Over the years one of the hottest topics (excuse the pun) in street rodding has been engine-operating temperature. Take an old car with a relatively small radiator, stuff in a big engine, cover it with a smooth hood, add A/C then go cruise around a fairgrounds and the results are often an automotive version of Old Faithful. Then there are those cars that cool fine when cruising but get hot on the highway, and we all know how annoying it is to drive at speed with one eye on the temperature gauge.

Regardless of when and where it happens, there are consequences to overheating an engine. The first sign is usually pinging and detonation or running on after the ignition is shut off. In severe cases the abnormal combustion that takes place in a hot engine under a load can blow holes in pistons, and damage rings and rod bearings. Overheating can cause pistons to scuff or even seize, exhaust valves may stick in their guides, and warped heads and blown head gaskets are common catastrophes. On the other hand, running an engine too cool has its own set of problems; among them, reduced performance and corrosive condensation collecting internally.

To look at engine operating temperatures from a technical standpoint we decided to pose a series of questions to experts in their particular fields--Bill Mitchell of World Products, manufacturer and supplier of blocks, heads, and complete engines; Scott Leon and Neal Matheson, both former GM proving ground technicians specializing in hot weather performance and reliability; Ed Newton of Amsoil, one of the most established names in synthetic lubricants; and Jeff Cannul of Stewart Components, manufacturers of high-performance water pumps. All took time from their schedules to respond--here's what they had to say:

Bill Mitchell
SR: How is the coolant path through an engine determined?
Bill: In earlier ('90s) engines the water went into the block, then up into the heads and back into the radiator. In later-style engines the water goes into the heads (while still cool) and then down into the block and back to radiator. This later method allows the cooler water to control ping and allows higher compression ratio by keeping the chamber cooler before firing. This is why our water jacket design of more water, fewer air pockets and less solid metal can take more compression, more timing and makes more power.

SR: How is coolant flow through the block and heads controlled?
Bill:The water holes in the block have another job long before it gets to be a block. Those holes in the foundry are called core prints. They hold the entire core package together and after the block is poured and cooled down the holes are the only ways to get all the sand out of the block. Therefore water flow and volume is controlled by the size of the holes in the head gasket.

SR: Are some engines harder to cool than others? Why?
Bill: It's all about real estate inside the engine. We need sand cores to make the water jackets. Close-bore centers, big bores and thick cylinder walls all take real estate away for the sand (water). If you look at our parts we hunt for real estate everywhere--bulges around cylinders in the valley, outside the block, everywhere. We connect all cores and take into account the opposition of the part installed to make sure there are no air pockets.

SR: Is there an ideal engine operating temperature?
Bill: There will be lots of opinions on this question and they're all right if it works for them. Generally 200 degrees, give or take 20 degrees is in ballpark.

SR: What's too hot? What's too cold?
Bill: All I can tell you is cold engines don't make power on dyno because cold pistons, oil, etc. rob all the heat you're trying to make in-chamber. Hot engines usually will ping and everything you do to get rid of the ping will make less power.

SR: What are the differences in cooling-aluminum versus cast-iron heads?
Bill: Aluminum robs the heat (like welding something in a vise). The consumer loves aluminum and we all need to give the consumer what they want, but cast iron usually makes more power.

SR: What are the differences in cooling-aluminum versus cast-iron blocks?
Bill: Same answer as with the heads, however we have found that using ductile-iron sleeves (exclusive in all our aluminum blocks) actually seals rings so much better that our aluminum blocks usually make slightly more horsepower (usually 10 to 15) more than cast-iron blocks. Only downside is ductile sleeves are two to three times cost of cast-iron sleeves.

SR: What causes steam pockets and are they a common problem?
Bill: Steam pockets form as a result of a water jacket that you can't get water into because there is no way to get the air out. Our new Warhawk LS1/LS7 heads have been designed with unimpeded water flow to eliminate dead ends and the steam pockets that result.

SR: Are there cooling problems with Siamese bores (cylinder walls touch at their widest point with no water between them)?
Bill: Again, it's all about the real estate. Siamese bores are a way of getting more bore size in the same space. Fortunately the head gasket manufacturers have a good handle on controlling flow with hole sizes and it doesn't seem to be a problem.

