[Diy_efi] RE: Timing and dyno pulls

[Adam Wade]

--- Grant Beaty <gbeaty at ufl.edu> wrote:

> I understand if the pressure were to lower, the cap
> obviously cannot regulate it, its just a relief
> valve. But if your cooling system is doing its job
> (no overheating and/or boiling), pressure should be
> more or less the same shouldn't it?

The cap basically limits the maximum pressure the
system can see.  Presumably, it is set to be right
around the ideal spot for the heat generation expected
in that vehicle.  I will make up a term here and call
it "heat load", which would be the total caloric
content of the coolant.  The manufacturer presumes
that the heat load of the coolant will very, very
rarely exceed a certain value, and designs the
pressure relief valve (cap blowloff) with that in
mind.  Obviously, it may well be possible to cause the
system to exceed that heat load under certain
operating conditions, and cause more than a very small
amount of coolant to be removed from the system
through the valve.  At that point, it's acting as more
of a protection system for the hoses than anything
else, I would think.

If you have a vehicle, say an econobox, and someone
decides to prep it for superstock racing, then I can
see where there might be benefit to a higher cap
blowoff pressure, since it would help control
localized boiling of coolant in the cylinder head. 
Moving the coolant past the hot spot more quickly
would also make a difference (and most likely a much
bigger one), and I would focus my attentions on that,
rather than the cap alone (which I would consider a
"band aid" if you were not getting sufficient
performance from the design of the stock system).

Conversely, you could certainly design a cooling
system that moved coolant sufficiently to prevent
boiling except perhaps "micro-boiling" on a surface
layer right in contact with the head (someone correct
me if I'm wrong on the physics, but it seems to me
that heating in a moving fluid should work roughly the
same way drag works with moving air...  A boundary
layer, and then exponentially increasing motion (or
cooling, in this case) as you move away from the
boundary), and has dufficient heat-shedding capacity
that the heat load could not be made to exceed the
design parameter under ANY circumstance, including
total destruction of the motor from metal fatigue from
excessive combustion chamber temperatures.  Excessive
design?  Perhaps, but within the realm of possibility.

>> I'd also point out that if your coolant never
>> exceeds its boiling point at a given pressure and
>> with the flow volume being X, then changing system
>> pressure isn't going to make a difference in
>> localized boiling.

> Wouldn't it always exceed boiling temp in the
> localized hot spots of the block? Obviously the
> total system temp shouldn't....

I think perhaps I was being overgeneral.  When I think
of boiling, I think of a significant vapor area that
prevents heat transfer as an insulator (compared to
the conductivity of coolant), which causes a
significant rise in system pressure from expansion. 
Obviously, there's an exponential curve in there for
boiling and system pressure, based on how much heat
can be moved away from the hot spot at what rate. 
It's the rate that becomes important, and the only two
real variables are flow and conductivity of the
coolant, as far as I can see.

And now that we are completely and thoroughly
off-topic, I can't even THINK of anything topical to
add as a tie-in at this point, but Merry Christmas to
everyone anyway.  ;)

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
|   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
| "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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