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From: John De Armond
Subject: Horsepower measurements
Date: Wed, 16 Feb 94 22:30:13 GMT

Speaking of dynos, here is the coast-down method of measuring HP.
You really only need a stopwatch and an accurate speedometer but I use a
radar gun for more precision in the speed measurement.

To measure the power available at a given speed, you first time how long
it takes to accelerate from a bit below to a bit above that speed.
Then you time how long it takes to decelerate with the car in neutral
through the same speed interval.  The first measurement measures
how much power over and above drag and rolling resistance is available
for acceleration.  The second measurement measures the total of all
parasitic losses including drag, rolling resistance, transmission
losses, etc.  The sum of these two is the power the engine is making.

If you're interested in the peak power of the engine, select the range 
of speeds to just bracket the RPM where the peak power is made.
It may take some experimenting to find out where this is.  Needless
to say, unless you compensate for it, this test must be done
on flat, level, dry road.  A run in either direction over the same
course and the results averaged is the most accurate.

The first thing to do is to weigh the car.  Many trukstops have 
scales and will weigh your car free with a fill-up.  I use a full
tank of gas because that is easy to replicate.  Include your weight,
of course.

The procedure is simply.  Get your stopwatch ready.  Accelerate 
at wide open throttle and when you hit your lower speed, start the 
stopwatch.  When you hit the upper speed, stop it.  Write down the
starting speed, ending speed and time.  Accelerate a bit more, reset the
stopwatch, put the car in neutral, coast down.  When you hit the upper
speed, start the watch and stop it when you hit the lower speed.
Record the time.

Here are the computations.

Sal 	= Speed, accelerating, lower
Sau     = Speed, accel, upper
Ta      = Time, accel

Sdl		= Speed, decel, lower
Sdu     = Speed, decel, upper
Td		= time, decel

W 		= weight of the car

net_ft-lbsA	=	net ft-lbs during acceleration
net_ft-lbsD	=	net ft-lbs during deceleration

HPa		= HP during acceleration
HPd		= HP during deceleration


0.0336 	= Magic number to unify the measurement units.
550		= ft-lbs per second in one HP

Step 1:  Compute net ft-lbs, accelerating

(0.0336 * W * (Sau)^2 ) - (0.0336 * W * (Sal)^2)  = net_ft-lbsA

Step 2: Compute net ft-lbs decelerating.

(0.0336 * W * (Sdu)^2 ) - (0.0336 * W * (Sdl)^2)  = net_ft-lbsD 

Step 3: Compute HP, accelerating

HPa		= (net_ft-lbsA / Ta) / 550

Step 4: Compute HP, decelerating

HPd		= (net_ft-lbsD / Td) / 550

Step 5: Compute total HP

HPd + HPa = total HP.


A car weighing 3200 lbs.  It accelerates from 60 mph to 80 mph in 5
seconds and decelerates through the same range in 20 seconds.
That works out to 68.4 hp accelerating and 17.1 hp decelerating,
85.5 hp total.  

This is a pretty sensitive test and can show, for example, the
added rolling resistance from driving in the rain with water standing
on the pavement.  Just remember that drag is a cube-law functio of
speed so don't try to "average" over too wide a speed range.

It's that easy.  I have this implemented in an HP95LX Lotus Spreadsheet.
I've also implemented this as an HP41 program that takes advantage
of the clock module.  I can make it available if anyone is interested.

The spreadsheet follows:
begin 664 hp.wk1

From: John De Armond
X-Source: The Hotrod Mailing list
Date: Aug 1992
Subject: Re: measuring power/performance

>awhile back there was a post on a cheap dyno substitute.  I want to increase
>my performance (lower quarter ET) in my 69 charger (440 with 727 a
>uto, 3.55
>gears).  I realize that i don't know squat about the engine hp and
>torquecurve.  From listing to some of the threads here, the first step to
>improving performace is to know what you have BEFORE you start trying
>i currently use the "make a change/make a run" method of testing performance
>increases/decreases. I have done most of the easy performance increase
>stuff (big carb (went from a 600 cfm holly to a 800 cfm Thermoquad), added
>a Jacobs Igniton box, Bosh platinum plugs, tried traction bars but they
>didn't help!, tuned the engine, played with the timing, ect)
>I would like to get the hp and torque curves for the engine as inexpensivly
>as possible, without pulling the engine.  Is there a good way to do this?
>I am starting to get into the higher $ stuff and have a limited budget and
>want to get the most bang for my buck... any and all advice is welcome.