SR: How does compression ratio affect engine temperature?
Bill: Simple, more compression makes more power, and power is made from heat. At some point the engine will ping, then all hell brakes loose, resulting in overheating, broken parts--you know the drill.

SR: Explain the theory behind using higher compression with aluminum heads.
Bill: If you cc the chamber in an aluminum head and then heat the head, the hotter you make it the bigger the chamber gets and the lower the compression ratio becomes--so just start it out a little higher. Besides all that, the aluminum is going to rob heat and make less power, so get it closer to the ping level right from the beginning by going with more compression.

Neal MathesonScott Leon
SR: Is there an ideal engine operating temperature?
Neal: 190 degrees is probably ideal, with a quick warm-up. That's hot enough to boil off moisture and prevent sludging, and low enough to provide a cushion for heat spikes.

Scott: 190-degrees to avoid sludge buildup and to remove moisture from the oil.
SR: What's too hot? Too cold?
Neal : Lower temperatures results in rapid sludging, can prevent closed-loop fuel injection systems from properly regulating fuel mixture, and results in decreased fuel economy.

Lower operating temperatures provide most people a false sense of security, but lower temperatures, either by thermostat or thermostat removal, do not result in increased cooling capacity. All it results in is a different starting point. Too cold results in unnecessary sludging of the oil, and may prevent engine/fuel/spark management systems from operating at optimum efficiency or power. Engine clearances are designed for normal operating temperatures. Thicker oil causes unnecessary drag.

Too hot is when coolant is lost. An operating, full, pressurized system (13-psi), with 50/50 glycol can safely go to 260 without engine damage from heat alone. The key is not loosing coolant, which results in hot spots, vapor pockets, warped heads, etc. However, 230 degrees and above can change the spark map required to prevent engine damage from pre-ignition/detonation. Most factory ECM spark-controlled engines automatically reduce spark advance as coolant temp increases. Mechanical distributors or non-ECM-controlled engines have problems with detonation at higher temperatures, and that can cause damage.

Scott: Much less than 190-degrees and moisture may start to accumulate in the oil. Up to 230 to 240 degrees is okay if there is no air in the system and a 13 to 15-psi radiator cap is used. Although most people don't like to see the temperature gauge this high, some new cars don't turn on the electric fan until the coolant temperature is 220 degrees.

SR: Is there a formula for optimum radiator size?
Neal: From a practical standpoint, the largest radiator you can fit in the opening. Street rods are notorious for having minimal grill area. Remember too, the radiator is only one part of a system. A bigger radiator is no bonus if you can't get air through it.

At the design level, manufacturers may have a formula for a starting point, but they also know all the numbers, which street rodders don't. Things like the history of previous similar models, air flow, opening size, baffling, fan performance, ram air at speed, vehicle aerodynamics, engine heat rejection, vehicle design weight and axle ratio. Even with all this, the bottom line is real-world or wind-tunnel testing to ensure it meets established guidelines. After all that, radiators are tweaked by adding or subtracting constant (fins per inch), number of rows and size of rows.

Another consideration--radiator capacity in a vehicle with A/C can be reduced by nearly a third if the condenser is mounted anywhere but in front of the radiator. That's why in an overheating vehicle the first thing to do is turn the A/C off.

Scott: When building a street rod I measure the space available and find a radiator to fill it.

SR: What are the differences in cooling of aluminum versus cast-iron engines?
Neal: Cooling wise there is not much difference. They both respond to the same things, and have similar limits. Aluminum engines, however, are not very forgiving of being overheated.

Scott: Cast-iron engines are much more forgiving of overheating.

SR: How would you compare aluminum and copper/brass radiators?
Neal: Aluminum is cheaper, lighter; they lend themselves to custom fabrication with welded tanks. Copper/brass is more durable and more tolerant of depleted anti-corrosion levels.

Scott: Both work well. Aluminum is lighter and cheaper, that's why the OEM's use it.

SR: How are water pump rates determined, and what happens when pulleys are used to increase or decrease water pump speed?
Neal: Water pump volume is initially established experimentally by matching the engines known heat rejection with the heat transfer required to remove that heat. As a practical matter, all properly-operating water pumps probably have some excess capacity. The limiting restriction in the system is the thermostat opening. As long as the pump supplies pressure, the coolant flow is determined by the opening.

