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L28ET Cooling system layout critique


Zcardude

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I'm looking for some feedback on how my cooling system is currently laid out. I got the car this way and have been tracking down one issue after another and have it running pretty good. One of my mechanic buddies (not a Datsun specialist though) said this layout is fine because it would essentially be the state of things when the heater valve is closed, but I am not sure that is correct as that is not what things look like in pics that I have seen. I am taking my car to the dyno on Thursday so want to make sure things are "right" before I have any issues there as I know dynoing the car will stress the engine to some extent. My Z does not overheat or anything but I have a large arizona Z car radiator and large electric fan, and the temperature does start climbing under load if I do a pull in 3rd higher gear, but comes back down relatively quickly after

 

I am running an L28ET with a P90A head, F54 block, water cooled turbo. I have no heater core, but some of the coolant lines that went to my heater are rerouted to the water cooled center section on my turbocharger. The coolant passage after port #6 on my head is just plugged which concerns me. 

 

Here's a drawing of how things are laid out in my car which I suspect is bad:

 

Untitled-1.png.32be0665bd5fd46ac79e0c343749fcea.png

 

vs this "correct" layout from the coolant system diagram thread:

 

image.png.ef688dfc5b9aff55ed8903e8d4ff5e59.png

 

 

What do people here think? It seems to me that at the least I should put some kind of a T valve in the back of the head, and route the coolant from the back of the head in to the hose going between the upper t stat housing and the water inlet. Has anyone else deleted their heater core and re-used some of the lines for a turbo? How did you do it?

 

Any and all feedback appreciated. Thank you!

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I'm not sure what running your turbo coolant lines like that is supposed to accomplish. As far as I can see, you've got a loop taking coolant from your lower rad hose (the coolest part of the system, but also the lowest pressure) and cycling it... right back to where it started in the water inlet...

 

This configuration makes me think the turbo isn't really being water-cooled at all, because I can't imagine the water circulating through any process except convection, and probably not very well at that.

 

The way I would run it is take coolant from the thermostat housing, go through the turbo, and then return through the water pump inlet. That way, theoretically, the water pump should circulate coolant through the center section of the turbo, and when you shut the car off convection can draw coolant through the turbo.

 

I would leave the plug at the back of the head if you're not running a heater, as this is the hottest coolant in the engine. In the future, you may wish to add the #6 and #5 coolant bypass mod which draws hot coolant from the rear of the head and brings it to the thermostat housing to reduce detonation.

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10 minutes ago, ZHoob2004 said:

As far as I can see, you've got a loop taking coolant from your lower rad hose (the coolest part of the system, but also the lowest pressure) and cycling it... right back to where it started in the water inlet...

 

I think the way it's actually working is that I'm pulling from the thermostat housing and going in to the lower radiator hose. I'll post some actual pictures for better explanation

 

10 minutes ago, ZHoob2004 said:

The way I would run it is take coolant from the thermostat housing, go through the turbo, and then return through the water pump inlet.

 

 

So do you mean actually an inlet on the water pump itself, or do you mean using the lower radiator hose? So that would actually mean tapping a part of the water pump then?

 

10 minutes ago, ZHoob2004 said:

I would leave the plug at the back of the head if you're not running a heater, as this is the hottest coolant in the engine.

 

So this plug is actually okay? I was thinking it might be trapping coolant in the back of the head, and I would be better off allowing it to flow out of the back of the head and back into the cooling system

Edited by Zcardude
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Also, looking at the cooling systems diagram thread a bit longer, I do see that turning off the heater on an earlier Z would essentially block coolant flow from the back of the head so now I am thinking this is likely a fine setup to run on the dyno Thursday. Just trying to cross my t's and dot my i's. :D

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1 minute ago, jonbill said:

Why does the line from the pump inlet to the turbo have water flowing in both directions? 

 

Well I guess I should have scribbled that out, really a lot of that is from the original drawing in the cooling diagrams thread. I don't know the actual direction of flow there so that is likely wrong in the drawing (or right meaning my hose routing is nonsense?)

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Based on the updated diagram I think what you have is about the best you can do right now. I believe the way the coolant is flowing is it's coming out of the thermostat housing, passing through the turbo and looping back to the water inlet, effectively doing the job of the original bypass line.

 

As-is I believe you're taking warmer coolant from the thermostat and looping it back into your water pump, but I don't know where else you could get coolant flow from. Maybe from the block coolant drain near cylinder #6 and return to the thermostat housing? Someone with more experience with turbo L series could probably shed some light on the best setup.

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9 minutes ago, ZHoob2004 said:

As-is I believe you're taking warmer coolant from the thermostat and looping it back into your water pump, but I don't know where else you could get coolant flow from.

 

That actually might explain why my coolant temp tends to climb slowly following load, then relatively quickly drop back to thermostat temp after returning to cruise.

 

Definitely interested in more opinions here if anyone has any

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  • 5 weeks later...

