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What is the pressure in the cooling circuit?

I'm changing from the trombone cooler to a rad style (not stock) and will have to fabricate some custom hoses. Anyone know what maximum pressures are involved in the front cooling circuit? Thanks
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Old 10-07-2017, 04:50 AM
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There isn't much pressure as pressure is resistance to flow.
Remember, the whole line is cooler so keep it all clean.
Bruce
Old 10-07-2017, 11:19 AM
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Thanks Bruce, that's kind of what I figured. And yes, agreed, clean is important for it to function.

Dave
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Old 10-07-2017, 02:07 PM
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Originally Posted by brighton911 View Post
I'm changing from the trombone cooler to a rad style (not stock) and will have to fabricate some custom hoses. Anyone know what maximum pressures are involved in the front cooling circuit? Thanks
Maximum oil pressure would be the setting on the system pressure relief valve/spring.

Without pressure, there is no flow. The cooling passages in an oil cooler/system and the frictional viscosity of hot lube oil pose some resistance. This means you should use oil line material that safely exceeds the above numbers. Do not be tempted to use low pressure, heat or petroleum sensitive plastic/vinyl/polyethylene tubing.

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Old 10-09-2017, 01:42 PM
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No cheaping out on this one for sure. I will using a single wire hose rated at 300 psi with AN fittings. Just about the last thing I want to have fail.
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Old 10-09-2017, 03:52 PM
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I can tell you this. The oil filter has a relief or bypass valve set IIRC around 47lbs/in2. I know that when the oil is cold in my car that the oil will bypass the filter. AHIK.

You need to use only factory or AN fittings. BTW, hose clamps will not pass tech at a track inspection.

You are wise to not cheap it out on one of these oil systems. BAT has metric hose ends that will work on the factory lines.
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Old 10-09-2017, 08:02 PM
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Edit: Please disregard this post as I didn't fully ready the initial question and assumed supply when pressure was questioned.

The oil filter bypass is the pressure drop across the filter element. Do not try to equate this to pressure in the system.

Just referencing the gauge which tops at 5 ATM (73 psi) my car typically pegs the gauge when above idle, so you can assume the system pressure is in excess of 73 psi.
for reference the min oil pressure at 5000 RPM is 5 ATM.

Last edited by 76FJ55; 10-10-2017 at 05:03 PM..
Old 10-10-2017, 07:21 AM
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The pressure in the scavenge circuit is very high when cold. But the key is that this section of the oil path with the trombone loop is blocked on the return side when cold. It's just a "stack" of oil that is waiting to move until the thermostat opens. So there is no flow and thus no pressure. The point I'm getting at, is this part of the oil system isn't under pressure until the oil at the thermostat is just above 180F

But for reference, here's an example of the pressure created by the pump elsewhere when cold. If you don't run a thermostat before the oil cooler, you risk damage to the cooler and other things when the oil is cold and thick

911 3.0L SC oil cooler system.
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Old 10-10-2017, 08:16 AM
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Scavenger side oil pressure.......

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Originally Posted by KTL View Post
The pressure in the scavenge circuit is very high when cold. But the key is that this section of the oil path with the trombone loop is blocked on the return side when cold. It's just a "stack" of oil that is waiting to move until the thermostat opens. So there is no flow and thus no pressure. The point I'm getting at, is this part of the oil system isn't under pressure until the oil at the thermostat is just above 180F

But for reference, here's an example of the pressure created by the pump elsewhere when cold. If you don't run a thermostat before the oil cooler, you risk damage to the cooler and other things when the oil is cold and thick

911 3.0L SC oil cooler system.


Kevin,

Why would the oil pump scavenger side be having very high pressure when cold? From the diagram, oil from the sump area is sucked by the oil pump and directed to the auxiliary thermostat. Where is the restriction point to produce this very high oil pressure? The cold oil by-passes the auxiliary thermostat and goes directly to oil tank. I have not dissected an oil tank and have no idea how the internal plumbing works. Thanks.

Tony

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Old 10-10-2017, 09:29 AM
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No flow doesn't mean no pressure. Differential pressure is what creates flow. If the outlet end of a tube is capped (by the thermostat) the entire length of that section will be at the pressure of the inlet to that section. I don't think the thermostat is specifically designed as a pressure regulating devise, though I could be wrong. I think the "bypass", refers to bypassing the cooler and delivering the oil directly back to the tank. This shorter path will of course require less pressure for the same flow, so assuming the outlet pressure is the same the peak scavenge pressure will be lower during bypass operation.
Old 10-10-2017, 09:58 AM
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I see references above to the pressure relief valve/spring and also pressure as indicated on the oil pressure gauge. Both of these things are for the pressure circuit, not the scavenge circuit. Pressure in the scavenge circuit should be less. A lot of what's pumped out of the engine is air. And aside from the filter and cooler, the oil just has to travel through the oil lines and then dump into the tank, which is vented to the atmosphere.

