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Water pump recommendations


Ultra Conservative. ULTRA!
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I read through some content and found a pretty in depth conversation about the fan clutch here:
I like the idea of electric fans, but I'm just not sure it's a good fit for me right now. It could be in the future, though. I have the single thermostat crossover.

What is a good choice for water pump? There are a couple types offered by vendors here and there is also the later HO style. What is recommended?

Right now, I have the bolt-on type (4 bolts), but can change to screw-on if that is better. I had in mind to at least convert to the Duramax fan.
the AC Delco pump from a 2000+ 6.5 will get you all the updates that GM did balancing the flow. This will get you the spin-on style pulley where you will need to use a different clutch. two good options here. If you go with the Hayden thermostatic clutch you will need ether the 9-blade steel fan (I have one available) or the d-max plastic fan. the second option and one of my favorites is the electro-viscous fan clutch coupled with a Hayden electric fan controller. Just lookup the application for a 2010-11 3500 express van with the d-max engine for both the clutch and fan blade.

Credit is due to @ak diesel driver for this as he put this together for us on our trucks. I can get you the part numbers if you need them, everything is available on RA using all AC Delco parts except for the fan controller. this setup will allow you to set what temp you want the clutch to engage and also have ether an override switch in the cab or have it wired into the AC system when the AC needs a fan but the truck might not be up to temp and a normal fan clutch wouldn't help. all of these mods are direct bolt on and no molesting under the hood required, allthough it's highly recommended to plan on installing new motor mounts and possible trans mounts due to the larger fan blade possibly contacting the fan shroud.
Thanks. Good information. I'll be using new motor mounts, as mine are originals, so likely ready to be retired.

Is there a direct comparison between the later pump GM produced vs. the Flowkooler model? The claim is better flow than even the improved design.

My concern is that the Heath Balance Flow system (the one with the ports in the rear of the heads converging into the stainless steel can) was supposed to have been a solution too, but after running that for years, I learned it wasn't really a good solution, so I removed it.
Thanks. Good information. I'll be using new motor mounts, as mine are originals, so likely ready to be retired.

Is there a direct comparison between the later pump GM produced vs. the Flowkooler model? The claim is better flow than even the improved design.
Not a suggestion just pointing out that the Flowkooler is not available anymore.
"Out of stock with no ETA available"?
If you search the flow kooler threads here- you’ll see we spoke about them and I called them up asking about the balanced flow issues. They knew NOTHING of it.
They have their twist on a pump impeller design and are making replacement units for all kinds of engines in cars and trucks with that impeller.

In the threads here you’ll notice when in their ads the LATER ADD about balanced flow. Sales pitching without actual testing anything.

Worse sales pitching to customers that haven’t worked around pumps professionally:
And they still have the thing about high head pressure. It an issue in the laws of physics.
“Head” pressure is just pressure on the output of a pump. But there is positive displacement pumps like your oil pump or fuel pump, and there is non positive displacement pumps like a car water pump which uses an IMPELLER not a PROPELLER. Technically pumps don’t create pressure. They create flow, and the resistance to that flow creates the pressure.

So actual head pressure is measured when you completely stop the flow of the fluid, then while at its median speed measure the pressure. And since this is an impeller- that number will be zero. The impeller is designed specifically to not allow pressure built from the pump. Here is why:
Engine cold, no pressure. Remove thermostat also remove radiator cap, start engine- no pressure only flow. As engine gets hot no pressure can build and at 212 you boil water away. If you add the cap- THE HEAT builds pressure like a pressure cooker on the stove.
You have a 15 lbs cap- and the engine gets hot enough with this engine to make 14 psi all by itself. Now- imagine if all the sudden you add a water pump that can add 1,2,3 psi pressure. POP goes the cap and water on the hiway!

Now think about that thermostat when it is put back in. Engine is cold, thermostat closed. Big 2” radiator hose feeding that water and it has to circulate somewhere or blow a hose off. So the impeller is designed to let the water “slip past” so that it doesn’t build pressure while cold. Most of us have seen the waterpump backing plate and noticed the pitting- its all chewed up like it is rusting away. That is not rust. That is cavitation. When you increase the flow it increases that cavitation linear. But when you increase pressure it reduces the cavitation. Why did gm go from 97 to 127 (not really the 100-130 estimates we round off to) snd not go to say 140 gpm? The cavitation is in reased dramatically. It isn’t hard to change impeller angles and counts. It doesn’t cost ten times more to do that. They had to pick how high they could go without creating a new problem.
And adding back in that thermostat- does that change the pressure that is created by the engine heat? Nope. That pressure when the thermostat is open and the cooling is really needed- what is the system pressure- is it magically higher head pressure for an advantage? Nope- it is what it is from temperature changes.

