Howard wrote:

Paul:
    Horsepower is clearly defined.....   in terms of force distance and
time.   This is easily translatable to engine horsepower at the flywheel.
Any scales that measure horsepower should not exceed this figure.   They may
be a net horsepower figure which takes into account accessory drag, but
there is absolutely no justification for a scale that produces a figure
higher than this.    The only possible reason for using a scale that gives a
higher figure is to cloud and confuse the issue.

I use SAE horsepower because it pretty much matches these numbers:

    33000 Ft-lbs per  minute = 1 horsepower.

    1 revolution of a 1' radius circle is 6.283'

    33000 / 6.283 = 5252

Thus horsepower = Torque * RPM / 5252

Any result that produces a number exceeding this is simply a lie.... there
may well be clever justifications for the number, but it is still simply a
lie as far as I am concerned.

Unfortunately I don't get Road & Track.   I checked on line for the article
you mentioned.   I presume it is in the current issue rather than the '99
issue.     I would appreciate the name of the article which I presume will
be available on the web before too long if I don't manage to find a copy
somewhere to read.     I drive pickups (old) and fly trikes (inexpensive
home built) ......  The latest Porsche and Ferrari performance figures don't
interest me much which is the thrust of that publication.    I did however
make a note of the fact that there was some stuff on GDI which is of
tremendous interest to me as I feel it is the future of fuel systems.

                   H.W.

vance wrote:

Indeed - horsepower is clearly defined - by Mr Watt as 550 ft lbs per
second = which equates to 33000 ft lbs per minute. However this does not
by itself define "SAE HP" The SAE designation or the DIN of Germany,
define the rules for correcting the values read from dynomometer tests
to standardized conditions of temperature and pressure - usually
resulting in a higher than measured value. Now those standard conditions
do not exist under the hood of your car, nor inside the cowl of your
plane - but engines rated this way compare apples to apples. Now heads
as gray as mine(and particulary those familiar with England and France)
will remember "Taxible","RAC", and "CV" Those were some of the names for
imaginary Bureacratic HP for taxation purposes, and were based primarily
upon total piston area, with some arbitrary BMEP, and piston speed,
(What do you do for a Wankel?).

Incidently gasoline direct injection is of course associated with
stratified charge techniques, and will probably result in less power per
displacement because the stratification will preclude the near uniform
stoichiometric mixture which you can get with good carburation or port
injection, and is a requirement for max power. They will run into the
same "smoke limit" problems that Diesels experience. You can see this in
the power ratings of the new outboards.
Vance

I like to say one HP = 550 lbs ft per second. Makes it clearer.
Force times a speed.

Part of the smoke problem with a diesel is the fuel is "throttled" and
not the air. Consequently when you slam open the diesel "throttle" the mixture is 
momentarily too rich hence the black carcinogenic smoke. It takes a while for 
the RPM to build up and pump more air to catch up with the increased fuel. 

The opposite happens in a gas engine. The air is throttled so you get lean stumble.  
The accelerator pump in a carb is designed to handle that. In a normal 
EFI system the mixture is momentarily enriched until the fuel flow catches up.

In a diesel there is no accelerator air pump so the problem is much
harder to handle. The electric motor boosted diesel turbo charger was invented
to mitigate this problem. There might be a rate switch connected to the accelerator
pedal. Throttle by wire could also be used. When the pedal goes to the metal the 
switch closes and speeds up the turbo charger. With EFI diesel injection this 
can be coordinated to keep the mixture in the right range at all times.  
Old fashion mechanical injection is not going to cut it.

Paul Lamar

 
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