Tom
The Olds 455 engine used in the passenger cars during the late 60s and early
70s used to over drive the water pump for extended idles and for high mass
flow at low rpm road loads due to axle ratios of around 2.43 to 1. When you
put that engine into a motor home with a gear ratio of 4.11 or lower you run
the water pump at a higher speed and you will have a tendency to erode the
tubes in the heat exchanger (radiator) because the velocity is too great for
the tube size. That is the reason that we used to have to slow down the
coolant pumps a little to prevent the erosion and cavitation at the pump due
to impeller design and pump rpm. (passenger vehicles tend to overdrive the
pump at ratios of 1.3 to 1.5 to one and trucks at about 1.1 to 1.2 to one,
due to the difference in gear ratios in the rear axles.) hope this helps
answer some of your questions. if you need a more in-depth answer let me know.
Mike>>>>>>
So, it appears that the reason they wanted to slow down the coolant was not
due to the residence time of the coolant in contact with the wall of the
engine block, (which is the myth that I could not believe) but rather due to
too high a flow rate in the radiator tubes and at the water pump impeller
which would then cause turbulence and cavitation in the radiator tubes and at
the impeller which in turn could then cause corrosion.
With the GMC we are not normally at 4.11 to 1 ratio but rather 3.07 to 3.7 to
1 (depending on which final drive we are using) so it might not be as big a
problem as with the 4.11 to 1 or lower ratios. Also, the GMC visually seems
to have a lot larger tubes that the typical automobile. Does anyone know how
the GMC radiator compares to a car or truck one?
I then inquired as to the best way to slow the flow if someone wanted to and
was told that rather than restricting the flow to or from the engine block,
the pulley on the water pump should be changed to a larger diameter.
If anyone has questions let me know and I'll request more info.
Emery Stora
77 Kingsley
Santa Fe, NM
The Olds 455 engine used in the passenger cars during the late 60s and early
70s used to over drive the water pump for extended idles and for high mass
flow at low rpm road loads due to axle ratios of around 2.43 to 1. When you
put that engine into a motor home with a gear ratio of 4.11 or lower you run
the water pump at a higher speed and you will have a tendency to erode the
tubes in the heat exchanger (radiator) because the velocity is too great for
the tube size. That is the reason that we used to have to slow down the
coolant pumps a little to prevent the erosion and cavitation at the pump due
to impeller design and pump rpm. (passenger vehicles tend to overdrive the
pump at ratios of 1.3 to 1.5 to one and trucks at about 1.1 to 1.2 to one,
due to the difference in gear ratios in the rear axles.) hope this helps
answer some of your questions. if you need a more in-depth answer let me know.
Mike>>>>>>
So, it appears that the reason they wanted to slow down the coolant was not
due to the residence time of the coolant in contact with the wall of the
engine block, (which is the myth that I could not believe) but rather due to
too high a flow rate in the radiator tubes and at the water pump impeller
which would then cause turbulence and cavitation in the radiator tubes and at
the impeller which in turn could then cause corrosion.
With the GMC we are not normally at 4.11 to 1 ratio but rather 3.07 to 3.7 to
1 (depending on which final drive we are using) so it might not be as big a
problem as with the 4.11 to 1 or lower ratios. Also, the GMC visually seems
to have a lot larger tubes that the typical automobile. Does anyone know how
the GMC radiator compares to a car or truck one?
I then inquired as to the best way to slow the flow if someone wanted to and
was told that rather than restricting the flow to or from the engine block,
the pulley on the water pump should be changed to a larger diameter.
If anyone has questions let me know and I'll request more info.
Emery Stora
77 Kingsley
Santa Fe, NM
