Cooling FAQ

1. Doesn't coolant need more time in the radiator to cool?

No. But a lot of people still think so.  We have come up with some explanations for the Doubting Thomas.

Debunking the I Can Have It Both Ways Theory

The water has to have "time to cool" argument is most common one we hear.  In a closed loop system if you keep the fluid in the heat exchanger you are simultaneously keeping it in the block longer.   Unfortunately, the block is the part that is generating the heat.   Sending hot coolant from your source (engine) through the heat exchanger (radiator) to the sink (air) will transfer heat as long as there is a temperature difference between the source and sink.   The engine is still generating heat the whole time so why keep the coolant there any longer than you have to.

Debunking The Conscientious Electron Theory

We hear that the coolant has  to stay in the system longer to cool but what is heat transfer really but conduction, convection and radiation of electrons.  The fluid in your system transfers those electrons based principally on the source-sink differential and the exchange material's transfer rate.  An electron moves at varying speeds - Bohr's model has it moving at 2 million meter/second and with a mere 11 million eV boost you can get an electron to 99.9% of the speed of light.   Though they move at varying speeds physicists accept that electrons move fast - really really fast.  Far faster than the flow rate of the water pump.   Your engine coolant's electrons do not know (or care) how fast you send them through the system - they just knows that the source is hotter than the sink and off they go.

Debunking Grandpa's Flathead Theory

"But wait a minute, I know Grandpa used to put washers in his flathead to slow the flow and cool his engine."  We know people did this too.  They still do it but the cooling benefit is not from the slower flow but the increase in dynamic pressure in the block that builds from the restriction.  Consider that Grandpa had two flathead water pumps sending twice the volume through the same size radiator core as the Model B 4 cylinder.  Too much flow in this no pressure system results in fluid loss.  Slowing flow rate helps prevent that.  At some point Grandpa maxed out the throughput and began building pressure in his block.  Increasing block pressure helps reduce the onset of hot spots on his cylinder walls and formation of steam pockets in his block. This is a real benefit and does help cooling but is only realized when throughput nears capacity or is at capacity.   While these restrictions may make sense when your rpm is excessive or your flow rate exceeds your heat exchanger throughput, they do not make sense for most applications.  If you doubt this thinking then try this simple Ask Dr. Science experiment; clamp off the lower hose while you watch your temp gauge.  Hopefully, you will debunk Grandpa's theory yourself before you experience vapor lock and melt your engine.

Flow restriction is not all bad if it serves to prevent cavitation.  Cavitation occurs when a pump turns so fast that you generate lower pressure and air bubbles or vapor forms.  These bubbles eventually implode and damage the engine block wall and impeller. Rapidly spinning the impeller can literally rip the air from water but may not actually move the fluid, it's tantamount to turning an eggbeater in a paint bucket.  Restricting the fluid flow to raise system pressure in the block may help prevent cavitation at higher RPM but is it necessary for most vehicles?  Probably not.

Most vehicles do not need  to restrict flow because they do not reach or sustain high RPM.  Additionally, thin aluminum radiators already restrict by design e.g. fewer rows of thinner tubes.  Restrict it further and you may as well hose clamp the lower radiator hose and we know how that works out.   When you face Grandpa on the track you may want your washers, otherwise, keep them in the toolkit.

Simply put, you have a far better chance of keeping your cool with greater flow rate through your heat exchanger and exiting the system than holding it in your heat exchanger while generating heat in your engine block.

2. Low Flow Luddites or..."the guy at the shop said"

When we are chasing an overheating problem, its common to seek help and who better than the guy sitting at the counter in a parts house or the countless experts begging to be heard on internet forums.    They are generally excellent credentialed sources but we sometimes meet a low flow believer who thinks heat exchange is determined by how long the fluid stays in the block and how long it stays in the radiator.  We fall back on simple logic. FlowKooler pumps achieve higher flow rates through a better impeller design.  The high flow rates are seen far earlier on the rpm curve than OEM and self-professed "performance" pumps which use OEM-like impellers.  The flow is more efficient builds block pressure sooner which helps in preventing cavitation. All good stuff which no one would refute. The low flow proponent' s argument of "too fast" begins to falls down when we discuss flow itself.  If spun fast enough, even the most poorly designed stamped steel impellers will achieve the same hi flow flow rates.  Granted they may cavitate, they may pump less efficiently and it will always occur at higher rpms. The point here is whether a discount design pump was spun at hi rpms or our well designed pump was spun under normal driving conditions; flow is flow.  Can it be logically argued that a hi flow pump flowed coolant too fast if an OEM pump achieves the same flow rates.  No, it cannot.

3.Why am I seeing erratic temperature swings?