All engines of every manufacturer has always only used an impeller water pump for this reason. They cannot run a pump that is positive displacement UNLESS they control the speed based on heat and pressure generated. A modern design that would be great is a positive displacement pump, no thermostat, heat sensors in a few spot on the engine as well as water pump that feed a computer to tell the pump when come on and go off as needed for cooling.

Yall remember me talking about the testing I did years back? We removed the belt driven waterpump and used two electric driven pumps to replace it. This was on an engine stand. And we did 2 pumps not one because we didn’t have electric motors that were strong enough to drive a single pump. That electric water pump idea should sound perfect- no wasted energy, better control of cooling- even one side vs the other!
And so long as you use a honda generator in the bed it would work.
Look up electric water pumps that can move 100 gpm and notice the hp they use. You are not driving that on 12 volts anytime soon. You need a Toyota starter motor running non stop for that much power. Water pumps steal a crap ton of power even without the fan. Thats why our old school drag cars had restrictors inplace of thermostats and the electric belt driven pumps- free hp. Just enough cooling to keep it alive then cool it down in the pits. They talk about the 130 gpm now from reading forums. But never learned the actual GM spec of 127 or would have referenced it. Then they never say what gpm their improved unit is. Just “It achieves greater flow rates, higher head pressure and a more efficient flow.” Ok, last claim of more efficient? Lets say sure- because they don’t say more efficient at what drops per revolution while using less fuel? More efficient at moving money from one store into theirs is most likely.

One last thing about their snake oil slinging speak- “color is due to Type II Class II military grade surface coating” ok- plenty of military folks here. Anyone know where to look that up? That is some bs there. I mean- they already are trying to argue against the laws of physics so why not the military nomenclature as well. Look at the pic and see what is meant. Then tell me who here had their impeller wear out as the issue why they replaced their 6.2/6.5 water pump. Instead of the front seal failure that sends coolant out the weep hole so you don’t seize the bearing and have your fan be the giant ninja star of death.

If that coating is all that and a bad of chips- why aren’t they selling it on the backing plate which actually surfers the damage they are trying to protect against.

Look- I am not saying this can not work, or that it can not work better.
But a couple folks said they tried it and had less results than the AC Delco.
And this company has done some shady advertising at minimal.
And they did admit to never having run a 6.5 on-a dyno or in a truck to do comparisons testing. They said they did tests on a small car engine (I can’t remember which right now) and the “technology” proved itself so well they started making pumps for many other engines including the 6.5.

Last take a look at what year vehicles this works on and doesn’t work on. They don’t even know the interchange years.

With the known problems that experimenting with 6.5 cooling has brought about- no way can I afford to blow up an engine verifying what they think will work with what we know works- after GM spent millions trying to figure it out.

