Cooling system concern

Rich Kruger

When I purchased my car secondhand it came with a set of well made stainless steel tubes from the engine to the radiator. Now that I'm at the point of finalizing the cooling system I realized the tubes are backwards. The radiator outlet is at the top of the rad. The issue being any steam/ gases at the top of the radiator could cause water pump cavitation as it draws from that point. Question- With a closed system where the rad has a steam port bleed at the top of the inlet sides tank (to the higher coolant tank) should I be concerned about the water pump suffering cavitation? Does anyone have any experience with this? Opinions on the internet are all over the place. The car will mainly be used on the street but with some tracks days during the year. And if this is a problem should I a) Bite the bullet and have the rad ports reversed and re pipe b) install an electric booster pump c) put the car in my living room and don't worry about it (lol)

Terry Oxandale

Skinny Man
Not the best person to answer, but as long as the system was purged well, that fluid levels are checked periodically, you should be fine. The greatest concern I see is for a loss of fluid causing the liquid level to drop below the outlet tube, and water pressure/flow being insufficient to push the water beyond this point. I don't know if cavitation is the correct issue here, because in this case it would be the loss of water flow, and not necessarily the loss of pressure? But, I don't know if either one is preferential from the impeller's perspective.

Howard Jones

What kind of car do you have? Aluminum radiator?

Ya I would reverse the inlet outlet on the radiator. Hot out of the top of the motor to the top of the radiator, then though the radiator to the to the other side bottom and then back to the bottom of the motor/ pump inlet.

Call around to radiator shops and find one with experience welding aluminum. I am going to change my SLC radiator that has both inlet and outlet on the bottom if it's necessary. I want to change things one at a time so I will learn something about what works and what doesn't.

Also read up on coolant system air bleed systems. Done correctly they will solve air in the system problems permanently. They are especially necessary in mid engine cars.


Personally it would not bother me that the inlet is on the bottom. I don't like the outlet being on top, but if the existing work is well done, I would run it and see what happens. But I would be sure I had working gauges I was confident in before driving any distance.

Once the system is purged, I cannot see having enough air in the system to cause an issue. If the 'air' is steam, it should have been condensed back to water before it reached the outlet side of the radiator.

As with any cooling system, it will be key to get practically all of the air out to start with.

You can expect some initial issues (I had a return side radiator hose collapse at high RPM) that will need to be rectified.

Rich Kruger

Thanks guys. Im curious to see how it works but I think I'm going to have the ports swapped on the radiator eventually. The big bugaboo in this case is the possibility of a negative pressure at the return side (like Franks return rad hose collapsing) causing the water pump to starve and making it easier to produce steam from overheating a low pressure liquid.


Lifetime Supporter
Howard, interesting diagram. Do you have any insight into the vortex separator? I was thinking about incorporating one when I have my expansion coolant tank made, but I haven't been able to find much information on how they're designed.

The most information that I've found is for Radium's product. According to their website:

Hot pressurized coolant from the cylinder head is sent into the vortex chamber's inlet tangentially and accelerated to a high rate of rotation. This is to help aid in deaerating the system.