to see is that it is easy to
measure the effect of a modification yourself. The trick to this is
measuring the speed with:

1. The same weight (should be obvious)
2. Do the test more than once on the same road in the same ambient
conditions and preferably by averaging the results of 2 runs in opposite
directions to minimize effects of grade and wind.
3. Be honest with yourself about speed maybe put a mark on the
speedometer which indicates the speed you are timing to or bring a
person to time.

Fortunately our vans are quite slow so timing errors are minimized, can
you imagine trying to accurately time a 10% improvement in power in a
911. Note that time, weight and power are all equally dependent on each
other so that a 10% increase in power will result in a 10% faster time.
So will reducing the weight by 10%, I'm looking again at that non
operational 300lb air conditioner back there.

The last comment I'll make is that unless you are drag racing another
van these figures can't be used to compare one van to another. Thats
because we all have different driving styles, gearboxes, tire sizes etc.
The figures can be used to determine if a change has made a difference
RELATIVE to your own baseline. I believe 0-60 times are a good measure
of how you perciive the performance of the vehicle since its represents
the real life situations of accelerating at full power through the
gears.

Tony