GM 6.5L V8 coolant system is of the constant bypass type, where waterpump circulates coolant into engine block, thru heads into coolant crossover\T-stat housing, thru the 1"dia bypass, and back into the engine block.

This provides the most constant and even thermal distribution within the system.

The original waterpump was rated at 85gpm.
The single t-stat, of dual-valve type, controlled flow to radiator, as well as the bypass flow returned to engine block.
Pump constantly circulated coolant thru engine block via the bypass when t-stat was closed.
As t-stat opened with rising coolant temp, secondary valve plate on t-stat bottom would seal off bypass, such that full coolant flow passed thru radiator.



That system worked well with the 6.2L series, of low thermal output, but the '93 - '96 6.5L turbomotor soon revealed the system shortcomings, by overheating rapidly under load.

The '97 coolant system upgrade consists of revised head coolant passages for the indirect injection combuston chamber and valves, new dual t-stat housing without bypass blockoff, dual Robertshaw hi-flow t-stats, and 135gpm HD waterpump.
The HD pump and constant bypass allows ~75% more coolant flow thru the block and heads.
The larger coolant passages in the heads reduce the hot-spots associated with the indirect injection chambers.
The dual t-stats allow ~15% more coolant flow thru radiator, without resricting or reducing bypass coolant flow.
A higher volume fan flows more air thru the radiator

By adding the D-max 11-blade 21" fan, and a lower-temp fan clutch, coolant temps are easily manageable for any loading, with factory 195deg t-stats.


Regarding t-stat temperature settings and useage -

All city, some hiway mileage - the 180deg may be best choice from frequent starting and heavy air conditioner-use requirements.

Mostly hiway mileage - the 195deg pair will return highest fuel economy.

Why? Heat is power - pull that heat off into the atmoshpere via the coolant, more of the energy in each new fuel charge is wasted into the cooler block and heads, leaving less energy to heat the air used to push the piston down.

Simple economics - keep the heat up for power, but increase the system ability to dissipate heat rapidly when required.


The 6.5L indirect injection combustion chamber - pre-cup - requires high coolant flow THRU the heads, and is the biggest drawback to using proven gasoline engine head-bypass coolant schemes.

Why? Gasoline engine combustion is in the cylinder, between the head chamber surface and piston. Each new intake air\fuel charge bathes the open combustion chamber with cooler temperatures. In the head, coolant passages surround valve and combustion chamber areas, allowing for quick heat removal when required.

Head\block temp is easily managed for any street-use type of loading.

Diesel head presents a flat surface to the cylinder, no valve-shrouding.

The indirect injection combustion chamber cup, about 1.5"dia x 1" thick, resides in a bore in the head, below the injector and glow plug - coolant never touches the chamber structure, itself.

It has a ~1/2" dia inlet hole facing the piston, where the air charge is forced inside by high cylinder pressure, where the piston is a few millimeters below flat head\valve surfaces - no place else for the air charge to go.

But the fresh, cooler incoming air charge does not flow thru that small opening, filling the large volume of the cylinder first.

The chamber heat from the previous combustion event is not quickly dissipated by incoming air charge, or circulating coolant, on the other side of the bore in the head, where the chamber resides.

This is acceptable, from a thermal energy = power standpoint, because the pre-cup, still near combustion temps, allows the fresh fuel\air charge to ignite more easily.

The constant-recirculation coolant system is required by these indirect injection combustion chamber heads.

Reducing coolant flow thru the head, with a gasser-type bypass, has not been reliably proven to be effective.

A temperature probe in each coolant outlet on each head, and one in each indirect CC coolant passage - available, plugged - under each intake port, would allow development of a suitable, reliably non-damaging system for street-use.

For now, the '97 coolant system upgrades have been proven effective and reliable by many people using the upgraded trucks in towing\hauling service.

Doesn't mean the problem does not exist, just that the factory 'patch' works.

That's my story, and I'm stickin' to it............

Thanks for the write-up GMCTD. You have again proven yourself to be an excellent asset to this forum. I had long wondered about the effectiveness of the bypass mod but didn't really have a good understanding of the mechanicas involved. As explained, it makes sense to look to other methods to increase thermal capacity and cooling efficiency.