That said, cold oil means higher pressure (even if not in the oil cooler). I haven't heard of anyone measuring pressure in the scavenge circuit because generally speaking it doesn't matter. If it were me, I'd play it safe and assume the oil lines you get (or have made) can withstand the same 70 psi or so that you see on your gauge. That will be more than they'll probably ever see.

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Old 10-10-2017, 11:22 AM
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Originally Posted by stownsen914 View Post
I see references above to the pressure relief valve/spring and also pressure as indicated on the oil pressure gauge. Both of these things are for the pressure circuit, not the scavenge circuit. Pressure in the scavenge circuit should be less. A lot of what's pumped out of the engine is air. And aside from the filter and cooler, the oil just has to travel through the oil lines and then dump into the tank, which is vented to the atmosphere.

That said, cold oil means higher pressure (even if not in the oil cooler). I haven't heard of anyone measuring pressure in the scavenge circuit because generally speaking it doesn't matter. If it were me, I'd play it safe and assume the oil lines you get (or have made) can withstand the same 70 psi or so that you see on your gauge. That will be more than they'll probably ever see.

Scott
The scavenge side is low pressure in normal operation, due to very low resistance downstream, as oil just flows back to the oil tank through the oil cooler line and front cooler. If the oil line is crimped at some point, pressure would skyrocket to match internal oil pressures in the proximal oil line. The thermostat in the right rear wheel well supposedly has a pressure bypass in addition to a temp bypass. That would make sense, as lots of folks have squished undercarriage oil lines yet have survived.

If the thermostat pressure valve failed (again pretty sure it has one) and you had a squished line, all bets are off...
Old 10-10-2017, 12:39 PM
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Quote:
Originally Posted by brighton911 View Post
I'm changing from the trombone cooler to a rad style (not stock) and will have to fabricate some custom hoses. Anyone know what maximum pressures are involved in the front cooling circuit? Thanks
Len Cummings on here makes nice custom lines. Search for my posts on "mazda oil cooler...."
Old 10-10-2017, 12:41 PM
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Have a look at what I did all those years ago...

Budget Oil Cooling - Myth Dispelled
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Old 10-11-2017, 02:11 PM
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Just because there's a free discharge to the oil tank, don't assume there's little pressure there. And the discharge to the tank isn't truly free. In the 73 to 89 tank, the oil goes thru the filter before it is dumped into the tank to re-feed the engine. But prior to the filter, there's a pretty significant obstruction by that thermostat. If you look at how the oil goes though the t-stat, going thru the temperature regulating unit's windows is not all that smooth. After the regulator, the passage to the return is pretty tight too.

So that's where I think the restrictions are. Plus cold thick oil is a big deal too. Note the thread I linked where Mike Bruns said that was a big issue on the one setup he observed
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Old 10-11-2017, 02:29 PM
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Here's a picture of the thermostat cut open. You can see the path through the thermostat is a bit convoluted. Picture credit goes to Grady Clay, RIP fine sir



And here's some additional details on how the thermostat works

How External Oil Thermostat Works - Illustration

And thanks for the correction by 76FJ55 regarding the pressure at the dead end when the thermostat is closed/cold.

Also note that the t-stat can be in open mode all the time. They do sometimes stick closed or stick open. I had one stuck open on my '87 Carrera and I have to assume when the oil was pretty cold the bypass passage was being tested. I could hear the oil gurgling in the oil lines on the passenger side of the car after startup
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Old 10-12-2017, 09:10 AM
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Auxiliary thermostat re-visited.........

Quote:
Originally Posted by KTL View Post
Here's a picture of the thermostat cut open. You can see the path through the thermostat is a bit convoluted. Picture credit goes to Grady Clay, RIP fine sir



And here's some additional details on how the thermostat works

How External Oil Thermostat Works - Illustration

And thanks for the correction by 76FJ55 regarding the pressure at the dead end when the thermostat is closed/cold.

Also note that the t-stat can be in open mode all the time. They do sometimes stick closed or stick open. I had one stuck open on my '87 Carrera and I have to assume when the oil was pretty cold the bypass passage was being tested. I could hear the oil gurgling in the oil lines on the passenger side of the car after startup


Kevin,

With all the information you have posted above, you should by now realize that there is no flow restriction caused by the auxiliary thermostat at anytime during its operation as you claimed. Find or locate a point or spot within the thermostat that is smaller than the cross sectional area of "A" to cause a flow restriction. You said you think there was flow restriction for the oil flow from the scanvenger pump going to the auxiliary thermostat and interested to know where.

I had done some tests and measurements of the auxiliary thermostats and found no where within it to have a reduction in cross-sectional area along the oil paths starting at point "A" (oil inlet to aux. thermostat). BTW, measure the X-sec. area of the inlet and the oil line/s and compare because I've done my homework. Share your finding.

