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Power output required, drag and extremely interesting point.

To: TMHEFFRON@aol.com, spridgets@autox.team.net
Subject: Power output required, drag and extremely interesting point.
Date: Thu, 27 Sep 2001 18:03:41 EDT
You make an extremely interesting point here that I have not seen made 
before.  I only know what bhp the engine in my car makes when I have it run 
up on the chassis dyno so all the numbers I see are RPM specific.  So, the 
bhp the engine produces to drive at 60mph in 3rd gear are in fact much higher 
than the bhp the engine produces to drive at 60mph in 4th gear because the 
bhp is always rpm specific.  I think seeing all my power figures in relation 
to dyno testing has blinkered some of my understanding because.........

On that basis your statement below would appear to be incorrect.  HOWEVER, as 
far as I know the chassis dyno readings are always taken under a full load 
which the dyno operator applies to the rollers (I guess this explains why the 
power figure as indicated on the dyno station jumps about while the rpm and 
speed remain constant (say 5000rpm, 6000rpm, etc).  It would also explain why 
for any given speed (say a steady 85mph cruising in 5th gear) my car consumes 
much more fuel with the softtop roof down than with it up.  What is happening 
is that because the aerodynamic drag increases the engine is required to burn 
more fuel (the max efficiency for any given rpm) to generate optimum power at 
that rpm simply because it needs to.

I guess that the chassis dyno power figure for any given rpm is the max rpm 
the engine produces is not necessarily what is required to drive the car at 
that given speed (as I had previously thought and always puzzled me).

Does all this make sense?  Have I seen the light or was I always thick?  Any 
comment?

Daniel1312

In a message dated 27/09/01 12:50:48 Pacific Daylight Time, TMHEFFRON@aol.com 
writes:

<< It doesn't matter what mod's you've made to the 
 engine, if you need 20 HP to go 60, that's all your engine is called upon to 
 produce when going 60. >>



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