://www.stewartcomponents.com/html/tech_support/techtip5.asp

    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.

    Cross flow radiators are superior to upright radiators because the

radiator cap is positioned on

the low pressure (suction) side
    of the system. This prevents the pressure created by a high-flow water

pump from forcing coolant

past the radiator cap at high
    RPM. As mentioned in the radiator cap section, an upright radiator

should be equipped with

radiator cap with the highest
    pressure rating recommended by the manufacturer. The system will still

force coolant past the

cap at sustained high RPM.

-Tracy Crook wrote:

This is interesting but it flies in the face of what is happening in
the auto industry in some regards. We shall find out shortly about
the flow through thicker rads. It is also not intuitive as aero drag
in a pipe is proportional to the length of the pipe.

Paul Lamar

I agree that there is something odd about the info on the Stewart site.
It's an odd mix of facts,  questionable information and especially, lack of
useful conclusions.

Can there be any logical explanation for their statement that there is very
little difference in airflow resistance between thick and thin rads?  None
is given so I'm not convinced that they really understand the subject.

The statement about thicker rads making less cooling than thin rads  was at
least explained by their explaination (due to less turbulent coolant flow).
They also explained that making the thick rad a double pass restored the
velocity and turbulance but did not say what the net result was.  That would
be the only useful bit of info we could use.  It's missing.

Then they talk about thicker rads with smaller cross section tubes that
restore the turbulance needed for effecient heat transfer.  Again, no bottom
line conclusion is given.  So what's the story, do those thick rads work or
not?

To me this gives the impression that they are trying to impress the reader
with their vast knowledge of the subject but not give out any useful
information.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

I also wonder about the methodology of their test. For example: I don't
know of a car in the world where the rad is not placed directly in
front of the engine block. Everything I have learned from K & M, 
Kayes & London and my own rad test box indicate that this will 
reduce the flow through the rad. 

(also see oblique rads on www.thecarricos/ACRE/)

Did they test thick verses thin rads with this configuration?  

Their results could be explained by:

A secondary effect (if it exist) is that something too close to a thin
rad is worse than something too close to a thick rad. This is because the
exit velocity is probably(?) higher on a thin rad and therefore the 
dynamic pressure build up on the face of the obstruction  and behind the
rad is higher for a thin rad consequently obstructing the flow more.

Paul Lamar
 
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