I wouldn't call it a false reading.  The reading at the outlet side of
the pump would be the highest reading you would see in the system.  The
pressure cap is near the inlet side of the pump, so that pressure would
be the lowest pressure you would see.  Both are correct pressure
readings.

If you don't have any air in the coolant system, there is an initial
pressure spike caused by the expansion of the water due to heating.
Pressure will quickly go to the cap pressure and a small amount of
water will be pushed into the overflow tank.  If you reduce power and
the water cools, pressure can go negative which will pull some
coolant back from the overflow tank.

The suggestion to use an expansion tank to measure the pressure is a
good one IMO.  If you have a tank with some air in it, the pressure in
the system will be related to the vapor pressure of the coolant which
will give an indication of the coolant temperature.  I don't think this
is a pure measurement in that the volume of air will be relatively
small compared to the volume of the water, so there will still be some
effect from the expansion and contraction of the water with
temperature.  So it's still necessary to learn what the water pressure
reading mean in different operating regimes.

I'm not sure of the internal water path, but it sounds like you have it
connected correctly with the water from the radiator going in at the
bottom and water out from the pump going to the radiator.

Bob W.



On Tue, 2 Oct 2007 13:12:49 -0400
"Jeff Whaley" <jwhaley@intldata.ca> wrote:

> Bill, where are you measuring your pressure? If you measure in the block or
> at the pump outlet you'll get unusually high, false readings.
>
> With a 22 lb cap you can't actually achieve 27 psi, as the cap will burp it
> to the over flow and reduce the pressure.
>
> I suggest you measure the pressure from an expansion tank connected to the
> inlet side of the pump.
>
> The stock Mazda cap is only 13 lbs . there is no reason to see even 22 psi.
>
> JWW
>
>  
>
>  
>
>  
>
>   _____  
>
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Bill Bradburry
> Sent: Tuesday, October 02, 2007 12:39 PM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Coolant Water Pressure
>
>  
>
> I just recently got my Renesis started again after finishing my cowl.  I
> seem to be getting very high coolant pressures.  I can only run the engine
> about 10-15 minutes before hitting the redline at 210*.  My water pressure
> is at 27 Lbs at that time.  I only have a 22 Lb radiator cap, so I assume
> that I am blowing into the recovery tank, but I have not confirmed that.  My
> oil temp has never exceeded about 165*.  It might have gone higher if I
> could have run longer???
>
>  
>
> This whole water pressure thing has me a little baffled.  Since this is a
> closed system and the only way pressure can build is due to the expansion of
> the coolant after heating???, I am confused by some comments that have been
> made from time to time.  I remember something that Tracy said about his
> pressure would build for a time, then go to zero.  It seems to me that the
> pressure should correlate to the temp pretty closely since it is a closed
> system??
>
>  
>
> Can someone enlighten me a little on the science of this pressure?  It seems
> to me that there could be some pressure build up on the positive side of the
> pump, but it would go negative on the suction side, so the net effect of the
> pump should be close to zero??
>
>  
>
> Also, my Renesis had only 1800 miles on it when I bought it, so I did not
> have to tear it down.  As a result, I am somewhat in the dark as to how the
> water flows through the system.  Could someone help me with that?  I had to
> remove the thermostat tower for height clearance , so I made an adapter
> plate that takes water from the top outlet of the pump and sends it to the
> radiator (double pass), then from the radiator, it returns to the lower
> inlet of the pump.
>
>  
>
> Thanks,
>
> Bill B
>
>
>
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