If you experience erratic temperature most likely you have air trapped in your cooling system.  Air rises to the top of the cooling system and gets trapped, potentially causing the cooling system to vapor lock.  When the radiator is made the higher point in the system, the air will escape into the radiator and it will be vented out through the radiator cap and the overflow system. There are a couple of ways to resolve this and free the air.
    1. Free trapped air via the vent plug Fill the system as normal with your antifreeze mixture.  Use a 50-50 mixture of antifreeze and distilled water.  Auto parts stores will sell in-expensive to determine the concentration of antifreeze in your system.
    2. Find the small threaded plug with a hex fitting at the top of the water box where the upper radiator hose enters the engine.  Loosen the fitting to the point where air starts to escape and inspect the thread sealant – re-apply as needed.  Be sure to run your heater to allow water into the heater core.
    3. Add antifreeze to replace the air being expelled through the plug.
    4. As fluid starts to seep out of the plug tighten it up, top off the overflow tank, and you’re done.  If you continue to get erratic temperature readings, or fail to get heat from the heating vents, re-open the screw and let any residual air to escape.
    5. To vent a system in this method raise the car such that the radiator is higher than the water box.  You can do this through jacks (and jack-stands, never support a vehicle by a jack alone), ramps, or a nice steep hill and parking with the nose of the car pointing up the hill.
    6. You’ll need to let the car warm up so that the thermostat opens for the system to vent in this method.  AVOID BURNS AND PROTECT YOUR EYES! - Never open the cooling system when hot.  Let the car warm up from cold with the radiator cap removed.  You may get some spillage while the coolant expands and the air bubbles out.  This coolant will be hot.
    7. Allow the engine to run, with the heater on (fan can be off or on low) until the thermostat opens and all the air is allowed to purge.  Once the thermostat opens you will see the coolant level inside the radiator bubble and drop.  Continue to add antifreeze to maintain fluid level.  Again be careful as the coolant and any steam released by the system will be hot.  Once the upper radiator hose becomes hot to the touch and no further air issues from the system carefully replace the radiator cap and ensure the overflow bottle is filled to the Max line.
    8. Continue to allow the car to run to allow the temperature to stabilize.  If you don’t have a temperature gauge allow the car to run until the fan cycles on and off at least once.  During this time ensure there are no leaks from the system and that the upper radiator hose gets hot to the touch (especially close to the radiator).  If not then allow the system to cool, and repeat the steps above to purge any remaining air.
  NOTE:  If you are not comfortable leaving the radiator cap off during warm-up then you can accomplish the same thing by leaving the radiator cap on and allowing the car to cool down after step four and then repeat steps one through four again, making sure the overflow bottle is maintained full.  The heating and cooling cycle will push the air out through the overflow bottle and then suck coolant in to replace the air when the engine cools.  The car should remain inclined for the whole procedure.

4. Why is my car overheating?

There is no one single answer for this question but here is a list of places to start to isolate the problem.

1. Before you do anything else, tune up the car. Many overheating cars are out of tune, running lean or with retarded timing. A lean fuel mixture will overheat your car. If your engine runs lean you can chase your tail looking for problems in the cooling system and never figure it out. The easy way to do this is richen your jetting a couple of steps. If the overheating is better, you're on the right track.

2. There is a lot of misinformation about ignition timing and cooling. Retarded timing contributes to overheating. Advanced timing helps cooling. Advance your initial timing a few degrees and see if it helps the car run cooler. However, if you advance to much you risk detonation and that too will cause you to overheat.  If you start to detonate back off the timing. Overheating cars should always run vacuum advance. Vacuum advance helps cooling.

3. Radiators: Your radiator is the primary means to bring the engine temperature back to the optimal temperature.  Radiators can be dirty, clogged, poorly designed, too small for the engine, fin density too great for the low rpm get the picture. We know a little bit about radiators too; visit The Brassworks FAQs page for more information.

4. Airflow.  Inadequate airflow can cause overheating. You have to get the air through your radiator and out of the engine compartment.  Obstructions to that airflow can cause a cushion of hot air to build around your block and engine compartment. A properly fit fan shroud sealed to the radiator helps to channel the air through your radiator.

5. Inadequate coolant flow. If you are overheating at idle, stop and go traffic, on grades or towing you might benefit from higher flow rates through the radiator.  reducing the cycle time between the engine and the heat exchanger provides more opportunity to shed heat.  Hi Flow water pumps and hi flow thermostats create these incremental opportunities.

6. Modified gear ratios: Generally  speaking lower ratios give slower acceleration, higher top speed and less braking power – Higher ratios give more acceleration, less top speed and more braking power.  Changing gear ratios may generate more heat in the engine and potentially cause overheating.

7.Coolant composition. Most people run 50/50 for the increased boiling point and the engine block preservation.  It is worth noting however that the specific heat capacity of ethylene glycol based water solutions is less than the specific heat capacity of clean water. For a heat transfer system with ethylene glycol manufacturer recommend that the circulated volume must be increased compared to a system with clean water.