With the circular pyro probes that sit under the sparkplugs on aircraft adapted to sit under the glowplugs or injectors, you could have an eight cylinder heat display, plus the two in the head. I would like to run this setup on a pre-'96 1/2 w/o cooling upgrades, Then with the HO waterpump,ect, and again with the rear coolant passages modified. With three pre '96 1/2 trucks, I have considered buying the HO water pump when the time for a new one arrives and obtaining the duel thermastats as they come to me.

There is an assumption made that may not be true and would change the theory of discounting the coolant bypass mod. First the crossover housing has a smaller inlet than the block does from the water pump. That difference in area is about equal to the area of the fittings I installed at the rear of my heads. All I'm doing is letting some of the hotter water from the rear of the block go directly to the t-stat instead of going through the head. The volume of coolant going through the head is still the same or very close to it and much higher than with the stock wp. The extra temp gauge at the rear of the head confirms a reduction in water temp and I feel that overall temps do not climb as high as before, although no more than a 5* change on the dash gauge. I'm sticking with my mod that has worked for quite a while now.

The block and heads are restrictive to coolant flow - no passage is a straight shot back to the t-stat crossover, nor is that desirable, as thermal dissipation would be lost.

Flow restriction - in and amongst cylinders, combustion chambers, exhaust ports, etc - is an inherent design of a coolant system, for maximum thermal dissipation.

Flow is into the block, where some coolant surrounds cylinders, some coolant passes thru small holes in the deck between each cylinder to the heads, with total flow exiting the front of the heads to the t-stat crossover manifold - repeat as necessary.

Coolant into the front of the engine is lower temperature than that exiting the cylinder case as it passes to the rear to the heads.
Coolant exiting the front of the cylinder case into the heads is cooler than that from the second, third and fourth cylinder.
Coolant exiting the heads to the t-stat crossover would reflect the sum temperature increase from coolant flow thru the block and heads.

Which is why the Engine Coolant Temperature ECT sender is placed there, and on the passenger-side of center, because of additional thermal consideration due to turbocharger heat radiation, averaging ~600deg.
Passenger-side head can run considerably warmer than driver-side head.

Providing an external path from the back of the head to the crossover, less restrictive than thru the heads, changes the block-to-head flow dynamic.

With less thru flow from the rear, the cooler flow into the head at the front of the block will be effected less by the temperature increase from the other cylinders.

Unfortunately, the gage temperature sender, located in the front of the head in an area of a block-to-head passage, would reflect this cooler front flow, and not the over-all coolant temperature increase, because of the bypass.

A temperature sender at the rear of the engine would reflect coolant temperature before it passed thru the heads, and would, of course, be much lower.

With that in mind, I would assume that assumptions should be assumed at one's own risk.................

You wouldn't happen to have done any research to prove that last point have you? Anyone I have ever talked to about cooling system performance has always told me that the rear of the block is generally the hottest area in the motor. The #1 cylinder has the coolest water around it and blends some of the, with a 3/8" hole in the 6.5, into the head where water has picked up temperature from flowing past all the combustion heat in the head. Each cylinder blends slightly warmer water into slighty cooler water in the head until it reaches the back of the block where a much larger passage allows for a larger volume of coolant to flow through the head. My temp gauge at the rear of my block has shown me that under light loads the front and rear of the engine are nearly equal, but as loads increase the rear temp climbs faster. Relieving some of that hotter water directly to the t-stat balances out the temps and I believe has also reduced overall temps. My research is based on actual application of theory while your's seems to be only based on theory. I have over two years of using this mod and haven't experienced any negative outcomes that many have cautioned me about. I have used it towing at higher elevations with dry, thinner air and above 90* temps. It has definately helped and I see no reason to give it up for some unproven theories of gloom and doom. :)

You can't argue with sucess.

I agree with your assesment - The front of the engine block is coolest, and the rear is hottest, as stated, and for the reasons stated.

The rear of the heads are coolest, and the front hottest, for the reasons stated.
Cracks occur most often at the front and rear cylinders, indicating a problem with flow around the valves and CC in those areas.

The scheme that grape recently posted - inlet at the center side, outlet thru y-manifold across front-rear of each head, would absolutely balance temps in block and across head.

But is it practically do-able for street-use?

The concept of these Forums is to allow folks to reach a better, an informed, decision by having both sides of a concept, or theory, or fix, made available.

I do have a temp-probe setup, as I suggested, only requiring some heli-arc operations to complete and install.

Until then, I will stick with the factory system, with it's inherent ineffciencies.
And, my experimentation has been only with the factory system - my knowledge of the system function precludes trying anything else, with this heavy truck.

