I have seen as small as 3/4" hose used between rad and pump,supposedly
sufficient for up to 200 HP. Just curious if you had any thoughts.

Pros     : weight ,  less cowl space/more clearance from hot exhaust? ,
tighter bending radius if req and less stress on aluminum rad connections.

Cons    : loss in flow but possibly greater heat transfer per volume of
water pumped ?


IMHO I would not do it. The car manufactures use 1 3/8" and 1 1/2"
hose for a reason. If they can save a penny they will do it.
I suspect it ups the HP consumption of the water pump. I know
Mazda went to a lot of trouble redesigning the water pump on the
RX8 engine doing away with a restriction in the flow.
Nobody uses a thermal couple in the aluminum next to
the combustion chamber so it is hard to evaluate those
kinds of changes. All air cooled aircraft engines
have these thermal couples (CHT) for a reason.
You don't want to over heat the aluminum. With restricted
flow of water to the block you won't know if you are affecting
the aluminum properties or not. The water temp may be no higher.

To get a feel for the problem calculate the area of a 3/4
hose compared to a 1 3/8 hose.

Paul Lamar ...No rotor no motor.
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Or take it to the next step; flow velocity.  At 40 gpm (which you would see
at 6000 rpm) the flow velocity is 27 ft/sec in 3/4" and only 5 ft/sec in 1
3/8".   5 is lower than necessary, but 27 is too high and will result in
some loss of flow and increased pumping power.  Best keep it below about
12-14 ft/sec.   One inch dia gives you about 15 ft/sec; 1 1/4" gets it down
to 10.

One man's engineering judgment . . .

Al (first flight expected in about 3 weeks)

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Good news Al. You have a very nicely done airplane.

Paul Lamar

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