[Diy_efi] RE: Timing and dyno pulls

M. Claywell clay0052 at umn.edu
Wed Dec 25 05:55:23 GMT 2002




On 24 Dec 2002, efi student wrote:
> Depends on whether it is venting?  If it isn't venting, then why do I =
> see a
> difference in the reservoir level between hot and cold on every single =
> 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 compressing,
thus the result is a resultant pressure. No cap on or 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 so =
> damn
> picky about where you put the cap in the system so that it isn't
> artificially affected by pressure from the water pump. 

The cap only relieves pressure, nothing more nothing less. If you get up to
the relief pressure of the cap, you leak some air, which means that 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 are going
to overheat, unless it was just a fluke. 

Where you locate your cap or your pressure relief valve does make a
difference, I agree. The reason is that the lowest pressure in the system
is right before the pump. You want the pressure relief device to see that
pressure. Most race systems have a header tank that has a tube that is
plumbed in to tap in to right before the pump. If you were to turn the pump
off while the system was still hot, there obvioulsly would still be
pressure, but then with the pump on, it provides additional pressure around
the circuit, with resultant pressure drops along each part/whatever.
Placing the radiator cap after the pump would place the cap under a higher
pressure, but the pressure the system sees at various points 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 screwed if you put it
after the pump. 

> Or even more =
> 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 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 =
> system
> volume because the coolant MUST expand when the engine starts =
> transferring
> heat, that means the level in the coolant reservoir goes up, but not =
> 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 about 60 or
70C then put it back on and keep driving.

> Let's press on with cooling system fundamentals.  I've seen the radiator
=
> for
> an engine producing 1000 hp all day long.  It's a lot bigger than I am, =
> and
> I'm 6'2".  What vehicle is capable of supporting a radiator that size?  =
> None

Depends on how fast it's moving and depends on the coolant flow rate 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 a
> production vehicle, car or bike, that had a cooling system capable of
> sustaining full throttle for more than a minute without being =
> overwhelmed.
> Sure, it's a design decision that all engineers have to make, how big =
> can I
> afford to make this radiator, but the point is they all insist that =
> higher
> system pressure gives better heat transfer both from engine to coolant =
> and
> coolant to heat exchanger.  You're saying they're not getting it, that
> cooling system pressure only applies if you are exceeding design =
> pressure?

Nope. There is pressure there. I believe the point that Adam and I made, is
that in an Non Pre-Pressurized system (basically what most people drive) if
you are NOT VENTING with a 16 psi cap, then going up to a higher pressure
cap is not going to make a difference. 

I totally agree that pre-pressurizing will definetely help in cooling in
many situations and will help to avoid or reduce nucleate boiling on inital
warm up and at higher temps. Obviously increasing pressure in a convential
sytem will help to reduce nucleate boiling. And of course you have to raise
the pressure to raise the boiling point. However, a convential system must
be heated to provide the pressure. The benefit of a pre-pressurized system
is that you reduce nucleate boiling right away / at lower temps. 


> Maybe I don't understand what you are saying, but all the evidence I =
> have
> both researched and experienced indicates that higher system pressures =
> give
> better heat transfer.  NASCAR claims they like it because they can =
> reduce
> the size of the radiator and therefore improve aerodynamics.  Are you =
> saying
> this is incorrect?

I agree with you, higher pressure should help heat transfer if it is
decreasing nucleate boiling, or it is allowing you to run a much higher
coolant temperature due to boiling point elevation.

Cheers,
Mark







> Oh, I do understand that an engine at idle is pretty unlikely to exceed
> design pressure unless the water pump is turning so slowly that the =
> coolant
> is boiling in the heads.  Completely different world at WOT.
> 
> Lance
> 
> -----Original Message-----
> From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org] On =
> 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 boiling
=
> 
> > 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=20
> 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.
> 
> 
> =3D=3D=3D=3D=3D
> | Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
> |   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=3D/   |
> | "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     |
> 
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