Air Conditioning Cooling Guide

20 Frequently Asked Questions And 90 Answers About Keeping Your Cool
By Ron Ceridono (as printed from Street Rodder Magazine)

Of all the street rod topics we deal with, the one that generates the most questions is cooling-all sorts of cooling. It's a big subject that can include everything from the parts under the sheetmetal to the passengers who ride inside.

To address a topic this large, we put together a series of the most frequently asked questions to present to experts in their various areas of expertise. This month, we begin with air conditioning and radiators-two subjects that are near and dear to most street rodders. In future issues, we'll deal with fans, fan controls, water pumps, insulation, and all the other hot topics concerning cooling.

Air Conditioning
1. How is the size of an air conditioner evaporator determined?

Hot Rod Air
The basic concept is to install the largest-capacity unit possible. We match evaporator coil to blower assembly and case design for maximum performance. Several factors also come into play. First and foremost is the size of the interior you are trying to cool (a coupe will require less Btus than a four-door sedan). The second factor would be the amount of physical space behind the dash-in some cases, you may need to have the evaporator hang down below the dash area a bit if space is tight. Keep in mind that when the system is on high and working to its maximum capacity-say, on a 100-degree day-it is easier to turn it down than to turn it up.

Southern Air
Our first heat and air unit was a mere 16 inches wide and 7 1/2 inches deep, and we soon learned that this fit tons of small rods, such as A models and most '30s cars. This unit would heat and cool these cars, but when it came to the '40s sedans, it was a little small so we began matching units to the size of the vehicle. We dyno-test all of our new units before we put them into production; so, if the customer gives us the car make and model, we have a unit for it based on the inside cubic feet of space in that car.

Vintage Air
We always recommend selecting the largest-capacity evaporator that will fit in the customer's vehicle. You can turn down the fan and adjust the temperature on a system if you get too cool inside your car, but you cannot make a unit that is too small cool a large area. Think about the climate-control system in the early stages of car construction. We manufacture mock-up units to not only determine fit, but to make it easy to fine-tune mounting brackets before hanging the actual unit. Many of our dealers loan these units for a nominal fee.

2. How is the size of the condenser determined?

Hot Rod Air
Condenser size is very simple: The larger the better; fill as much of the radiator core size as possible. You cannot oversize a typical street rod condenser. You should have a minimum of 230 square inches of condenser.

Southern Air
The size of the condenser is determined by the core size of the radiators. We use only parallel-flow condensers, and the salesperson will ask the year and model of the car when the kit is ordered. We will then supply a condenser that will fit.

Vintage Air
All of the components must be sized properly and matched. The evaporator absorbs the heat from inside the vehicle, and the condenser dissipates the heat to the air. We recommend selecting the largest condenser that will fit within the dimensions of the radiator core, and the use of parallel-flow condensers.

3. What engine/cooling system modifications are necessary to add air conditioning?

Hot Rod Air
Most important is making sure the motor is positioned in the framerails so you can run a good-size fan. Keep in mind that the newer blocks are designed to run warmer temperatures than earlier motors (so don't be alarmed when you see 210 to 220 degrees

Southern Air
In a nutshell, you need a really good radiator fan, thermostat, and medium-volume water pump. Stock-size pulleys are your best bet, as GM and Ford have better engineers than any of the aftermarket sellers. I have an aluminum, dual-row radiator in my '46 woodie with 1.50 tubes and a 430hp Vortec 350. It will not overheat, but I also have one of our 16-inch, 225-watt fans with our adjustable thermostat.

Vintage Air
If an engine-cooling system has adequate components and is operating at its proper efficiency, no modifications should be necessary just because air conditioning is added to a vehicle. If an engine-driven fan is on the vehicle, we always recommend a shroud to maximize the efficiency of the fan. If a customer is using an electric fan as his primary cooling fan, we strongly recommend the addition of a trinary safety switch, which allows A/C system pressure to engage the fan in conjunction with and independent from engine temperature.

4. Do you recommend mechanical or electric fans with air conditioning?

Hot Rod Air
Engine-driven fans with a shroud will give you the most consistent cooling. Depending on the vehicle, if you can also run a pusher fan as a secondary fan, this will increase the air-conditioning performance at slow speeds. A second scenario is that anything less than a 17-inch engine-driven fan usually does not have the cfm to be used as a primary cooling fan; at that point, you should look at using a 14- to 16-inch electric fan and shroud combination. With air conditioning, you will need at least 2,500 cfm of airflow.

Southern Air
We highly recommend the six-blade Flex-a-lite stainless with our radiator shroud; however, this combination doesn't fit all cars, so get the best and biggest fan you can on the car. For instance, '33-34 Fords don't have much room between the water pump and the radiator, but one really good 12-inch fan and one 10-inch will fit on the grille side. If you're building a chassis, set it up with the motor back far enough to make room for a good fan.

