[Diy_efi] RE: Timing and dyno pulls

efi student efi.student at sbcglobal.net
Wed Dec 25 06:13:00 GMT 2002 

Exactly the point I was raising.  While I know that the pressure at the =
cap
isn't the same system wide, it does set pressure at a known point in the
system.  Unless I am completely misunderstanding the physics of the
situation.

I agree that a prepressurized system will minimize nucleate boiling =
sooner,
but I don't see how a properly functioning system at operating =
temperature
wouldn't do exactly the same thing.  And certainly I agree with the =
"remove
the cap" test.  Removing the cap after the coolant has reached operating
temperature takes all the pressure out of the system, reduces the =
boiling
point to atmospheric, and the system will never recover without =
additional
coolant expansion to repressurize the system.  Or am I missing something
really basic here?

Lance

-----Original Message-----
From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org] On =
Behalf
Of Grant Beaty
Sent: Tuesday, December 24, 2002 10:03 PM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] RE: Timing and dyno pulls


>But air is compressible. I don't understand what point you were making=20
>about the coolant being incompressible. Not trying to be an ass, I just =

>don't see what your are trying to say.

Your well-maintaned cooling sys has almost zero air correct? Lets say it =
is
zero. So as the car is warming up, coolant is expanding. Since coolant
doesn't compress, any gain in volume would produce an infinite gain in
pressure unless something gave way. In our case, the cap vents and gives
way. The spring pressure in the cap determines cooling system pressure.

Whats wrong with my logic?

Grant Beaty

----- Original Message -----
From: "M. Claywell" <clay0052 at umn.edu>
To: <diy_efi at diy-efi.org>
Sent: Tuesday, December 24, 2002 10:51 PM
Subject: Re: [Diy_efi] RE: Timing and dyno pulls


>
>
>
> On 24 Dec 2002, efi student wrote:
> > Depends on whether it is venting?  If it isn't venting, then why do=20
> > I =3D see a difference in the reservoir level between hot and cold =
on=20
> > every single =3D car
> > and bike I have worked on?
>
> I believe you answered you own question a few lines down.
>
> quote: "Coolant expands when heated."
>
> If we say that the total volume that the coolant and air in the system =

> is constant, and if the coolant is expanding, then the air is=20
> compressing, thus the result is a resultant pressure. No cap on or=20
> leaky hose means you don't get pressure.
>
>
> Seems a bit ludicrous to imply the system
> > doesn't pressurize to the cap pressure, or why system designers are=20
> > so =3D damn picky about where you put the cap in the system so that =
it=20
> > isn't artificially affected by pressure from the water pump.
>
> The cap only relieves pressure, nothing more nothing less. If you get=20
> up
to
> the relief pressure of the cap, you leak some air, which means that=20
> you will lose some pressure without a doubt. Most likely in just about =

> all cases, once you get into a situation where you start venting, you=20
> are
going
> to overheat, unless it was just a fluke.
>
> Where you locate your cap or your pressure relief valve does make a=20
> difference, I agree. The reason is that the lowest pressure in the=20
> system is right before the pump. You want the pressure relief device=20
> to see that pressure. Most race systems have a header tank that has a=20
> tube that is plumbed in to tap in to right before the pump. If you=20
> were to turn the
pump
> off while the system was still hot, there obvioulsly would still be=20
> pressure, but then with the pump on, it provides additional pressure
around
> the circuit, with resultant pressure drops along each part/whatever.=20
> Placing the radiator cap after the pump would place the cap under a=20
> higher pressure, but the pressure the system sees at various points=20
> would be no different. So if the pressure is 35 psi after the pump and =

> 31 before the pump, and you have a 34 psi cap, you are going to be=20
> screwed if you put it after the pump.
>
> > Or even more =3D
> > simply,
> > coolant is relatively incompressible.  Coolant expands when heated.
>
> But air is compressible. I don't understand what point you were making =

> about the coolant being incompressible. Not trying to be an ass, I=20
> just don't see what your are trying to say.
>
> > Unless
> > there is air in the system, the cap must be the limiting factor for=20
> > =3D system volume because the coolant MUST expand when the engine=20
> > starts =3D transferring
> > heat, that means the level in the coolant reservoir goes up, but not =
=3D
> > until
> > the cap pressure is exceeded.
>
> Hook up a pressure guage or transducer close to your cap and watch the =

> pressure. Then vent the cap (if you have one with a venting lever) and =

> see what the pressure does. Better yet take the cap off when it's=20
> about 60 or 70C then put it back on and keep driving.
>
> > Let's press on with cooling system fundamentals.  I've seen the=20
> > radiator
> =3D
> > for
> > an engine producing 1000 hp all day long.  It's a lot bigger than I=20
> > am,
=3D
> > and
> > I'm 6'2".  What vehicle is capable of supporting a radiator that=20
> > size?
=3D
> > None
>
> Depends on how fast it's moving and depends on the coolant flow rate=20
> to a very large degree.
>
> > that I have seen, except one of those Terex dump trucks, but they're =

