The high output 130gpm water pump on a <95 single stat x-over is just barely slightly better than running the from the factory 80gpm pump. Simple mathematics of Bernoulli's Principles of Hydraulic Dynamics.
Since fluids (such as water) are uncompressible, the smallest restriction in the system limits the volume that can pass through that point. Trying to cram more volume through that point (higher flow water pump) only results in a slightly higher volume passing through that restriction, and HIGHER back pressure behind the the restriction (as high as 30-60psi with a partially open/stuck shut t-stat OR a t-stat not designed to flow the volume of water the pump is designed to flow) - that will actually SLOW the movement of fluid behind the restriction. Which will keep heat in the coolant behind the pump, as well as limit the amount of even now hotter coolant that makes it to the radiator.
The point is, you want to balance the flow rate of the pump (gpm) to the flow rate of the most restrictive point in the system - which is always the thermostat - to limit back pressure to a minimal amount (a little back pressure is good, it allows more heat transfer to the coolant it's in contact with longer, too much back pressure is bad because the hot coolant has nowhere to go).
With a thermostat, the flow is NOT determined by the diameter of the flange of the t-stat, but the diameter of the valve seat inside the t-stat and how high the valve is lifted off the seat at full open. The actual area for the coolant to flow through is the circumference of the valve seat x's the height it is lifted. So, circumference is the diameter multiplied by pi.
Let's use this pretty easy example, typical for say a 2.2L gasser:
Circumference x Height = Area
Circumference = Diameter x pi
Valve seat diameter -> 1.25"
Lift height wide open -> 0.25"
So, [1.25" x 3.14(pi)) x 0.25" = Area
3.925" x 0.25" = Area
0.98"² = Area - just less than one square inch, 3n open 3/4" hose/pipe has 0.442"² area.
By comparison, at a standard 60psi pressure of a municipal water system, with 3/4" house pipes, faucet and 3/4 garden hose, you flow 113 gallons per minute. A vehicle cooling system runs nowhere near that psi of pressure, it is designed to run at just a few psi difference on either side of the t-stat at its wide open and a very low system pressure of 15psi overall.
The 130gpm water pump you're running on your son's '94 was designed as an answer in the '96 production on the assembly line to the overheating in the rear of the passenger head/cylinder, and to flow almost double the earlier 6.2/6.5 80gpm. The engineering answer was the early, "temporary" large single t-stat housing and the large diameter, high lift (nearly ½" at wide open) valve t-stat in it. Look at
@J_dude 's photo of his 96 special t-stat x-over. Look at how large and deep the "bowl" is and how tall the outlet housing is to the spigot - that was to both allow enough coolant to flow into the bowl behind the t-stat AND enough depth/height for the high lift t-stat. It was the "fix" to allow the new 130 gpm (unbalanced flow) water pump to move the volume it was designed to. But that casting of x-over and outlet housing was difficult to both cast and machine (expensive). The Bean Counters quickly intervened with the Engineering Department and the result was the easier to cast and machine (cheaper) rectangular box with two runners attached and simple outlet cover using two common standard t-stats (cheaper) that combined flowed close to the "temporary" design.
Want your son's 94 to run cooler? Remove the restriction that's preventing the 130gpm from flowing as it was designed to, and is creating backpressure which is also holding heat in the engine by doing so. Swap out the "choke point" 94-95 x-over and put a 97+ one on with AC Delco/GM t-stats in it so that 130gpm water pump can "breathe" freely and move the coolant it was designed to through the engine, out of it past the t-stats and through the radiator.
Also, as
@Will L. pointed out earlier and I agree with him 100%, Water Wetter does make a difference. It is a surfactant that enhances the transfer of heat from metal to water and while you may not see a difference on the temp gauge (because the t-stats regulate the coolant temperature to a couple of degrees of their operating temperature) but it does help pull more BTU's out of the engine into the coolant and then rids those BTU's from the coolant to the core of the radiator which then rids the BTU's to the air flowing through it. The temperature drop they claim to coolant temp comes from running a racing engine, which does NOT run with a t-stat in it, at speed and measured the difference without and with Water Wetter in the cooling system. Why do they call them "warm up laps" before a race? Because that's exactly what they're doing, bringing their t-statless engines up to operating temperature and maximum performance efficiency before starting the race.
Remove the restriction and put a 97+ dual t-stat x-over on your son's 94 and you'll see the coolant temps drop down to where your 99 runs now that the 130gpm pump can carry all that heat away from the engine unrestricted.
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