I asked this on a different thread but it was sort of an off topic, and not much discussion took place, so I figured i'll make a separate thread.

Has anyone ever thought as to why the connecting holes inside the water jacket are so small? Wouldn't it improve the cooling if the holes were actually larger, thus improving the coolant flow through the engine?

You can clearly see what i am talking about on the left side, where there is more lighting on this photo.



I spoke about it with Ray as we were working on my motor, but never came to any conclusions.
I could assume that Mazda left the holes as they are in order to slow the speed of flow, and induce more turbulence, so that all of the coolant's capacity to absorb heat is utilized, but could it be that it actually may be the reason why the housing developed the tiny cracks around the spark plugs? Because of the insufficient coolant flow?

Another memebr mentioned that it may actually be the opposite of what i thought of, and the smaller holes will increase the speed of flowing coolant...

When I saw that my first thought was to take the dremel tool and hack those holes to a double of the size, but decided to ask around first.
I am wondering if anyone has done this before, and if so were there any improvements?

 

I was the other member, for reference.

rotorocks, do you have a photo, or can you post one, of the holes? Just to make sure everyone understands what's going on?

Essentially, my answer was based on fluid dynamics (without giving the math, 'cause it's assuming way too much to do any good) in that when you've got the a certain flow rate from the pump, a smaller diameter tube will increase the pressure, and move the fluid faster, even though the overall flow (gallons per minute, or whatever unit you want to use) stays the same.

So my thinking is that the smaller holes make the coolant move through faster, which would promote more cooling by circulating the water faster than if the holes were larger.

However, I didn't mention on the previous thread, because I hadn't been thinking about it before... But there's probably a potential that the higher pressure generated by the smaller holes could be what was causing the cracking, and not the higher heat... I'd be surprised, but I guess in theory that'd be possible.
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In my head, I'd want the holes in the block to be as small as the pressure would allow, so you get high-speed fluid moving through, carrying the heat out as fast as possible. But in the radiator, I'd want as large of tubing as possible, to give the fluid the most surface area, lowest pressure (a temperature reducing function right off the bat) & more time to cool down before it got shot back through the hot engine block.

 

probably a combination of structural support and fast flow.

To get the wall to support itself well you would probably have to induce a zig zag which would slow coolant flow too much at high rpms.

 

The answer is actually very simple. Think about where the hottest part of the engine is. It's around the spark plugs. This is where the coolant passages are largest. In order to try to equalize material temperatures around the engine, you need more cooling in some areas and less in others. If all of the coolant passages were the same size, you'd have unequal cooling. This would be bad. You've got the least cooling at the intake ports. As you go around the engine towards the spark plugs, you have small holes in the middle. Air is still cool here. As you get towards the spark plug holes you have the greatest cooling benefit with the largest passages. Heat is a little less on the bottom of the engine than it is at the spark plugs. The coolant jacket in this area has a little less volume than around the spark plugs but much more than the top of the engine. Your heat goes out the exhaust ports and the coolant passages are smaller here again.

Now you know why it's done that way!

 

Brilliant!

 

So do you think the current setup can be improved? There has to be a way to run the engine cooler. Not mentioning the oil delivery system.

 

the is the thing, If I am not mistaken, the holes around the spark plugs were also rather small. Whether it was done intentionally (for greater suppport), or whatever...
Take a look at this pic:



At the plugs side, the jacket is almost shut solid, except for the tiny holes that go through.

 

My 13B engines don't look like that. Maybe that's why we've seen a couple of reports of housing cracking around the spark plug holes.

 

That is what i am talking about.
I remember seeing the cracks around the leading spark plug hole. hair thin tiny cracks. If i am not mistaken Ray has called you about them. And when i looked at the water jackets i was like WTF? I wanted to drill the holes bigger, but chickened out. On the next rebuild (hope it doesn't happen anytime within the next 50K miles ) I am drilling the crap out of that thing, unless someone comes up with a reasonable explanation as to why it is done the way it is done.

 

It's related to turbulent flow. Promoting turbulent flow constantly brings "new" coolant in contact with engine surfaces resulting in much better heat transfer. Large coolant passeges result in a "smoother" traveling coolant. This means it travels in a laminar manner. As a result, the engine is heating up, essentially, the same coolant areas as they pass thru the engine. It's then dependent on the heat transfer within the coolant to move the heat away from the hot surfaces to the rest of the coolant volume.

I don't really have a good analogy but when you put ice cubes into a glass of water, it eventually gets cold. If you put ice cubes in and stir the water, it gets colder faster.

Really hot portions of the engine, like the spark plug and exhaust areas need turbulent flow AND volume.

 

Then drilling a couple of smaller holes in the particion there should promote turbulent flow, and increase the volume ...At least is sounds somewhat logical.

 

You must consider that if the coolant flow is increased then the amount of time the coolant has to "soak up" heat is diminished. Therefore the coolant is not as efficient as it could be.
Slowing the coolant down will allow it to absorb as much heat as possible in a given time and be most efficient. Also increasing turbulence makes more coolant molecules come in contact with the hot surface. Displacing heat faster and more efficiently.
There is a fine line as to the speed and amount of coolant that passes through the engine at a given location to maintain an even temperature around the block.

 

DaveFZR just posted his engine going bad in the issues forum.

He had perfect compression but when they did a pressure test coolant came out of cracks in that area.

 

I have a question relating to the coolant flow around the engine. Can anyone describe the path the coolant flows as it goes around the engine, from leaving the pump to returning back to the pump?

Where/how does it return to the front of the engine after reaching the back? (its too cold out for me to go climb around under my car and look)

I understand the basics of how water cooling works, but I am having a hard time finding information on the path it takes around a rotary. specifically how the rear side housing diverts the coolant.

Would you also be able to include a pictures of the engine with a simple MS paint of lines showing the major flow patterns.

 

to promote turbulence as well as surface area in areas you want more cooling try dremel/machining 'ribs' into the jacket in that area.....

 

yes some have "waffled" this areas--but whether or not that actually helps is still in question. What is NOT in question is just keeping the coolant at a safe temp level---UNDER 225F. In doing that the "cracks" around the lead plugs do not develope. The renasis actually has much better cooling jackets around the plug area than earier models.
OD

 

If I remember correctly RB in the tech manual offered to machine ribs in the colling jacket to promote better cooling in higher HP app's. I will try to find my old manual.