DeTomaso Mailing List: June 2001, Message #114

Andy,

You wrote:

We're not doing any of the traditional "work" like lifting the water
> or accelerating anything of significant mass.  That doesn't sound like 10
hp
> of work to me.  A fraction of a horsepower sounds sufficient.

I suspect you are an innocent victim of the public school system, the folks
at Dyno Jet, Pantera Tech articles or some combination of all three.

True, the water is not being lifted nor does it need to be accelerated.
But, as with most things, there's no free lunch.  It's going to take some
power to move that water.  How much?

Well, having invested all of 5 minutes with a calculator, here's the way it
looks to me:

Power is force X distance / time.

Using familiar units, that would be lb-in / sec

In the case of the pump, throwing in some arbitrary numbers, say 5 psi
across the pump and 2.5 gallons per minute:

5 lbs / in squared X 2.5 gal / minute X 231 cubic inches of water per gallon
X one min / 60 sec = 48 lb in / sec

Since man long ago decided to call 550 lb - in / sec  ONE Horsepower,  48 lb
in / sec is a bit less than 1/10 a horsepower.

Now again, I only spent 5 minutes on this and never cracked a textbook ( nor
a 40! ) and I'll die of embarassment if I'm off by a factor of 10 or a 100.

But my gut thinks this is about right, because these numbers aren't far from
what an electric fuel pump can do and I would expect that same electric fuel
pump to soak up about 1/10th of a HP.

In conclusion, NO, I don't think 1/10th of a HP is going to pump enough
water to cool that 351.

Simultaneously, I'm not convinced 10 HP can be transmitted to the stock
water pump by a single V-belt wrapped around 90 degrees of pulley.  Not
because I did any calculation, but because I have never seen a 10 HP motor
connected to anything with only a single V-belt wrapped around 90 degrees of
pulley.  So the truth I suspect lies somewhere between these extremes.

Hope this helps, but mostly I hope I didn't make a mistake with those
numbers!

Guy D.

----- Original Message -----
From: Andy Poling <andy@realbig.com>
To: Multiple recipients of list <detomaso@realbig.com>
Sent: Tuesday, May 29, 2001 8:42 PM
Subject: Re: SV: What I learned about cooling systems this week


> On Sun, 27 May 2001, Tomas Gunnarsson wrote:
> > Richard,
> >
> > 1 hp would require around 60 amps, 12*60 is closer to the 736 W equal to
a
> > horsepower (here in Europe). But, strangely enough those pumps seem to
work
> > anyway!
>
> I think maybe this is a case of apples and oranges.
>
> The electric pump impeller can be designed specifically to be efficient at
the
> speed of the electric motor under load (easily predicted).  The mechanical
> water pump's impeller, on the other hand, has to work over a huge speed
range.
> It's design is therefore necessarily a compromise.
>
> Also, the way I reckon it, the only "work" done is the creation of the
head
> pressure needed to force water through the radiator and the rest of the
system
> (i.e. countering the friction of the water flowing through the restricted
> system).  We're not doing any of the traditional "work" like lifting the
water
> or accelerating anything of significant mass.  That doesn't sound like 10
hp
> of work to me.  A fraction of a horsepower sounds sufficient.
>
> Can someone show me where my reasoning is faulty here?
>
> -Andy
> #3822 (making 0 hp at the moment)
>
> 72 Pantera - Rocky              91 Miata - Steve               84 RZ350 -
Sting
>
>
>