Here's another side of the story....

The '95 system overheated to 240deg at steady 55mph cruise, no ac, with the first trailerload I pulled, and would not cool down.
That evening, with same trailer, empty, on the flipside, 70mph, no ac, temps never over 210deg.
Fluid coolers in radiator tanks, 5-blade fan, 85gpm waterpump, single 195deg t-stat with bypass block-off valve.

With only '97 twin t-stat manifold and external fluid coolers , same trailer same load same route, 55mph cruise netted 180deg, ~205 with ac, and ~200deg at 65mph, empty and ac, on the flip-side.
5-blade fan, 85gpm waterpump, 180deg t-stat pair.
Added 7-blade fan later.

As a side note, the big-block straight front axle trucks always run hotter at 70mph, than at 55mph.
The axle sets up a hi pressure area in the engine compartment, reducing air flow thru the radiator.

Slow down, temps drop.

Bernoulli's law was based on a head of pressure forcing a volume of water thru a hole into open air (Q = Area x sq rt of 2gh).
Pipeline measurement uses that formula (Q = sq rt of Pressure x Diff press) for calculating flow throughout the industry, petrochemical and other wise.
Still, a pressure head into an open-ended system.

And, the pressure drop is measured in inches of water, not psi - 27"H2O = 1psi - where head pressures of 650psi and volumetric flow rates of 1.5 million cuft can give differential pressures of 150"H2O to 270"H2O, or 10psi.
Rather insignificant pressure drop when compared to 650psi head.

Fortunately, that cannot be applied here - the head pressure in this system is generated by a pump, of which pump intake is from the other side of the restriction.
That restriction being the engine block and heads, not a round orifice into an open-ended volume.

Surf back up a few posts to the first sentence in Bowtie's second opening gambit for the truth of the matter - an impeller-type pump (and others) flowing in a circuit with no restriction generates no pressure, only flow.

Further, if any volumetric output of the pump is less than the flow thruput of the circuit, any little pressure will be developed due only to the friction of the circuit containment walls - plumbing, and such.

The diameter of the impeller, the span, width and number of vanes in the impeller determine the pumping volume.
Those factors and the impeller circumferential and vane-to-wall clearances in the pump determine the amount of head pressure which will be developed against the restriction - in this case, the engine coolant passages.
The t-stat(s) have a small 'bleed' orifice, which eliminates initial system air bubbles and equalizes system pressure on both sides of the closed t-stat(s).

When max system design-pressure is reached, the coolant bypasses around the impeller clearances to the impeller intake, preventing further pressure increase.

The pump will develop system pressure within 30 seconds of cold engine starting, providing no air is in the system, say, from empty coolant reservoir.

The small pressure increase over system pressure due to coolant expansion from thermal input is easily handled by the Coolant Overflow Reservoir, which does have a finite volume.
A volume easily exceeded when coolant temps are allowed to reach vaporization temperature (steam), and one reason for which the coolant system is pump-pressurized - temperature at which water boils increases as pressure increases.
Anti-freeze additive also raises boiling point, as well as prevents oxidation\corrosion, insuring excellent thermal transference.

The HO pump and dual t-stat crossover manifold increased overall system flow from max85gpm to 130gpm - 75% increase thru the block and heads, 15% thru the radiator.

Having rebuilt many engines bare-block up, I have seen original freeze plugs that externally appear functional, but when knocked out show to be rusted nearly thru, so I am skeptical of the 'freeze plugs popped-out after upgrading to the HO pump' scenario.

And having purchased new factory items which were ruined in production, I am aware that factory testing of the improved coolant system, with resultant popped-out freeze plugs and block heaters, may only have 'illuminated' some ongoing productivity problems.

And '75% improved flow thru block and heads' could also mean that the original flow was 75% deficient.

But, even with the improved flow capabilities, the larger fan and lower thermal cut-in fan clutch are also required to control engine temps under sustained loading, proven time and again.

If the vehicle has the bypass block-off single t-stat, the HO waterpump can 'illuminate' any potential system problems, in short order.

If the vehicle has the '96 single t-stat constant bypass system, the upgrade requires only the pump and\or manifold - the factory installed hoses fit either system.

The upgraded system is an inter-designed package, with intrinsic caveats not to be ignored.

That's my story, and I'm stickin' to it...............