Paul,

I was planning on using two 1.5" thick heat exchangers (U-shape) with a
one inch gap between them for my cooling arrangement. Now I am not sure
that this has such a large advantage after I read the following from
Stewart Components Racing concerning the newest water heat exchangers on
the market.  One 3" thick unit might be adequate after all. Total
exposed face  on the water heat exchanger equals 900+ cu.in. with a
dedicated forced air K and W duct before and after.

Paraphrasing the following below text:  Concerning only the heat
exchangers themselves,  two in series and stacked as described above
would have more cooling efficiency when X volume of water is going
through them, but the pump would be pushing against increased line
resistance or HEAD and therefore the coolant volume going through the
exchangers would lessen or the total system pressure would have to go
up.   Key cooling requires adequate turbulence within the heat exchanger
tubes and turbulence contributes to increased resistance whether induced
by high flow rates or mechanical turbolators restrictors.  Double pass
radiators in effect lengthen the amount of time a certain volume of
water has to travel through the restrictive pathways.

[I agree these double stacked heat exchangers are good for getting rid of
max BTUs but worse  for maintaining water flow.  Getting the hot water
out of the block is directly effected by water flow rates.  Ergo slow
the rate through the heat exchanger and the flow through the block slows]

Now any  increase in system pressure would only be the result of the (in
effect) longer heat exchanger, but this factor can be significant.  If
we measure pressure resistance through a heat exchanger, and use the
example given by the author, then a  12.5 psi increase through a given
heat exchanger would increase 16X times if we  double the length of
those tubes.

Guys, these numbers seem to be incredibly large and run counter to my
test experience. What gives?  Am I reading the advice incorrectly?  I
would be more incline to believe that line resistance (or pressure)
triples or quadruples, as line length doubles.

Why am I concerned?  Again long term cooling resulting from running a 13B
at power settings around 200 to 220 hp have me concerned that the stock
pump can't flow enough coolant.  Long term to me doesn't mean ten or
even thirty minutes.  It means fifteen to twenty hours. The real point
here is higher pressures involved with moving a higher amount of flow
through the block and heat exchanger.  Everyone can agree that more flow
is a good thing but comes at the expense of a greater wall resistance
and hence psi. and of course horsepower. The limiting design factor then
becomes the ability of components to withstand greater pressures.  Hose
connections and the pressure release cap become critical items.
Placement of the cap and tank become critical as well.  Stewart
Components recommends the cap on the pump inlet side after the heat
exchanger.  That position would see lower pressures then before the heat
exchanger naturally.


http://www.stewartcomponents.com/tech_tips/Tech_Tips_5.htm

Tech Tip #5 - Radiators & External Plumbing

Radiators
Thicker radiators do have slightly more airflow resistance than thinner
radiators but the difference is minimal. A 4" radiator has only
approximately 10% more airflow resistance than a 2" radiator.

In past years, hot rodders and racers would sometimes install a thicker
radiator and actually notice decreased cooling. They erroneously came to
the conclusion that the air could not flow adequately through the thick
radiator, and therefore became fully heat-saturated before exiting the
rear of the radiator core. The actual explanation for the decreased
cooling was not the air flow, but the coolant flow. The older radiators
used the narrow tube design with larger cross section. Coolant must flow
through a radiator tube at a velocity adequate to create turbulence.

The turbulence allows the water in the center of the tube to be forced
against the outside of the tube, which allows for better thermal
transfer between the coolant and the tube surface. The coolant velocity
actually decreases, and subsequently its ability to create the required
turbulence, in direct relation to the increase in thickness. If the
thickness of the core is doubled, the coolant velocity is halved. Modern
radiators, using wide tubes and less cross section area, require less
velocity to achieve optimum thermal transfer. The older radiators
benefited from baffling inside the tanks and forcing the coolant through
a serpentine configuration. This increased velocity and thus the
required turbulence was restored.

Radiators with a higher number of fins will cool better than a comparable
radiator with less fins, assuming it is clean. However, a higher fin
count is very difficult to keep clean. Determining the best compromise
depends on the actual conditions of operation.

Double pass radiators require 16x more pressure to flow the same volume
of coolant through them, as compared to a single pass radiator. Triple
pass radiators require 64x more pressure to maintain the same volume.
Automotive water pumps are a centrifugal design, not positive
displacement, so with a double pass radiator, the pressure is doubled
and flow is reduced by approximately 33%. Modern radiator designs, using
wide/thin cross sections tubes, seldom benefit from multiple pass
configurations. The decrease in flow caused by multiple passes offsets
any benefits of a high-flow water pump........."

Doug in Japan

Yes I read that. All the many factors in heat exchanger are trade offs. It is hard to make an over all best compromise given all the factors. Improve one
and something else gets worse. Read Kays and London Compact heat exchangers if you have the book. I forgot to mention it when you were here. Do you have a copy? We pretty much have to use what is readily available.

Paul Lamar