> > electro-motive so it's apples and oranges there.  I have yet to see=20
> > a production vehicle, car or bike, that had a cooling system capable =

> > of sustaining full throttle for more than a minute without being =3D =

> > overwhelmed. Sure, it's a design decision that all engineers have to =

> > make, how big =3D can I
> > afford to make this radiator, but the point is they all insist that =
=3D
> > higher
> > system pressure gives better heat transfer both from engine to =
coolant =3D
> > and
> > coolant to heat exchanger.  You're saying they're not getting it, =
that
> > cooling system pressure only applies if you are exceeding design =3D
> > pressure?
>
> Nope. There is pressure there. I believe the point that Adam and I=20
> made,
is
> that in an Non Pre-Pressurized system (basically what most people=20
> drive)
if
> you are NOT VENTING with a 16 psi cap, then going up to a higher=20
> pressure cap is not going to make a difference.
>
> I totally agree that pre-pressurizing will definetely help in cooling=20
> in many situations and will help to avoid or reduce nucleate boiling=20
> on
inital
> warm up and at higher temps. Obviously increasing pressure in a=20
> convential sytem will help to reduce nucleate boiling. And of course=20
> you have to
raise
> the pressure to raise the boiling point. However, a convential system=20
> must be heated to provide the pressure. The benefit of a=20
> pre-pressurized system is that you reduce nucleate boiling right away=20
> / at lower temps.
>
>
> > Maybe I don't understand what you are saying, but all the evidence I =

> > =3D have both researched and experienced indicates that higher =
system=20
> > pressures =3D give
> > better heat transfer.  NASCAR claims they like it because they can =
=3D
> > reduce
> > the size of the radiator and therefore improve aerodynamics.  Are =
you =3D
> > saying
> > this is incorrect?
>
> I agree with you, higher pressure should help heat transfer if it is=20
> decreasing nucleate boiling, or it is allowing you to run a much=20
> higher coolant temperature due to boiling point elevation.
>
> Cheers,
> Mark
>
>
>
>
>
>
>
> > Oh, I do understand that an engine at idle is pretty unlikely to=20
> > exceed design pressure unless the water pump is turning so slowly=20
> > that the =3D coolant is boiling in the heads.  Completely different=20
> > world at WOT.
> >
> > Lance
> >
> > -----Original Message-----
> > From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]=20
> > On =3D Behalf Of Adam Wade
> > Sent: Tuesday, December 24, 2002 5:52 PM
> > To: diy_efi at diy-efi.org
> > Subject: RE: [Diy_efi] RE: Timing and dyno pulls
> >
> >
> > --- efi student <efi.student at sbcglobal.net> wrote:
> >
> > > What about the effect of a higher system pressure on localized=20
> > > boiling
> =3D
> >
> > > and surface wetting for thermal transfer?
> >
> > That is a specialized example of the general rule in
> > the post you're responding to.  So IOW, I already
> > covered that.
> >
> > > Isn't a higher cap pressure going to reduce the size
> > > of the steam bubbles around the exhaust ports and
> > > improve thermal transfer?
> >
> > That depends on whether the system is venting=3D20
> > through the lower pressure cap or not.  As I
> > mentioned, if the cap is not venting, then getting one
> > with a higher venting pressure won't make a difference
> > in system pressure.
> >
> >
> > =3D3D=3D3D=3D3D=3D3D=3D3D
> > | Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
> > |   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=3D3D/   =
|
> > | "It was like an emergency ward after a great catastrophe; it  |
> > |   didn't matter what race or class the victims belonged to.   |
> > |  They were all given the same miracle drug, which was coffee. |
> > |   The catastrophe in this case, of course, was that the sun   |
> > |     had come up again."                    -Kurt Vonnegut     |
> >
> > __________________________________________________
> > Do you Yahoo!?
> > Yahoo! Mail Plus - Powerful. Affordable. Sign up now.=20
> > http://mailplus.yahoo.com
> >
> > _______________________________________________
> > Diy_efi mailing list
> > Diy_efi at diy-efi.org http://www.diy-efi.org/mailman/listinfo/diy_efi
> >
> >
> > _______________________________________________
> > Diy_efi mailing list
> > Diy_efi at diy-efi.org http://www.diy-efi.org/mailman/listinfo/diy_efi
> >
>
>
>
> _______________________________________________
> Diy_efi mailing list
> Diy_efi at diy-efi.org http://www.diy-efi.org/mailman/listinfo/diy_efi
>



_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org http://www.diy-efi.org/mailman/listinfo/diy_efi


_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi

More information about the Diy_efi mailing list