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All this talk about how an instant infinitesimal rise in water temp can cause a pressure spike in a closed system seems to ignore a lot of stuff. Like why does it go away so fast? Also, since the combustion temps are around 2k deg, the block will change size and shape, to gawd knows what, but it too will not shrink back in an instant and we don't know if the volume of the water jacket shrinks or expands on the first firing. Don't think we can get an answer focusing on incompressible water and ignoring thermally (and pressure) changing the shape of the block. I think the real answer will resolve all of these factors, not just one. In any event, Al's idea of leaving a small air bubble in the highest point in the system is sound. I'll definitely do mine that way ... Jim S.
Ed Anderson wrote:
Hi Tim, I think something like you mention is occurring. In fact, thinking further about it, it may be that the initial heat of the combustion chamber causes the water in the side housings next to the combustion chamber to elevate in temperature almost immediately and expand in a constrained space with no air. Thus as you suggested, giving rise to the rapid pressure increase. Then after a few minutes of running the metal of the block (and other parts) will have heated (and expanded) increasing the coolant volumetric area resulting in more volume and therefore less pressure.
Ed A
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