Al Gietzen wrote:

Vince:

I have a system quite similar to what your are planning on my 20B in a
Velocity RG.  It has a "filler" bottle which is just a section of 2"

tubing

with a 25 psi 'system' pressure cap and a overflow bottle (with 13 psi
pressure cap) made from a aluminum fire extinguisher that I picked up

free

at a extinguisher service shop. Photo attached.  There are 2 rads, and

an

air bleed line comes from each to the filler bottle.  The line out the
bottom of the filler bottle connects to the line just upstream from the

pump

return inlet.

What is the minimum size (volume) of the overflow bottle?

The over flow bottle is close to 3" i.d. and 11" tall - which is just

under

1 1/2 quarts.  I've measured about 3" of coolant in the overflow bottle

at

ambient before engine start, and about 2" from the top at 205 F coolant.
That is just over 3/4 of a quart (24 oz) volume expansion in a 20B.

This is

operating at low power level, so there is no nucleate boiling in the

engine.

I expect that at higher power there will be more expansion.  Haven't
operated there yet.

How many lbs of pressure should the overflow bottle hold?

I have a 13 psi cap on the overflow bottle.  Note that with pressure

caps on

both, the system pressure can get to the sum of them both. With a 25 psi
system cap, I pressure tested to a measured 40 psi.  So for low altitude
operation I may just keep the overflow bottle cap at the first notch.

Also note that a quality racing cap is need for the system cap.  A stock
automotive cap is likely to leak around the secondary seal before

pushing

the 13-15 psi into the overflow bottle.

With the fill air bleed at the pump oulet, and the bleeds from each rad

tank

there was no problem purging the air from the system.  Interesting that

on

the very first run I did without a prop, a rapid increase in rpm would
generate enough negative pressure at the pump inlet side to draw coolant
back into the filler neck from the overflow bottle.

Al

Thanks for the response.  This is all very good information.

Is there any merit to placing a check valve in the 25 psi bottle between the
high pressure section and the lower pressure bleeder line feeds?   I'm
thinking of something similar to the what you see in the pressure caps where
there is a solid washer (no hole in the center) that drops over the outlet
port where the pressure keeps it shut under normal operating conditions.
You could add a small spring to help hold it shut.  If you have vapor
pressure developing through the bleeder lines the valve would open and the
vapor would pass through to the pressure cap.  Under normal conditions the
check valve would prevent the coolant from reversing flow down through the
bleeder lines to the low pressure side of the system.
Del 

 
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