You're fine on that setup. The lines in the same direction on the turbo are actually correct, depending on what state the engine is...

What size is your turbo cooling line? 10mm? or larger?

I'd eliminate the external bypass straightaway, ESPECIALLY if you have larger than 10mm hoses through the turbo. If you are running 15mm hoses, I'd plug the internal bypass on the block (you can JB Weld in the entry hole in the front cover if you remove the water pump.) There is plenty of recirculation through the turbo to prevent cavitation and aid in warm-up with the turbo cooling line.

As to my first statement "depending on engine state"- what you have is flow from the lower thermostat housing when the engine is running and thermostat is closed (this is why blocking the bypasses can be done--the turbo is now your bypass line.) Once the thermostat is opened, there is still more pressure at the lower housing of the thermostat than at the lower radiator hose, so you will get flow through the turbo.

Once the engine is shut off though, what happens (and this is CRITICAL TO TURBO ANTI-COKING) is that the cooler water at the lower radiator hose is thermally siphoned through the turbo as it is heated and rises to the thermostat housing. This will continue as long as the center section adds heat to the water and the thermostat is open. This is why having a 2mm weep hole in the thermostat disc is important---if the engine cools and thermostat closed rapidly...the turbocharger has the potential to boil the coolant, and that weep hole would allow steam bubbles out...or if not turning to steam still allow the hot water rising off the thermal-siphon effect to rise up through the turbo and out to the radiator.

As you have stated, the plug at the rear of the head is fine as it represents the same flow present when the heater core is shut off. Now, if you like, you COULD take that 15mm fitting and run a 1/2" hardline around the back of the engine to the lower thermostat housing directly. This will let water flow off the back of the head which is quite stagnant, and leads to detonation on L28ET's. There is of course the cooling mods you can do...but in a pinch, running that line aroudn to the thermostat housing to get some flow back there helps quite a bit in keeping temperature on the 5/6 cylinders  in-line with what they should be...

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2 hours ago, Tony D said:

You're fine on that setup. The lines in the same direction on the turbo are actually correct, depending on what state the engine is...

What size is your turbo cooling line? 10mm? or larger?

I'd eliminate the external bypass straightaway, ESPECIALLY if you have larger than 10mm hoses through the turbo. If you are running 15mm hoses, I'd plug the internal bypass on the block (you can JB Weld in the entry hole in the front cover if you remove the water pump.) There is plenty of recirculation through the turbo to prevent cavitation and aid in warm-up with the turbo cooling line.

As to my first statement "depending on engine state"- what you have is flow from the lower thermostat housing when the engine is running and thermostat is closed (this is why blocking the bypasses can be done--the turbo is now your bypass line.) Once the thermostat is opened, there is still more pressure at the lower housing of the thermostat than at the lower radiator hose, so you will get flow through the turbo.

Once the engine is shut off though, what happens (and this is CRITICAL TO TURBO ANTI-COKING) is that the cooler water at the lower radiator hose is thermally siphoned through the turbo as it is heated and rises to the thermostat housing. This will continue as long as the center section adds heat to the water and the thermostat is open. This is why having a 2mm weep hole in the thermostat disc is important---if the engine cools and thermostat closed rapidly...the turbocharger has the potential to boil the coolant, and that weep hole would allow steam bubbles out...or if not turning to steam still allow the hot water rising off the thermal-siphon effect to rise up through the turbo and out to the radiator.

As you have stated, the plug at the rear of the head is fine as it represents the same flow present when the heater core is shut off. Now, if you like, you COULD take that 15mm fitting and run a 1/2" hardline around the back of the engine to the lower thermostat housing directly. This will let water flow off the back of the head which is quite stagnant, and leads to detonation on L28ET's. There is of course the cooling mods you can do...but in a pinch, running that line aroudn to the thermostat housing to get some flow back there helps quite a bit in keeping temperature on the 5/6 cylinders  in-line with what they should be...

 

 

Thanks for the critique Tony! I really appreciate the detailed response and the suggestions

 

Currently, the hoses to the turbo are 15mm hoses so I will look in to blocking off the internal bypass on the block. Are there any symptoms that I should expect to be seeing at the moment since I have the internal bypass open still, or is this merely a precaution?

 

I'd not heard previously that re-circulating the rear coolant port into the lower part of the thermostat housing could help with rear cylinder heating issues. I was actually already looking in to having the costly head cooling mods done as I am noticing that cylinders 5 and 6 are definitely slightly leaner looking when I pull the plugs, which I suspect is due to hotter combustion chamber temps. I am going to try re-recirculating the rear coolant port to the lower part of the thermostat housing before removing the head and going through additional headache having it drilled out, etc, and see if this helps. 

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It will help move out stagnant water for sure! It will help till you can get the flow off the top of the combustion chambers proper.
That big hose will give you plenty of recirculation, no need for the bypasses.

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