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Cooling System Goals
Most people seem to think that all a cooling system needs to do is keep the engine from overheating. But what is not realized is if the engine runs too cool, thermal efficiency is lost and power is reduced. Many will argue that an engine has more power when it is cold, but that is only due to the fact that the intake air is colder and denser, actual BSFC is higher. Remember that an engines whole job is to make heat and turn it into mechanical energy. Running the engine as hot as possible (limited by the coolant boiling point) will increase power and provide a lower BSFC. If the coolant begins to boil, steam pockets will form and detonation will limit power (by forcing you to retard timing to less than optimum or run the engine cooler). Most of today's high output street motors using a water/ethylene glycol mixture will be limited to a maximum of 200° F before detonation becomes a problem (unless other steps are taken). Another goal of modifying the cooling system is to even out the temperatures of the whole engine, which is not easy to do. All it takes is one hotter cylinder to run into detonation to limit the engines power. Most high performance engines are close to detonation to begin with, so a good cooling system is a must.
Nucleate Cooling Phase
As coolant flows through the system it absorbs heat from the parts it contacts. As it does this some of the coolant will boil and form tiny steam bubbles (absorbing a lot of heat) on the internal engine surfaces. When these bubbles get larger they become a flow restriction and the flowing fluid pushes them away from the surface and that process starts over again. This process is called the Nucleate Cooling Phase. When the coolant boiling point is too low or the flow rate is too slow, these bubbles can become too large and form steam pockets that insulate that part from being cooled. This usually happens around the combustion chambers. Once the steam pocket forms the surface can rise in temperature (even though the coolant is not overheating) and cause that part to overheat, which can cause detonation or other problems.
Types of Coolant
I'm sure that you've read or heard somewhere before that water is the best coolant. This is true as far as being able to absorb heat for a given flow rate, water does do that the best. Water also boils at a lower temperature than other coolants and can develop steam pockets easier, so it's not the best coolant in that respect. A water / ethylene glycol mixture will boil at a higher temp and resist steam pockets better than plain water, the down fall is that it has to have a higher flow rate, but that is easy to accomplish. The 3rd common (and least common) form of coolant is propylene glycol, which had the highest boiling point and can run higher than 250° F (average temperature as seen on a gauge) without forming steam pockets, but it must flow at more than twice the speed of a water / ethylene glycol mixture (which means major changes to most cooling systems).
System Pressure
The pressure in the block is higher than the radiator pressure, this is because the pump is building pressure due to the thermostat being a restriction. This pressure raises the boiling point of the coolant and reduces the chance of steam pockets, so never run with out a thermostat (or some form of restriction). The radiator cap will usually hold 15-18 psi, if the radiator holds the system at 15 psi, the boiling point of plain water will be raised to 250° F. The water pump can then make an additional 40-45 psi in the engine and bring that boiling point close to 300° F. So as you can see, pressure is important.
Stock Cooling Systems
Most stock cooling systems pull coolant from the radiator and push it through the each bank of the block, it then goes up through holes in the head gaskets to the heads and out the front of the heads to a common exit point. This ok for a stock engine that has no problems with detonation, but the cooling is very uneven. The front cylinders will run coolest and the front combustion chambers will run the hottest. Most stock pumps will also favor one bank. The stock pump used on a small-block Chevy for instance will always favor the passenger side bank. This means that cylinder 2,4,6 & 8 get more flow, so the 1,3,5, & 7 bank runs hotter. With the center exhaust ports right next to each other, you can see that combustion chambers 3 and 5 will run the hottest, it is in these two cylinders that detonation will usually first start. It seems a little backward to start the coolant at the block instead of the heads; it would make more sense to bring the coolest coolant to the hottest parts first. This type of reverse flow system has been tried with much success, but it is harder to get it working properly and not worth it for car companies to research when the stock system worked good enough on a stock engine.
Water Pumps
As I said before, stock pumps rarely flow evenly between banks. On the small-block Chevy you can restrict 1/2 of the block inlet to the even cylinder bank to get more even flow, but the better solution is to use an aftermarket high volume pump that has worked out such problems. Stock pumps have a stamped steel impeller and tend to cavitate easily when turned more than 6000 rpm, so overdriving the stock pump offers little to no advantages and can actually aggravate any cooling problems. Most aftermarket pumps will use a cast iron or an aluminum impeller that better resists cavitation. Weiand, Howard Stewart and Milidon make very good water pumps for most popular applications, which improve flow, resist cavitation better, and require less power to drive than stock pumps.
Electric Water Pumps
Many aftermarket companies offer electric water pumps. These pumps do not flow well or build sufficient pressure in the block. They are only good for limited drag racing use, and when used they need a high pressure cap to help prevent steam pockets. At best these pumps can flow 30 GPH and only build about 5 psi in the block. An electric pump should never be considered on a street or any type of endurance engine.
Coolant Flow
Different coolants require different minimum flow rates, but contrary to popular belief, you cannot make the coolant flow too fast. This rumor was started because people removed the thermostat to gain flow, because they had an over heating problem, and it aggravated the problem. The real reason they ran into problems is that removing the thermostat also removes the restriction that builds pressure in the engine, so they gained flow, but reduced the boiling point of the coolant in the block. Running a higher flow thermostat and a higher volume pump to maintain pressure, will give no such problems. If you think about it, making the coolant flow twice as fast will also make it flow though the engine twice as often, so there will be more even temperature across the engine.
Basic Flow Modifications
Most stock systems will have a common outlet for both banks. The outlets of each bank flows directly at each other than must take a 90° turn to return to the radiator. If one side gets hotter (which is sure to happen) the pressure of that side will increase. The increased pressure will increase flow in the hotter bank and decrease flow in the cooler one. The faster moving coolant will cool the hot bank better and the slower moving coolant picks up more heat in the colder side. As you can see, the hot side is getting cooled and the cooler side is heating up. This happens until the banks reverse, the side that was cooler is now hotter and has more pressure. The cyclic flow will continue until the engine is shut off. Smokey Yunick was the first to do studies on the cyclic flow and traced the problem to the outlet. By tapping the front of the heads, and bringing the coolant together in a Y eliminated the cycling.
Radical Modifications
To truly equalize temperatures throughout the engine, you must start fresh and build totally custom cooling system. The first step is to tap off the pump and put coolant to the back of the block so the coolant enters at both ends. This helps equalize the cylinder temperatures, but the heads will still be hotter toward the front. To equalize the head temperatures you must tap outlets at the back of the heads so that all the coolant does not have to pass the front combustion chambers. To further equalize, you can tap inlets and outlets in the center of the block and heads also. At that point the coolant will be flowing basically from bottom to top and is about the best you will get without reversing the flow.
Reverse Flow Systems
As I said earlier, it makes sense to put the coolest coolant to the hottest parts first to bring the temperatures down as much as possible, the already heated coolant can help bring the temperatures of the coolest parts up and make everything more even. To do this the coolant must flow in reverse (compared to most systems). The problem with reverse flow systems is that the pump tends to cavitate easier (even with a good aftermarket pump). To limit cavitation, a higher boiling point of the coolant helps and so does a higher system pressure.
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1967 Firebird 400 conv T400 3.08, 1989 Formula 350 700r4 3.27