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The Highs & Lows Of A Cooling System
Paulsen, McFarland Discuss The Concept Of Flow Rates In The Cooling System
By Meghan Frazier
Photography: Meghan Frazier
Chris Paulsen is president of C&R Racing, Inc. based in Indianapolis, Indiana. C&R Racing manufactures racing radiators for sprint cars all the way up to Formula One machines. Paulsen is an expert source on all things “cool.”
Could you explain the concept of controlling water speed and how it affects the cooling system?
First of all, “controlling water speed” is not something a lot of attention is paid to. What I mean by that is, the cooling system in a race car—that being a water pump, a radiator and the means water flows through the block and cylinder heads—are all kind of a given thing by each manufacturer. We build our radiators to basically reject heat from the cooling system in the most efficient way possible given the race car and the racetrack it is going to run on.
Let me start out by saying flow rate through the cooling system is a very important thing. With recent technology, we have pretty well proven that the higher flow rates we can get, the more efficient the cooling system is. Many people still believe in the old technology of slower flow rates. However, we are changing them every day. They believe by slowing down the flow, there is more time to absorb the heat and energy from the block and heads. This gives the water more time in the radiator. Therefore, the heat will be pulled out of the radiator better.
Quite honestly, with today’s technology, we have proven it is quite the opposite. We tend to prefer high-flow-rate systems. Pump manufacturers nowadays, like Stewart Components and Edelbrock, are trying to build pumps that will flow as much as 100 gallons a minute. This rate is really more like 60 gallons a minute.
With that in mind, you have a water pump that is trying to perform a high flow rate through the system. Well, the cylinder heads can be restrictive with their water passages, which in turn slows the flow rate down to a certain degree. This restriction also raises the pressure in the heads and the block, which is actually a good thing. Pressure in the heads and block helps reject heat and keeps the air molecules and water compressed so they don’t form steam pockets. With lower pressures, air will actually expand and begin boiling. This results in local hot spots in the combustion chamber.
One of the main reasons we like to see higher flow rates is higher flow rates will actually absorb more heat and BTUs of energy from the metal surfaces. They will actually pick up heat more efficiently and carry it out of the engine and into the radiator. When it comes to the radiator side of things, the higher flow rates push the water through the radiator tube much faster so it creates a much higher velocity in the cooling tubes. This higher water velocity actually creates more turbulence in the water, which rejects heat much better. So, that is why we say the old rule of thumb “slow down the water flow so it spends more time in the radiator in order to cool it better” is really not correct at all. The higher flow rate and higher water velocity through the radiator core really goes a long way in helping that radiator reject the heat.
What kind of performance Advantage will you see when you go to a higher flow rate?
The biggest performance advantage of the higher flow rate is keeping your temperatures in check and cooling your engine better. As far as any performance other than that, there really isn’t any. The critical part of a cooling system is the combustion chamber area in the cylinder heads. This is where the combustion is taking place and where the bulk of the heat is generated. We have seen higher flow rates help keep these combustion chambers cooler. It keeps the air from separating out of the water and causing local steam pockets and localized boiling in those combustion-chamber areas. It also keeps good water-to-metal contact in those areas, which is the secret to keeping everything cool.
What happens if those metal combustion chamber surfaces get too hot? The engine will actually get a mild detonation. With most race engines, if it starts overheating, the driver will come on the radio and say the car is starting to lose power, is running poorly and doesn’t have the horsepower he had before. What is happening is the temperature increases more and more and those hot spots in the cylinder heads will actually start creating a detonation situation in the engine—it is usually just a mild detonation, but it can make the power drop off.
What can an average racer do about controlling water speeds or flow rates? Is it just a matter of what type of water pump they install?
Yes, pretty much. The flow rate is not something racers can really control. Basically, you want the maximum water flow rate you can get. My suggestion is don’t worry about doing anything to physically control the flow. The engine itself is going to control flow rate because of the passages and the water system. It is only going to allow so much water to flow through because it only has so much area to work with. Winston Cup teams use AN line that goes from the engine back to the radiator. The hose size they pick can be somewhat of a restrictor. In other words, it could be the thing in the whole system that is restricting how much water is flowing throughout the whole system. If that is big enough that it is not the restrictor, well then, the radiator becomes the next restriction point.