One method is the so-called "coast-down" method which involves accelerating
the car between two speed points, timing the interval and then timing
the interval for the car to freewheel coast back through the points.
A little quick math computes the average horsepower over that range.
The coastdown neatly cancels out a whole bunch of complicating factors
such as grade.  See Hugh Macinnes' "Turbochargers" book for the details.

You really want more detail than this provides in that you want to see
the torque curve at all points along the track.  One expensive way is
to get a G-analyist or RacePack with the accelerometer option and record
your run.  Neglecting air resistance, torque and acceleration are directly
related and proportional.  Of course, you could wait until the mag does
the build-it-yourself data acqisition system article. :-)

Another method is to fit an encoder wheel to the speedo cable (if you have
one) or to the driveshaft (if you don't) and record the pulserate.  A
very easy way is to bring the pulses to the ACK* pin of the PC parallel
port, write some software to timestamp each interrupt generated and
compute acceleration at each instance from the change in rate.  A fairly
simple Excell (or *gag* lotus) spreadsheet can do this and plot the

Another method, my favorite, and the subject of a future magazine article,
is to use a radar to do the same thing.  The easiest method, and one I've
used for years, is to take the doppler output frequency, pipe it to a
national semi frequency-to-voltage converter (LM39somethingoranother)
and use the voltage to drive a surplus stripchart recorder.  The radar
can sit at one end of the track and cover the entire length.  The chart
output is a trace of speed but acceleration can be computed manually
by differentiating the chart at a point of interest.

Even better is to pipe the doppler into a PC and do the analysis as with
the speedo encoder.  K-band doppler is too high a frequency to stick it in
on the ACK* pin but X band will work fine.

If you can't get your hands on a surplus radar gun (check out any hamfest),
get a hobby electronics or ham radio magazine and contact Advanced Receiver
Research.  They sell the GunnPlexer for about $150. This is basically the
RF section of a radar gun and is small enough to fit inside a Cambell's
soup can.

Sorry I can't point you more specifically but I'm doing this kinda on the


From: John De Armond
Subject: Re: How 'bout Electric Shaft Brakes?
Date: Mon, 03 Jan 2000 18:17:00 EST
Newsgroups: rec.outdoors.rv-travel

Alan Pilcher wrote:

> So, I've looked at alternatives and am down to exactly two:
> 1) drag a large tree trunk on a tow line behind the trailer.  Colorful
> and the price is right but it's noisy, unsightly and attracts an
> excess of official attention.  Also, as the rough edges get knocked
> off and the underside of the log gets shined up the coefficient of
> friction goes down and braking force is reduced.  Last thing you'd
> want trailing you down the mountain is a 60' log with a shiny
> underside!
> 2) The Jacobs Electric driveshaft brake.  It's a little pricy but
> looks to be a real gollywhopper of a brake with no noise and very
> little maintainance.  If I understand the explanation properly, one
> could actually dial in the amount of braking force desired so as to
> precisely regulate the downhill speed.  Also, it's completely
> independant of the transmission/torque converter so no heat issues
> with those parts.

The eddy current brake works like a champ.  The only disadvantage is
the cost and once you get past that, it's wonderful.  No wearing
parts and no friction.  And yes, it is variable.  It is basically a
generator with shorted windings.  A DC magnet supplies the field. 
The more field, the more drag.  Simple as that.  The heat is
dissipated in the rotor.  Water cooling is available for those
really BIG  hills.  I have the engineering data on these units
around here somewhere if you need anymore info.

> So, I'd love to hear comments/tips/praise/warnings from anyone who's
> used/is using the electric shaft brake or knows anything about them.
> It'd also be a hoot, I suppose, to hear from anyone who's used Option
> #1.

Er, well a variation thereof.  Back in my hotrodding days I needed a
dyno but I could not afford one and had no room for one so I built
something that would do the job.  It consisted of a small trailer
with a truck axle under it.  Attached to the pinion of the
differential was a sprocket.  This sprocket drove a chain that drove
a surplus airplane propeller (think it was from a DC-3).  The
propeller faced backward and generated thrust against the direction
of travel.  The propeller was variable pitch so all I had to do to
vary the drag on the trailer was to vary the pitch of the prop.  A
strain gauge measured torque and a tach pickup, RPM.  A computer did
the calculations.  All I had to do was latch this thing to my test
vehicle and go driving. It was wonderful for tuning my homemade
electronic fuel injection systems because I could hold any speed at
any load I desired. Needless to say, it generated some interesting
looks, not to mention comments on the CB :-)


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