Engines operated under heavy load and slow speeds may benefit from a faster ratio, but usually it goes the other way. Engines operated at high speed can run a slower ratio. Not necessarily for flow reasons, but for reduced pumping losses and increased power output. Impeller blades can also be removed to reduce pump flow.

Scott: It depends on the demand of the engine; a small-block Chevy with A/C typically has a ratio of 1.25:1 while a non-A/C six cylinder may have a ratio as low as .95:1

SR: How does timing and air/fuel ratios affect cooling?
Neal: In general, both retarded timing and a lean fuel ratio produce more heat. Proper state of tune will optimize cooling. However, rich mixtures have a greater affect on lowering exhaust gas temperatures than cooling.

Scott: A lean mixture will run hotter that a rich mixture.

SR: What's the best coolant to use?
Neal: 50/50 ethylene glycol and distilled water is the best all around. Besides the obvious benefits of preventing damage from freezing and providing anti-corrosion properties, the atmospheric boiling point is raised from 212 to 220 degrees. (It always amazed me people with a cooling problem replace their coolant with water. In doing so, they lower the coolants boiling point and pretty much guarantee the overheating will continue).

Higher concentrations of glycol actually produce higher freezing temperatures, and concentrations greater the 60 percent become highly flammable. Long-life coolant has the benefit of more stable anti-corrosion protection, but has been reported to become gelled if the cooling system contains any air.

Propylene Glycol is more politically correct nowadays because it is safer for pets; cooling wise it not much different than ethylene. Corvette tried a reverse-flow cooling system, and intended to use 100 percent Propylene. Propylene is highly flammable.

Scott: A 50/50 mix of ethylene glycol and water.

SR: Do you suggest engine-driven or electric fans on street rods?
Neal: Electric fans are usually a good choice, as they are easier to adapt and maintain a constant airflow at idle speeds. Short bursts of power are usually manageable. However it is faulty to think that a 750-watt (one horsepower) electric fan can move as much air as an engine-driven fan consuming 15 hp. When an engine is working hard, the A/C is on an engine-driven fan is hard to beat.

Some considerations for engine-driven fans:

Flex Fan - Simple and effective, but noisy and consumes power all the time.

Viscous - Torque limiting, basically just a fluid coupling. Could be a good choice for street rods if the space is available. Provides some drive all the time.

Viscous - Temperature-controlled (not thermostatic). May be a good choice for street rods, as fan speed is increased as temperature increases.

Viscous - (Thermostatic) turns on and off at a predetermined temperature. The best of both worlds, it doesn't run until needed. The rest the time it idles, conserving energy while providing idle air. It is not uncommon for Mid-western drivers visiting the desert to comment they think their transmission is slipping because the engine sounds like it suddenly revs up. They have never heard the roar of the clutch turning on before.

Thermostatic clutches are not normally recommended for street rods. The problem is, temperature calibration is critical. Production OEM fans are normally calibrated to turn on when the coolant temperature reaches 220 degrees and every application gets its own calibration. A change in the radiator constant with exactly the same coolant temperature will change the clutch air temperature, requiring a change in the released clutch temperature. A street rodder doesn't have the option of going into the parts store and asking for a 178-degree clutch, for a 16-inch fan. Even if he did, he would have to know what temperature cut-in he needed before he asked for it. Too low a cut-in it would run all the time, too high and it will never turn on. Calibrations low enough for non-A/C vehicles are hard to come by.

Scott: Electric fans do an all around good job, especially at low vehicle speeds. For a car that's working the engine hard it's difficult to beat an engine-driven fan with a clutch (however most street rods don't need that much fan). With mechanical and electric fans, proper size is important. Electric fans should be sealed to the radiator with a shroud, and they work better pulling the air through the core than pushing it.

SR: What kinds of problems cause engines to overheat at low vehicle speeds?
Neal: If you have an overheating problem at idle or low speed, then you have an airflow problem not a cooling problem. Since you are not making any power to speak of, you are not making any heat either. You just need to get rid of the heat you have which means you need airflow.

Engine speed creates heat. I have seen people with a heating problem rev the engine, or downshift to a lower gear to cool it down thinking the higher fan speed and increased water flow will help. It doesn't. Higher fan speed might be a good thing in this instance, but he would be better off to turn of the A/C, shift up a gear, and reduce highway speed.