Jim McFarland is not only a theorist on making power, he is president of Autocom, an engine performance consulting firm in Austin, Texas, as well as a member of the Circle Track Technical Council.
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Can you explain how flow rates affect the cooling system?
A combustion engine is basically a heat pump. Heat is the forerunner of power. You need heat to make power, and you have to be able to control how much heat is contained in the combustion space. If you don’t, component damage can occur.
The cooling system is basically there to absorb heat. Because of the passages in the cooling path throughout the engine, you are dealing with areas where flow tends to stagnate. Part of this is the physical passages available for water to move through. Another part, which is very important, is the fact that the temperature in the coolant is not uniform throughout the entire cooling system. You have hot spots, cooler spots and steam pockets. These variations in temperature cause the movement of the coolant to be different in different locations. The control of the temperature variations or extremes within the system is important. You would like for the range of that temperature to be as narrow as possible. As you narrow it, you begin to move away from the problems that steam pockets and hot spots cause. Problems may include excessive cylinder bore distortion, which can lead to cylinder head warpage and cause inefficient ring seal.
The rate at which the temperature will move through a substance, whether metal or coolant, depends on the temperature difference. The more extreme, the faster it moves. What you are trying to do is stabilize dimensions in and around the combustion space. If it is too cool outside the combustion space—if the water is moving so fast that it is too cool—heat will be liberated from the combustion space into that coolant because the temperature grade is too extreme. You raise the temperature of the coolant and it begins to reject some of the heat loss. You would like for the coolant to maintain a reasonably high level of temperature so you don’t lose a lot of heat out of the combustion space to a cooler environment. But you don’t want that temperature environment outside the combustion space and the cooling system to have extremes in temperature. So, the rate at which you move coolant through the system will affect the temperature that is contained around the combustion space. When you have extremes in that coolant, you run into problems of periodic heat rejection and/or absorption in the coolant that is irregular and affects how much heat you are retaining in the combustion space. The whole idea is that you want to be able to hold as much temperature in the combustion space as possible because heat is power. But, you don’t want it to be excessive. Excessive temperatures can lead to detonation, pre-ignition, lost power and serious problems in terms of part damage. The compromise comes in providing an environment for the combustion space that is not excessive. You would like it at 180 degrees F there and 210 degrees F here. To do that, maintain the proper kind of flow and introduce water or coolant in areas where there is not much flow by nature of the block design or head design. That is why extra lines are run because areas have been identified, and coolant is put into those areas to help reduce the temperature extremes due to lack of movement.
Has the process of flow rates changed through the years?
The early work legendary engine builder Smokey Yunick did was find out what are the extremes, where are the hot spots and where are the steam pockets. The first thing was to find out where these areas are. Then it was a matter of if we can open up the cooling passages inside the block or heads or if we can modify the flow rates of the existing passages.
Some of these changes couldn’t be made internally, so we brought the coolant externally into these areas. There has been a progression of first recognizing that there were problems and then finding out what can be done through internal passage modifications or core changes. The manufacturers have addressed the problem from a casting prospective by changing the internal passages. The racers have said, “That’s fine, you have helped. But I still find areas that are trouble, so I am going to bring external sources of coolant in because the internal passages are not adequate.”
What are the advantages to either having a high flow rate or a slow flow rate?
If you move water too fast, it will be too cool and will lose heat from the combustion space. If you move it too slowly, you may have too high a temperature and develop steam pockets that restrict the flow path. The middle ground is to get the flow rate sufficiently high enough so it doesn’t absorb too much heat from the combustion space. There is a see-saw effect here where you are balancing the temperature of the combustion space with the temperature in the cooling system so it doesn’t lose too much heat if it is too cool, and it doesn’t reject too much if it is too hot.
How does the average racer know he has the right combination in his cooling system?
First, they should be measuring the cooling temperature. If that is excessive, he knows he has a cooling problem. Then it is a matter of talking to an engine builder to find out if he is doing anything to address the problem. If not, he can talk to other racers to see what they are doing to solve this sort of problem. Or, he can start putting temperature gauges elsewhere in the engine. Some racers put them at the back of the cylinder heads to find out if there is excessive temperatures elsewhere.
It is almost an “after the fact” issue where you see the results of a problem like detonation then you try to identify what is causing that. That is the Saturday night guy’s approach. He sees the results of the problem first and then tries to figure out what is the cause.