In a 50% solution with operational temperatures above 36 degrees F the specific heat capacity is decreased with approximately 20%. The reduced heat capacity must be compensated by circulating more fluid.

An ethylene glycol mixture does raise the boiling point and will lubricate the water pump to prevent corrosion in system. Some people change the ratio of these fluid in hot season and when the weather cools return to 50/50.

8.Cooling the transmission is added work for a radiator.  Do not obstruct airflow or heat the air passing over the radiator by placing the cooler in front of the radiator. Transmission and engine oil coolers constructed within the radiator tanks can also tax your cooling system by introducing a heat source in the tank.  Using external coolers may help because they take the heat out of the cooling system.

9.  The elusive manifold vacuum leak. Trouble at idle may point to a manifold vacuum leak. If you find you're too fast an idle speed, rough idle or stalling, misfiring on acceleration or adjustments to your carburetor seem to have limited effect than you may have a vacuum leak causing overheating.

10.Use a better grade of gas. If you are not running premium fuel and experience overheating a higher grade may help. If there is no improvement, try advancing your timing a few degrees. A little extra octane will allow you a little extra timing without getting into detonation. Earlier engines were designed for better gas than is sold today.

11.Head gasket leaks from the cylinder to the water jacket are a definite cause of overheating. That's bad news and a whole lot of work.

12.A stuck thermostat. Many thermostats are designed to fail in the open position but thermostats have also been known to stick partially open and impede coolant flow.

13.A slipping water pump belt can slow the flow rates from the water pump and impede coolant flow resulting in overheating.

14.A fan blade with insufficient blade count or incorrect pitch may not draw enough air.  The fan can also be too far from the core or located too far inside a fan shroud which then traps the air, moderates the heat and heat exchange is diminished.

15.Headers without thermal coating can cause overheating.

16.A stretched timing chain or belt can cause overheating.

5.Why is my engine overheating on the highway?

  • Your waterpump could be cavitating
  • The path of fluid through the radiator could be constricting the coolant e.g. double pass or triple pass
  • The fin pattern in the core could be so dense you have a block of aluminum or copper
  • You tried to save money and bought inadequate radiator
  • Your lower hose may be collapsing at higher rpms choking flow
  • Something may be obstructing your airflow e.g. winch, polished aluminum tank
  • Air could be routing around your radiator core

6. If I am overheating; do I even need a thermostat?

Running without a thermostat is a slippery slope.  The thermostat provides drag on the water flow which increases the backpressure the water pump.  This additional pressure, over and above the nominal static pressure of the radiator cap raises the boiling point of the systems coolant. The higher temperatures helps suppress localized film boiling at hot spots such as around the exhaust port.

The transition from nucleatic boiling (bubbles of steam originating from irregularities on the surface) to film boiling (where the hot surface is coated with a film of steam) is called Departure from Nucleatic Boiling or DNB.  DNB is very very bad because steam is a good insulator compared to water.  Once DNB occurs, the area under the steam gets hotter because the steam fails to remove engine heat.  The adjacent metal, which is still wetted, heats therefore from conduction.  DNB happens there.  The process spreads until substantially all the coolant-wetted surfaces are insulated by a film of steam.  Uh oh - the engine overheats.  To make matters worse, this steam buildup in pressure will forces the radiator cap open spilling coolant to a recover can (or the street).  The loss of coolant from the system escalates the whole problem and the situation get progressively worse.

The second issue is that of water pump cavitation and surge.  Operating a pump at high RPM with insufficient head pressure provided by the frictional losses in the coolant passages and the thermostat creates a greater likelihood that the pump will either cavitate or surge.  Cavitation is the condition where localized boiling or degassing occurs as the fluids exits the impeller vane and pressure changes.  Surges are the result of unstable flow rates.

Both cavitation and surging are destructive elements to the engine block and cylinder wall. How destructive? Cavitation's collapsing bubbles function like a sand blaster that will eventually erode away impeller material and block wall surface.  Surge can do the same thing and the added vibration can stress the impeller enough to break it. What often looks like corrosion damage to the impeller when the housing is intact may actually be cavitation damage.

7. When should I check my cooling system?

The best time to inspect your heater and radiator hoses is cooler weather.  Less obvious signs of decay can be seen and felt by grabbing hold of a cold radiator or heater hose and giving it a good squeeze. Brittle or cracking material, a spongy feel, or a hose sticking to the inside of itself are bad signs.

8. Can I block off my BBC bypass to the intake manifold?

Blocking off the bypass will route the coolant directly to the radiator which is good for heat exchange but remember the bypass circulates water until the t-stat opens.  This routing helps circulate the fluid and raises block pressure which prevents the formation of hot spots in the engines.  Failure to run bypass may result in excessive pressure buildup at the t-stat which can cause it to open early or potentially cause gasket leak.