Scott: Poor performance at low speeds is a result of not enough airflow.

SR: What kinds of problems cause engines to overheat at moderate to high vehicle speeds?
Neal: Again, engine speed creates heat. Running a grade part-throttle in a lower gear will always run hotter than running wide open throttle in a higher gear. A higher numerical gear ratio will always require a bigger cooling system than a lower one. If you look at vehicles around 1985 when four-speed with overdrive transmissions became standard production equipment, cooling systems required suddenly became smaller.

A chronic overheat condition at highway speeds could be due to the following:

Inadequate system capacity - radiator/fan too small.

Cap not holding pressure - 13 psi raises boiling point of water to 245 degrees.

Reduced coolant flow - Plugged radiator, water pump flow (e.g. loose or damaged impeller), slipping belt, and inoperative or partially-closed thermostat.

Fan drive - bad clutch, slipping belt.

Scott: Camaros used to run two fans, but instead of turning on one fan then both, they employed a three-relay system. Both fans first came on at low speed--when necessary both switched to high speed. This allowed for quieter operation at low speeds,Additional comments from Scott and Neal:

* A significant part of engine cooling is from the engine oil. Anything you can do to reduce engine oil temperature will help the cooling system. Ideally, oil temperatures should be kept below 280 degrees for improved performance and reduced oxidation.

* Proper thermostat operation is critical. Any engine which has experienced a serious overheat with coolant loss should automatically have the thermostat replaced. Overheated thermostats tend to stick or no longer open fully, which perpetuates the overheating.

* Properly-operating pressure cap with recovery bottle is a must. Pressure raises the boiling temperature. Working together, the cap and recovery bottle maintain a packed system by returning coolant from the recovery bottle and purging air from the system.

* Good airflow requires proper baffling. Any air that passes through the radiator should make a one-way trip. Good baffling prevents hot air from recirculating back into the system. Almost as important is adequate space for the hot air to exhaust through the engine compartment without restriction. Production cars normally have closeout panels and air dams to keep air from returning to the front of the vehicle

* Good fan performance (electric or engine-driven) requires proper shrouding. A spinning fan moves at least as much air radially as it does to the rear. With the radiator as a restriction, airflow can more easily escape to the sides and recirculate unless redirected into a one-way stream by the shroud.

*Additives: We tested an additive once, which supposedly made water wetter and therefore improved heat transfer. Stabilized results were at the lower end of test repeatability.

* At a stabilized W.O.T where the thermostat is fully open and the fan is fully on, the difference (delta T) between top tank and bottom tank is normally 14 to 16 degrees. A delta greater than this indicates either system not packed, (air in system) or else flow is restricted. Normal causes of reduced flow are things like thermostat not fully open, plugged radiator, loose water pump impeller, slipping belt, etc.

* What to do when your car overheats? Any, or all, of the following: Turn off A/C, slow down, shift to higher gear. Spray the radiator with water. If overheat is from a sudden loss of coolant, like a burst hose, it's best to pull over and shut the engine off.

If all else fails, shut off the engine. After-boil will usually result from shutting off an overheated engine.

* Problems with after-boil when engine is shut off without an overheat condition? Replace the cap. Use a higher-pressure cap if necessary.

* The radiator cap should always be at the highest point in the cooling system.

Ed Newton
SR: Other than combustion, what are the sources of heat in an engine?
Ed: Other sources of heat include friction, heat recovery or exhaust cooling devices, electrical generating systems and normal heat-releasing mechanisms from other fluids doing work such as power steering systems. There is also quite a bit of heat transferred through the hood via sunlight or from radiation from road surfaces.

SR: How and where does oil absorb heat?
Ed: Part of the purpose of engine oils is to absorb heat that is generated from the engine combustion process. The oil then carries the heat through the lubricating system, which is then collected back in the oil sump where it releases heat and cools off. From there, it is circulated back to through the engine. Lubricating oils absorb heat by collecting it from a number of different surface areas and taking it away to an area where there is a large difference in ambient temperature. The oil is then able to yield this collected heat up to the cooler area. This process in addition to the radiator cooling system takes care of keeping the engine in the correct operating zone.

