I was reading your book, How to Cool
Your wankel, and have
 a question about Fig. 2 on pg 33. The cited maximums of
 length and height are not empirical maximums, but numbers
 used to illustrate a relationship of the diffuser,
 correct?
 C. Smith

 Yes. That statement is based on Fig 12-11 on page 30.
 It was meant to illustrate the impracticality of a
 vertical
 mounted, large frontal area, efficient heat exchanger
 mounted
 under the cowl.

 A: You need to cool the engine at take off and climb out.
 B: The dynamic pressure forcing air through a heat
 exchanger
 at 100 MPH is not much. (.18 psi)
 C: Therefore you need a thin heat exchanger with large
 frontal
 area to keep the airflow volume and speed through the
core
 high.

 Since you need a 500 to 600 cubic inch thin heat
exchanger
 to cool the water it must have a large frontal area.
 A 24 inch by 24 inch by 1 inch thick heat exchanger
 is 576 cubic inches. Water cooling is 2/3rd total
cooling.

 It is hard to find that much unobstructed frontal
 area under the cowl. You will also need a long duct to
feed
 it.

 According to Fig 12-11 that duct must be 2.5 times
 as long as half the core height or 2.5 times twelve
inches
 or 30 inches long. Not practical in most cowls.

 You also need about 12 inches clear space behind the core
 or the air flow through the core will be restricted.
 Fig. 12-13 through Fig. 12-16.

 That is why I recommend the Kays & London
 wedge shaped diffuser on page 36 Fig 4. The restriction
 on the back side still applies.

 BTW I have a limited special on my book.  Send $30
 plus $5
 handling and mailing cost to:

 830 East Santa Maria St.
 Suite 303
 Santa Paula CA
 93060

 Cash only.
 Sorry U.S. and Alaska only.

 Paul Lamar


Has a particular model of two pass heat exchanger been
established as the weapon of choice?
C. Smith

Howe is pretty good as it is all brazed and not epoxied.
Unfortunately two pass rads are now hard to find
but it is easy to make one if you can find some one to
tig weld.

I see Racing Beat now has a modern, thin, large area
core for rotary engines like a late model Dodge pick up
trucks.
http://www.racingbeat.com/FRmazda4.htm

You are almost better off going to a rad shop and buying a
thin Dodge
truck rad and remove the plastic tanks and weld on your
own
with a two pass design.

Find a tig welder in your neighborhood and give him a thin
.032" or .049"
aluminum test piece and see how he does. Then give him the
rad job.

Paul Lamar

Bud Warren had an article in EAA Experimenter on converting a
single pass radiator to a three pass unit.

http://www.eaa.org/experimenter/articles/2009-06_radiator.asp


Tom

--------------------------------------------------------------------
Snipped from the article:

HOW IT'S DONE
For this illustration we are using a new, all-aluminum,
Chevrolet-style cross-flow radiator measuring 31 inches wide by
19 inches high by 3 inches thick, which can be purchased for
around $200 from a racing speed shop. Two good online sources
for off-the-shelf aluminum radiators suitable for this
modification are StockCarParts.com and SummitRacing.com.
Two-pass radiators can be purchased off-the-shelf, but you can
achieve superior cooling by modifying your own stock radiator to
a three-pass by following this procedure. So if you have access
to a shop, are fairly handy, and are a pretty good aluminum
welder or have a buddy who is, you can save money by doing the
modifications yourself.

To understand the process, consider that prior to these
modifications, water will enter the radiator through the inlet,
trickle down into the water reservoir on the inlet side, and
make its way rather passively (but still under pressure from the
water pump) across the core and into the water reservoir on the
outlet side of the radiator.

Our objective with this modification is to cause as much of the
water as we can to route purposefully through specific sections
of the radiator core once, twice, then a third time before it
exits the outlet and back into the engine. The theory is that
more of the water will spend more time in the core, which will
exponentially increase cooling capacity. It is important to
understand that when making this modification, a larger than
normal radiator is needed, as this modification will add coolant
flow restriction.

GETTING STARTED
To create your own three-pass radiator, you will need to install
two small baffles into the water tanks: one in the inlet side
and one in the outlet side. To start, count the cooling tubes in
your radiator, marking the sections into thirds. Mark your first
cut at one-third of the way below the inlet and the second cut
at one-third above the outlet.

End of quote.

-----------------------------------------------------------------

All well and good but don't forget we want a thinner rad so we can get
more air through it lower speeds. We also only need two pass
just to get the inlet and outlet on the same end of the rad.

Bud mentioned coolant flow restrictions but the up side to that is
higher speed water in the passages which enhances the heat transfer
from the coolant to the metal. Like almost everything else in cooling
aircraft engines there are up sides and down sides so sometimes you
will not notice a difference when you make a change. One must fall back
on basic principles to figure out what works best. It took Kays & London
30 years to write the book "Compact Heat Exchangers". In effect it took
me ten years to learn enough about it to write the book :)

MANY car racers are still not getting it.

Paul Lamar