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Discussion Starter #1 (Edited)
So I’m cleaning up the passenger side of the engine compartment. New hoses and rust prevention. This little stub pipe that comes out of the head and over to the heater core line on the fire wall. Does anyone have a good reason/idea why it’s 1/4 I’d on one end and 3/8 I’d on the other? In my mind there is enough meat to drill this out to 3/8 to increase flow. Thoughts. Pics attached.
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smaller hole means more pressure?
 

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you're right, the engineers are wrong. drill it out
Heck yeah! You don't need that stinkin' HVAC to work right anyway... 🤪
 

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If you still have the cooled EGR, Leave it alone.If you’re cooled EGR is gone and you have the Rube Goldberg bypass you can plug the stand pipe and drill that heater nipple out to 3/8 and that will drop the pressure in the water jacket back down to a sane level. Craig
 
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Thanks for the array of responses guy's. As most of us know the engineers are always right even when it does not work and you show them. My simpleton mind was thinking more flow at the rear of the block would regulate temps a bit better on the rear cylinders.

Sean
 

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A heater HWS nipple restricted to 3/8 was normal for decades on everything I turned wrenches on. An ME friend, Mike, who also owned a radiator shop, told me it was to keep from popping the heater core before the tstat opened. Poor Mike ended up dieing from lead poisoning... RIP, Mike! Anyway, I believe Cummins went down to 1/4" because of the addition of cooled egr, to push more through the cooler. Well guess what... before cooled egr, the CTDs also had a 3/8 nipple and no such thing as a stand pipe. When I discovered the 1/4" on my deleted 12, I decided to go back to the pre cooled egr configuration (to clean up the stupid, hidious mess) and I used pressure gauges and TCs every step of the way to be sure nothing had too much pressure or hot spots. BTW, jacket pressure was kinda high, with the stand pipe plugged, but drilling that nipple from 1/4 to 3/8 dropped it 10 psi. Craig
 
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Big block Mopars use a 1/2 and 5/8 heater hose nipples. So why didn’t Cummins use at least 3/8 on the 6.7’s?
 

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I think it was to push more water through the EGR cooler. But when I completely plugged the EGR flow and capped the stand pipe the jacket pressure went to 63 psi until I drilled the heater nipple out to 3/8. Craig
 

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There's a lot more to pump flow than discharge pressure and just that nipple won't change flow much, if at all. Something more important to think about, all the HWRs... heater core, egr cooler, welded plate exchanger (on the max cool trucks) and turbo cooling are directed back to the pump return and right back into the engine. Doesn't that give you a warm fuzzy feeling knowing that super heated water is bypassing the radiator? But, the EPA forced most of that.
Back to the jacket pressure... So many guys were worried about too much jacket pressure and blowing out plugs but I don't know how much is too much, it all depends on what the weakest point can tolerate, and drilling the nipple to 3/8 dropped it nicely. 63 is a lot more than necessary but that's all in the pump curve and total head. The thumb rule for setting an industrial closed loop fill regulator is, measure the system height in feet, divide by 2.307 to get the psi to keep the system filled and add about 3 psi for safety margin. That equals the minimum psi to set the fill reg to keep the system filled when the pump is off. Engine closed loops keep the system filled a little different way, radiator cap/recovery tank, but the head calc. is the same. A closed loop only needs enough psi to keep it filled. The pumps on a typical TWO STORY industrial plant system, chilled or hot water, run ~ 40 psi discharge, when running. So, 63 psi is a lot for a little engine. That's in the engineering nitty gritty and pump impeller design.
Craig
 
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