SR: How do valve springs create heat?
Ed: There are two sources of heat from valve springs. This first source of heat comes from the spring being compressed and released. The second source is the friction generated from the spring pushing against the valve, returning it to its closed position.

SR: When are oil coolers necessary?
Ed: In most cases, oil cooler is not necessary. The engine's cooling system is usually adequate to keep oil temperatures within safe limits. However, in applications which create extreme conditions and large peak demands on the existing cooling system, such as racing, oil coolers may be required to help keep the oil temperature at a safe level.

SR: Would you agree that generally a larger volume of oil or more oil pan capacity will not lower oil temperatures because oil is such a good conductor of heat; rather, it just takes it longer to get up to the same temperature?
Ed: A larger sump may slow down the temperature increases and the larger surface area may help disperse the heat better than a small sump, although in the end, an equilibrium temperature will be reached over time. Given the constraints of how large a sump can be used in a motor vehicle, the sump size has little effect on oil temperature after the vehicle is up to operating temperature.

SR: Does oil viscosity affect engine oil temperature?
Ed: This is a very complex question. Oil viscosity is a very important factor when determining oil's ability to absorb and transfer heat away from critical engine components. First, given two different viscosity levels, it takes more energy to pump the oil around the lubrication system with a thicker oil because it provides more resistance to flow and will build up heat faster than a thin oil being pumped through a restricted area.

If oil is too viscous and difficult to pump around the system, the ability for the oil to get to the critical engine components may be compromised creating more friction and heat build-up. The opposite can also take place if the oil is too thin for the application. In this case there may be an insufficient layer of lubricant to protect the metal components and the result is more friction and heat build-up. The bottom line is to make sure to use the correct viscosity for the application. We recommend synthetics due to their high film strength and resistance to breakdown which compromises viscosity.

SR: How hot is too hot for engine oil?
Ed: This usually depends on the oil being used. For example, synthetic oils usually have the ability to withstand higher temperatures than conventional oil. As a general rule, engine oils will begin to degrade when the oil temperatures begin to reach 250 degrees F, though premium synthetic engine oils resist this breakdown to a much higher degree due to the uniform polymer chain and superior additive package.

SR: Is there a minimum oil operating temperature?
Ed: Normal oil temperatures are going to be between 200 to 220 degrees F in most engines. It needs to evaporate water condensate that may have been built up in the system due to hot engine oil cooling down in cold ambient temperatures.

Jeff CANNULSR: What is the biggest challenge in controlling an engine's operating temperature?
Jeff: Probably the biggest challenge is sufficient airflow.

SR: What is considered to be proper engine operating temperature?
Jeff: That varies from engine to engine; some engines run better hot than cold. You have to find that fine line between temperature and horsepower loss.

SR: How are water pump flow rates determined?
Jeff: At our facility if we can dyno the cooling system we can determine the optimal flow and design around that.

SR: What controls the rate of water flow?
Jeff: The overall design of the motor--the block, heads, radiator. All water passages affect flow.

SR: Does increasing flow improve cooling?
Jeff: Increasing flow does sometimes increase cooling. Mostly when a vehicle is air-side limited.

SR: Can water flow through the radiator too quickly or too slowly?
Jeff: I'm not sure who came up with this theory but water cannot flow through the radiator too fast (that is a myth). Any time spent out of the engine allows the water to cool.

SR: What happens when pulleys are used to increase/decrease water pump speed?
Jeff: That is the most common way to increase or decrease water flow, but you need to be careful not to send it through the engine too fast for fear of cavitation.

SR: How much cooling system pressure should there be?
Jeff: Pressure in an everyday situation system should be in the range of 10 to 15 psi. In racing applications more pressure will work better because of the engine material makeup which allows for higher temps--NASCAR systems run 30 psi or more.

SR: When conducting tests, what type of coolant do you use?
Jeff: We use straight water when dyno-testing a system.

SR: What are the most frequently asked questions about cooling you deal with?
Jeff: Can coolant run through the radiator too fast? No.

Should I take out my thermostat and put a restrictor in to improve pressure in the system? No.

SR: What are the common problems/mistakes that cause engines to run hot?
Jeff: Lack of water flow on the coolant side and restricted airflow.

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