Dump the heater, keep the fan?

[Stanley]

Yep, now that I see where the leak is I'll fix it. Just need the valve and hoses. That cheapo set of little stubby screwdrivers will come in handy again. And plenty of liquid wrench.

Also plan to do the mod that runs a hose from the heater outlet on the head to a tee on the upper radiator hose, so I need a tee and valve (to run it when needed) in the line before the heater.

BTW, saw an AC evaporator like mine on e-bay for $235, without the enclosure or thermostat line and control. Is that for real? When you could probably put a complete system together from much newer used parts for about the same?

[Zed Head]

I'm no expert on how these engines work but the guys who modify for more power and/or race have shown that allowing coolant to flow at a high rate out of the back of the head back to the radiator will cause imbalanced cooling and increase the chance of overheating. The heater core only bleeds a small amount of the flow that is passing up through the block to the back of the head then forward to the thermostat housing. When a hose is run directly from the back of the head back to the radiator, flow through the head is diverted.

Most recommend blocking the port at the back of the head instead of rerouting it. This forces the coolant to pass through the head as designed, removing heat along the way.

Just passing on some knowledge from other forums.

[Stanley]

Interesting. I'm no expert either. Checked the head temps with the IR thermometer - #6 was hotter than #1 (by about 10-15 degrees) even though the exhaust temp was a little higher at #1. So it appears to confirm the theory that some extra cooling is needed at the back of the head. The point about flow being diverted makes sense too. Maybe use a restrictor or a smaller hose?

A restrictor could also be used inline when the heater hose is bypassed (looped) from the head to the water inlet.

[LeonV]

I'm no expert on how these engines work but the guys who modify for more power and/or race have shown that allowing coolant to flow at a high rate out of the back of the head back to the radiator will cause imbalanced cooling and increase the chance of overheating. The heater core only bleeds a small amount of the flow that is passing up through the block to the back of the head then forward to the thermostat housing. When a hose is run directly from the back of the head back to the radiator, flow through the head is diverted.

Most recommend blocking the port at the back of the head instead of rerouting it. This forces the coolant to pass through the head as designed, removing heat along the way.

Just passing on some knowledge from other forums.

Wha???

I have NEVER heard this.

Shorting the back of the head to the pump inlet creates issues. Creating a path from the back of the head to the radiator only helps to improve things. There's a huge thread on HybridZ detailing this.

[Stanley]

Not to complicate the issue - but it does... my intake manifold heater (hard) line is shot. Was going to just plug the outlet at the lower thermo housing and the end of the tee on the other side of the block, but been reading a number of sources that you need that circulation (while the thermostat is closed) or hot spots will develop and the water pump will suffer. Dunno if it's B.S. At the dragstrip it may take an hour to get through the staging lanes. Last year a dragster blew up and the lanes sat still 45 minutes for the clean-up. So the runs might be with the thermostat closed. Don't want to sit there idling a long time and maybe foul new plugs (with a slightly richer mix for the evening). I like to get the z over a hundred at the strip (though it happens after the light). I'm thinking run a new heater hose around to the back of the tee on the other side. But then I'm putting more hot coolant back into the intake, raising temp after the thermostat opens. Damn. Maybe a restrictor in it?

[Zed Head]

Sorry Leon, that's not the way I interpreted the results. People are porting the driver's side of the head to reroute coolant flow, but nobody is just running a big hose from the heater core supply line to the radiator. There's a dead zone in the head. Bypassing the heater core, wherever you end up directing the coolant, makes it worse. The coolant comes up through the block and leaves before passing over the combustion chambers. That's my take on it. I could be wrong.

Unless all of those guys just missed it and they're drilling all of those holes for nothing when there's a big one already there.

Edit - here is Tony D's post on the comparison of blocking the outlet versus looping (bypassing) it. I may have added my own twist above by inferring (through lazy thinking) that the bypass direct to the radiator would cause the same issue. But that's part of an ongoing separate discussion. This work applies to what you're doing - Looping the Heater Hose - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z

Edited April 9, 2013 by Zed Head

[LeonV]

I may have added my own twist above by inferring (through lazy thinking) that the bypass direct to the radiator would cause the same issue.

Yes, that's exactly it.

The whole reason for the head being hot in the back is that the thermostat is in the front, as is the water pump. The path of least resistance is to the front of the head, thus the back gets less cooling. By creating a path from the back of the head to the radiator, you are working to equalize temps, not the other way around as you infer.

What Tim and other guys are doing is a better solution as they tap into multiple (or "better") spots in the head, but that doesn't make routing a small line from the back of the head to the radiator inherently "bad".

With that said, it will definitely not "increase the chance of overheating"...

[Zed Head]

I can't agree. You're inferring from very little evidence. There's no real difference in flow through the head determined by where the coolant goes, whether it's back to the pump inlet or back to the radiator. In effect, going to the pump inlet pumps less water through the radiator. If the flow through the engine and thermostat was barely enough to remove the heat produced, you could argue that diverting hot water back to the inlet pushes things over the edge. But I haven't seen enough real evidence of why the overheating occurs. Most of the theories suggest that "spot boiling" over 5 and 6 is the initiation of a reduction in cooling capacity overall which then leads to overheating in general. The hole drilling and plumbing is intended to move more water over 5 and 6.

I understand what you're saying I just haven't seen real evidence for it. Even in all of the pages of the head cooling thread on Hybridz. Lots of conjecture, some experiments with hole drilling and re-plumbing and some subtle results, but little understanding/explanation of what's really happening in the engine and head.

If what you're suggesting is correct, then my statement about running the heater core supply back to the radiator directly would be a good idea, and cheaper than the hole-drilling an re-plumbing . Maybe it should be suggested over on Hybridz. It would be a lot easier than what's being done now.

Edit - Sorry Stanley. This topic has generated a ton of discussion on a different forum. I'll stop now.

Edited April 10, 2013 by Zed Head

[Stanley]

No problem, this is good stuff. Probably doesn't make much difference for me - don't have a high power motor, and there's a big difference between 15 seconds down the strip and running above the yellow line for an hour or more on the track. The z never overheats anyway (Aluminum 3-row, oil cooler, shroud & spook). Just doing some plumbing work anyway so figured I'd do the mod while I'm at it. If I do, I'll put a restrictor in the line, they're cheap.

Rather than plug the thermostat housing, I plan to run a hose around to the tee, like the stock system, for better circulation, maybe. Will put a restrictor in that line too, between the tee and the water inlet (after the heater return branches in), so as not to recycle as much hot coolant.

[Zed Head]

You would most likely be fine to just block both passages off completely. I started a new thread with pictures to show how the coolant gets to the heater core supply passage, for mine and Leon's discussion, and anyone else who's interested. The heater core supply passage is really most like an after thought to get hot coolant to the heater core. If the passage is blocked the coolant just flows back up the head to the thermostat with the rest of the coolant.

http://www.classiczcars.com/forums/engine-drivetrain-s30/49177-head-cooling-what-i-talking-about.html#post428557

[darom]

Stanley, if you do decide to install the heater, here is the one I put on my 76:

Vintage Air Compac Heaters 50515-VUH

It hid nicely in the center, I had to fabricate 2 mounting plates. It freed up a lot of space behind the dash.

Regards!

[Stanley]

Thanks darom, I was looking for a small heater. Extra space behind the dash would be good for a hidden sound system. But now that I see it's just the hose connection that was leaking, and that big useless evaporator (that I thought was the heater core) is gone, I'll just get some new hoses and a heater valve, and stay with the stock heater.

Maybe some wd-40 will free up that heater valve....