We could argue and contradict, but at the end of the day, we should still stay as friends. I highly value your knowledge and experience. Thanks.

Tony
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If I realized that, I wouldn't post the things I did? I'm not arguing or trying to contradict the actual areas within the thermostat. I'm simply noting the path(s) the oil must take to get where it wants to go.

This sketch made by JRSIII in the other thread I linked in post #16 is a REALLY good sketch to show what's going on inside the t-stat when cold



With that picture, here's some locations within the t-stat to consider when cold. Cold is the issue we're most concerned with here, since the pressure will drop considerably when the oil heats up. Although there is the restriction of the oil flowing through the cooler but let's not get into that.


1. Note the alignment of the regulator window and this is most notable in the actual photo of the cut thermostat. The oil is encountering an edge and has to deal with this edge after it goes around the annulus of the t-stat casting and then upward 90 degrees to enter the regulator. Wouldn't this cause a pressure increase?

2. As the oil continues up through the regulator, it encounters the projection of the spring seat machined into the cap, and the spring itself. That said, the cap itself is indeed relieved around the seat and also up into the cap. These reliefs allow a more free passage of oil around and into the next annulus here. Again the oil has to change direction somewhat abruptly to exit the t-stat. But the change in direction is well managed by forcing the oil around that spring seat area.

Also worth noting is port/location #9 in the photo, which is the exit to the tank. Note how this diameter appears to grow as the oil exits the t-stat. That's a nice design to help minimize the losses as the oil leaves the t-stat.

Don't get me wrong here. I'm not implying the factory thermostat is a bad design. We'll not find a better thermostat when you consider its inlet and outlet port arrangement and the bypass/overpressure section it contains. I only wish that the t-stat had steel fittings like the later 964 t-stat.

I'm just saying that in the spirit of compactness, the thermostat has some tight areas that appear to create some pumping losses. The only reason I shared any of this is to point out to people that there isn't a free release back to the tank from the engine case oil sump. The thermostat has definitely has some tight turns in it. These turns are not smooth bends like a typical hose fitting. They're rather abrupt turns.

Take my comments for whatever they're worth- just observations and nothing more. I'm not saying my analysis of this contraption is based on flow testing and measurements. Just visual observations and my own understanding, which may be flawed in more ways than one, of how the oil is distributed throughout the t-stat.

Side notes:

I've said it before and i'll keep saying it. Anybody buying one of these t-stats used should very strongly consider opening it (BOTH caps) to replace the regulator and clean the inside. These units can contain a lot of garbage, especially in the bypass section. Oh, and have fun reinstalling the small cap atop the bypass section. That small diameter spring is a really stiff SOB.

Speaking of the caps, consider getting some replacement caps with hex heads. Mostly because it's difficult to get the caps off without nearly destroying them. I have needed to use a large pipe wrench carefully placed on the edges of the cap and a lot of force (heat included as well). The original caps with slots in them are also a cruel joke as far as using a tool for removal and re-installation.

Sure, you can make your own tool with some strong (emphasis added) sheet metal contoured to fit the curved slots, along with some decent welding skills if you want to make a socket. Or you can grind a pair of thick chisels to the contour of the slots and use a wrench on the chisel. That's the tool I made until I came across the caps with hex heads. I say chisels plural because as luck would have it, the slots are NOT the same size on these two caps and therefore you need to make two tools.

If nothing else, at least you can still buy replacement slotted caps for not a lot of money. Or just contact Jimmy T here on the forum who sells some really nice custom machined & anodized hex caps for basically the same price as the original slotted caps, along with other cool things he makes. No affiliation with Jimmy on my part other than being a very satisfied customer.
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Old 10-12-2017, 02:20 PM
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Well this thread is growing into quite the discussion, many thanks to all for the input.
I was going to ask which line was the supply and return for the cooler (more on that) but thanks to Kevin's post re Grady Clay's diagram , I already have my answer.

So time for some pictures of the "plan". You can see the cooler, one of the 12 AN 90 elbows, the 12 AN hose (good for 350 psi) and the adapter (work in progress) to attach the custom hose to the hard lines. What I did was buy one stock cooler flex line, cut off the ferules (much tougher than expected) and now had the barb and female fitting to hard line in hand. The barb was too large for the 12AN hose (by 2mm) so I cut it off and you can see the new barb in picture two. It is a hydraulic hose style that will be welded to the female fitting cut off from the cooler hose. Once I determine the final length, I will have it crimped on at the hydraulic shop. The reason I wanted to know which was the feed and return lines is I want the fan thermostat (brass fitting on the lower right hand side) to be on the exit of the cooler not the infeed side. One feature I like on this cooler is a drain so if you are really OCD, you can drain the cooler during an oil change.

I'll post further pictures once I get it all together, the season is soon drawing to a end so it will be the time to dive into the many planned projects during the downtime.





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Where did the cooler come from?
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