9. Will the high pressure affect my radiator?

Its not likely.  The increase pressure is an ancillary benefit of higher flow rates.  The pressure you experience will on par with rpms greater than 3,000.  No problem there, no problem with the flow rates and pressures when we shove them down the rpm curve.

10. Should I change my pulley sizes?

We recommend stock pulleys and no changes but engine builders being engine builders…they’ll do what they’re going to do. Enclosed is a thoughtful response to this question.  It was borrowed from from, a mustang related forum.  The contributor who submitted this response ultimately arrived at the same conclusion e.g. no change to pulleys.

All of the accessories take more power to turn as rpm increases. That power - parasitic loss - is provided by the engine. If you can reduce the power it takes to turn them - you free up some of that power and make it available to go to the crankshaft instead of using it to turn accessories. In other words -- you reduce the amount of parasitic loss.

 One way to reduce the amount of power needed to turn those accessories is to turn them more slowly. One way to turn them more slowly is to DECREASE the diameter of the DRIVE (crank) pulley. Doing so reduces the speed at which the accessories turn by the same ratio (or %) as the ratio of the diameter of the pulleys. If the underdrive crank pulley is 20% smaller diameter than the pulley you're replacing, it will reduce the speed of the accessories by 20%. Reducing the speed those accessories are turning means less power is needed to turn them -- which frees up that power to go to the tranny input shaft.

 Now -- since there is no free lunch, there are sometimes prices to pay for this approach. Frequently, underdrive pulleys slow down the accessories so much that at idle, performance is impacted. Slowing the water pump down also slows down the mechanical fan that is attached. This means less air across the a/c condenser and radiator, and less water being pumped through the engine. In some cases - overheating in traffic during the summer is the result. The a/c blowing less than cold air can also result - both from less air moving across the a/c condenser and from the compressor turning at a lower rpm. Slowing down the alternator may mean it can't provide enough current to meet the sytem loads -- which means you draw the battery down more. Sometimes this leads to starting issues, or dim lights, lower quality idle because of lower than optimum voltage in the ecu circuits, etc. Slowing down the power steering pump sometimes leads to problems when tight turns are made at slow engine speeds as in a parking lot -- squealing pumps are not uncommon.

 Now - these symptoms don't happen to everyone. Each car/driver/accessory use/driving pattern/geography-weather patterns/etc. are different and unique. Just because someone else has a problem or doesn't have a problem with underdriven system IS NOT predictive as to what will happen to someone else's car.

 Most underdrive sets therefore have a small DRIVE pulley to slow down all accessories, combined with different size pulleys for the water pump and the alternator to SPEED those 2 accessories back up - to avoid the potential for cooling/charging/a-c issues described above. For DRIVEN pulleys, a smaller pulley speeds them up. So a full set of underdrives typically has a smaller DRIVE pulley to slow everything down, and smaller DRIVEN pulleys for water pump and alternator to partially speed them back up. Slowing everything down 25%, except the water pump/fan and alternator - which get slowed down (after all the math is done) only about 10-12%.

 Then there are race pulleys which are even smaller on the drive pulley -- and usually consist of JUST the drive pulley. Because most racers are only driving the alternator and water pump -- and they're running at MUCH higher engine rpm. So turning those two accessories slower isn't as much of an issue when the engine is operating 4000-6000 rpm.

 So - what will happen with your car? The ONLY way to know for sure is to try it. But fear not -- if you have charging, cooling, or other issues - there is always a market to sell the underdrives used -- to people that want to find out what's going to happen when they put them on.

 After experiencing problems in all the areas described above (a-c, cooling, charging and power steering squealing) I pulled off my underdrives and put the stockers back on. For a racing only car -- they're a good investment. For most street cars -- the challenges they can cause at low speeds are rarely worth the gains you see; and the biggest part of the gain occurs over 3,000 rpm -- where we don't spend much time on the street anyway.

11. What is that Knocking sound?

This is not really a cooling question but it made the list...

Knock or pinging or detonation or spark knock is caused when the air/fuel mixture ratio in the cylinder causes the fuel to burn unevenly. Fuel normally burns in pockets and when each pocket of fuel burns, a shock occurs that burns the next until all the fuel is burned in that stroke. When a knock is present, the pockets don't burn evenly, causing the cylinder wall damaging shock waves that can damage the piston itself. The pocket formation also creates the common "pinging" noise that is often described when knock is present.

You also can get a knock sounds from the following:

  • piston slap
  • worn piston bearings
  • worn wrist pins
  • loose or worn lifters
  • loose or worn rockers
  • low octane gas
  • carbon deposits on cylinder walls
  • incorrect spark plugs
  • intake leak
  • bad bearings on the crankshaft
  • thrown rod
  • there is a body in your trunk