It just pisses me off when any company wants to pedal snake oil and and acts like it is king daddy when they haven’t even actually tested it on that engine enough to see their holes don’t even line up. Yeah if you do remember that part- ya might search it - but to be fair that might have been hummer forum not here.
I have one of those flow-cooler coolant puims setting right here. Never before installed.
I just wish that I had done some research before I tried to cure the cooling system woes I had with My truck.
I would send this pump to anyone if they wanted to actually run it, please pay the postage, I`m already out the price of the pump, plus postage to get it here and I do not want to have to cut anymore losses.
I had already replaced the coolant pump on this engine. I needed the truck now, not a week from now, so, I went with the NAPA coolant pump and after doing the reading of the posts it would have been nice to have been able to wait for the Delco coolant pump to get here.
So far the NAPA coolant pump has worked well, even with the temp gauge in the RR cylinder head, that area has never been a digree or two higher than the rest of the engine, I did however, install the coolant restrictor fitting into the coolant pump, along with the Champion radiator and the @Twisted Steel Performance thermal coating to that radiator.
Do all of the recommended upgrades, as has been told about in this forum and Your engine will run at the temperature that it was designed to run at.
The only complaint I have about all of this is, that I never did the reading of the threads and posts in this forum, and, that I did not ask about that flow-cooler coolant pump that now sets in a yellow kitty litter bucket on the garage floor. After all, I paid a big price for such a pretty impeller, I would not want that getting all messed up.
I have actually thought about throwing the pump untio the scrap steel barrel and sending it off to the scrappers, but, I keep stopping myself from doing that, just in case, someone, some day, might develope another see through engine block and want to run the tests on this pump to see if it would actually improve the cooling capacity of the components that I have already installed.
Oh yeah, on fan clutch devices, if You get a Kennedy Diesel fan clutch for the 6.2/6.5 engine. Do not order that during the cooler months, then wait for the warranty period to run out to install it.
I also made that mistake, it worked worser than the wore out fan clutch that I had replaced for this one.
I contacted kennedy diesel about it, explained the situation and was told, too bad, its yours.
I then found out about his bad batch of clutches and the one I was sent fell into that category, so, My bad for not immediately installing that FC and seeing if it was any good. No, I will never do business with KD again just because He knows about his bad batch and would do nothing to alleviate My pain. :laff
Most don't know but there is a factory high flow 4 bolt w/p, the later clutch with the big nut was a assembly line thing, faster than 4 nuts, the pump body is no different.
The last time I did side by side testing of spinon and the H.O. 4 bolt was about 10 years ago. Douglas wouldn’t let it go because he didn’t want to “have to” change everything on his hummer. So he bought both pumps, we bolted them up to my spare 6.5 on the stand and drove them with a 120v motor from his bad air compressor he was replacing.
We rigged up a little surge tank and hose looped as if a heater core. Faked the radiator with hose from my pool. Used the block off plates with threaded holes and put barb fittings with hoses, each one into its own trash can.

I was hoping it would work also- would prefer to had the 4 bolt stay on mine.
It was better than the original 4 bolt that I had removed from my hummer, but still pushed way more water through the driver side.

Then we tried eliminating the second heater hose that the 4 bolt uses and tried that. Again it showed improvement- but still the drivers side gets more flow. The only thing I couldn’t replicate to compare is the restriction fitting for dual thermostats to see what difference that made. But back when we did the testing in the 90s we added the flow meters inline and it showed up instantly.

The impeller: clearance; vane count and angle, passage cut, etc all looked identical between the 4 bolt and spin on. My theory is the heater hose port on the passenger side that is not present on the spin on pumps somehow causes an Eddy and disrupts the flow going to that side. See pic below. Basically water goes easier where there is no resistance and the port close to where it is entering the impeller has that added resistance. Then by the time the water goes fully through the impeller to the other side of the pump it is flowing better to not be disturbed.

If they have updated the 4 bolt since then - awesome. But I would have to see a video of it pumping to believe it.
Here is an interesting read, also.

I'm not sure why this pump seems to be so expensive, but maybe someone here can explain. Same part number Hink gave also.

Based on what I'm seeing from your post, Will, it looks like the spin-on style still flow better and is the only true "balanced" flow maker on this engine based on what you've witnessed.

Twisted Steel Performance, have you been able to acquire any test data to prove one design over the other?

I just noticed in Hink's pictures, he is removing the 4 bolt HO and replacing with the spin on HO
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My crossover has a welded plug in the top of it leftover from Heath's original Balance Flow kit, as I mentioned. With the proper water pump, is there any need for me to plumb anything into this while I'm working through this project?

I guess I'm asking due to what TSP did on his P400 build, although I'm using a stock manifold.
IIRC someone determined that a AC Delco professional grade 99.5+ water pump had the same housing as a 96.5-99 4 bolt water pump and to get the proper housing tweaks for the balance flow one had to stick with the OEM grade Delco water pump.

No idea which thread had that info unfortunately.
Get a GEP water pump from a reputable distributor if you want to be on the safe side of quality. Real4WD.com gets direct from GEP, I'll be sourcing my water pump through them for a future build. Their website isn't the easiest to navigate or up to date so just call and order directly
I don't recall which thread it was but I think it was @WarWagon that did some testing on this in a fleet of trucks back in the day. iinm the winner was the 99+ WP from GM that has the spin on style pulley.

it would be awesome if someone could actually do a flow test on a mock up engine, post a video on youtube for all to see :D Maybe using various pumps for test subjects. I have always though the passengers side suffered on flow rate due to the cab heater plumbed into that side of the engine.

