Date: Sat, 30 Oct 1999 22:36:04 -0600
Reply-To: John Rodgers <inua@SCOTT.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Rodgers <inua@SCOTT.NET>
Subject: Re: Power Measurement and Comparison of Modifications
Content-Type: text/plain; charset=us-ascii
Tony, this is cool stuff. Glad to know there are those amongst us that can
apply the good principles of physics to everyday applications. I'm about to
rebuild my 2.1L engine. Will follow through on what you have written here
and see if I get any performance gain, as I perceive it. I'm sure others
will appreciate this info, I know I do.
Nice refresher, since my Newtonian Physics classes were 40 years ago.
Thanks
John Rodgers
'88 GL Driver Wannabe
"T. Thompson" wrote:
> 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
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