Also wondered if the flow would equalize if there was an additional crossover passage on the rear of the block. IIRC the gasser SBC factory intakes allow flow from one side to the other.

I do recall on the 305/350 engines that there was a certain intake gasket that when installed caused overheating issues when that area was blocked off. I could be wrong though.
Yer talking about if someone would- but we did and did it years ago. Gm tried every configuration conceivable using the rear head ports and heater hose routing. Some folks figure they just wanted to spend beyond ten million bucks just for fun. Chris has a descent theory they swapped just for the speed of assembly line but it doesn’t account for the plumbing alternations, all the different thermostat crossovers tried, etc.

On the mock up- i mentioned i did it already.

Bill heath said he ran tests on a few different ones- i tried two and they were identical. I remember asking here which was the one Heath mentioned bought it specifically.
I'm hoping it's ok to post this. Below is from what is probably a forbidden forum and is not typed by me. I don't believe I can post a link, but here is the body of the post with part numbers. Maybe this is helpful. The conversation has been going on for years and has proven to be controversial and confusing. Below is a portion:

OK, I've been watching this thread and waiting for someone to take it a step farther and bring up the fact that there is TWO spin-on fan AC Delco HO water pumps.
'97 - '99 trucks spec out to a P/N 252776 or 88926290.
2000 and newer 6.5 truck and other applications call for a P/N 251603 or 12456231, 19168609 or 88894035.
Both are HO spin-on fan water pumps. Except the 251603 is $120 more roughly and it carries claims by the over-industrious (in my opinion) 6.5 aftermarket crowd that it is THIS ONE that is balanced!!!

My mind is blown on this thing! I have bought both...the '99 version is still hanging on my truck on the 2009 6.5 Optimizer I just installed (tested) used and turned out to be a dud!
I don't see a difference in the new GEP take off I bought 5-6 years ago from Peninsular and had on my GM block for 3 years. By the way, when I pulled it off the impeller had signs it was rubbing the backing plate and had some radial play in the bearings. So it did not get re-installed with only 20,000 miles on it.

I've been a 6.5 owner for 21 years now continuously and have seen a lot of products and claims made for and about the "performance" upgrades of this and that. There are core improvements that can be made to increase user satisfaction to be sure. Cooling is one of them. This "balanced" water pump thing seems unsubstantiated when it comes to one spin on fan HO water pump to another. Likewise, I can't believe the bolt-on 137 GPM pumps are any different.

Now.. I just need to finalize the deal to get a good (new I hope) long block from the seller I'm working with who sold me the dud, and who shall remain un-named at present, and redo all the gut-wrenching, sleep losing work I just did...AGAIN!!

From this point on, it's me again - TSP, were you every able to cut the different castings apart to compare? It was mentioned just after the post above.

This may seem like a petty question, given the technical aspects of the rest of the conversation, but...
...with the spin-on type pump, how does a guy paint it with the pressed-on pulley already in place? I guess it just doesn't get painted under the pulley.
I honestly have had my own theory on the coolant flow and balance from the pump. I somehow don't think it's so much the balance but more on the flow pattern within the block it's self. there are coolant channels in the block that would allow flow from the left side of the pump into the block up around the first couple of cylinders up into the head and back into the front port where the crossover is mounted. leaving the coolant in the rear of the block and head somewhat stagnant only to get pushed around due to the turbulence flowing in the front portion of the block area.

we all know that like anything flowing, water or coolant will take the path of least resistance. I don't thing there is much in the block or the head that would directly channel the coolant flow fully throughout the engine where it can't just take an easy path. having that said, my theory is it's not the pump that needs more flow or balancing but a better pathway is needed to direct the flow around the furthest two cylinders keeping them cooler.

I think Chris is on to something with his build he's doing showing all the coolant lines he installed under the intake. I am almost thinking that if one was to also relocate the heater hose connection from the crossover to the rear block off plate near the #8 cylinder giving some coolant flow out of the block and head in the rear, this would create a flow path out of the rear of the head eliminating my theory of partially stagnate coolant in that area of the block.

I'm not saying that this will work or have any benefit, just something I would like to test out.