Vintage Air
The key is airflow, and it doesn't matter if you achieve the airflow with an engine-driven fan or an electric fan. When using a mechanical fan, we like to see a minimum of a five- or six-blade fan with 2 inches of pitch. Be careful when selecting an electric fan as the primary cooling fan-many just don't have the capacity. We recommend selecting a fan with at least 2,600 cfm when the vehicle is equipped with air conditioning. Both fans will benefit from a good shroud and a smooth air path.

5. What interior modifications do you recommend when adding air conditioning (tinted windows, insulation, etc.)?

Hot Rod Air
Keep one thing in mind-air-conditioning systems do not make cold air; they remove the heat that is in the air, so you must seal all holes and door seals to keep the forced air from entering the interior. Insulate any surface in the interior, doors, firewall, headliner, upper cowl, kick panels, etc. If there is a surface, insulate it. This will also give you a quieter ride. Tinted windows, especially in the South and in warmer climates, will help-nothing more so than purchasing an evaporator large enough for your vehicle cab.

Southern Air
Tinted windows help, but nothing beats door seals and floor and door insulation. If the doors are not sealed, it creates a vacuum cleaner effect and out goes your cool air.

Vintage Air
The single most important factor in improving the efficiency of a climate-control system is how well the vehicle is sealed and insulated. You must keep the engine heat and outside heat load out of the interior of the vehicle. Tinted windows help reflect the heat soak from the sun, and this helps the air conditioner cool the vehicle more efficiently as well. Proper weatherstrip and door/ glass seals will also help isolate the interior and lessen the load on the A/C system. Be sure to seal any holes in the firewall, along with the clutch and brake pedals.

6. Do you offer air-conditioning compressor mounting brackets?

Hot Rod Air
Yes, for just about any motor out there. If there is not a specific bracket available, we also offer a universal bracket that gives the customer a cradle to start with and all they will need to do is build a baseplate for attaching it to the motor.

Southern Air
We carry a full line of Alan Grove steel brackets and our exclusive polished stainless steel brackets. We also make many billet aluminum brackets and carry all of Street & Performance LS-series mounts.

Vintage Air
We offer a wide variety of compressor mounting brackets, ranging from basic steel assemblies to our Front Runner drive systems that package all engine accessories.

7. Do you recommend remote condensers?

Hot Rod Air
Only as a last resort. Keep in mind that remote condensers pick up road heat and exhaust heat (it's not getting fresh air that is available when the condenser is installed in front of the radiator), so if you use one, be sure to run a trinary safety switch-just a little piece of insurance. That way if your system should develop higher head pressures, the switch will cycle the system off so you will not blow an A/C hose. It will also act as a low-pressure switch in case the system develops a leak somewhere. Remote condensers work fine around town or at slower speeds, but once on the open road, the system starts working harder and building up pressures, thus becoming less efficient than a front-mount condenser.

Southern Air
Our remote condenser will perform as well as the radiator-mounted one, and even better in traffic situations. It has a high-performance fan, pulling air across it constantly to stay cool, as opposed to the front-mounted ones that have very little air flowing through them at idle. Our remotes have a full shroud also. In some cases, the remote is a must on Model As and cars with exposed radiator cores through the grilles.

Vintage Air
Our first choice is always to mount the condenser at the front of the vehicle where it gets the best airflow at the coolest possible temperature. We look at remote condensers as an option if there is no way to package a condenser at the front air stream of the vehicle. Remote condensers must be mounted in such a way to direct an adequate amount of the coolest airflow available through the condenser to remove the heat from the refrigerant. A trinary safety switch is an absolute must with a remote condenser to protect the compressor from excessive system pressure and engage the condenser fan.

8. Do you offer heat-and-air/air-only/ heat-only packages?

Hot Rod Air
Approximately 98 percent of what we sell would be heat and air-combination systems, but we do offer air-only and heater-only units.

Southern Air
We offer heaters, air-only units, as well as air and heat units. We have them to hide behind the dash or hang under the dash, and have our new TrimLine Heat Air unit that hangs under the dash and has defrost also.

Vintage Air
We offer several cool-only underdash systems, as well as a variety of heaters and heater/defrost systems, but our most popular systems are our Gen II heat/cool and heat/cool/defrost systems.

9. Do your systems include defrosters?

Hot Rod Air
Yes, all of our air-conditioning systems are available with or without dehumidified defrost.

Southern Air
Yes, all of our MaxiKooler units and most TrimLines feature our electric-servo motor-controlled defrost.

Vintage Air
Most of our systems are available with dehumidified defrost. Our Gen II-series system features a dedicated defrost mode.

10. What are the most common air-conditioning installation errors?

Hot Rod Air
1. Not preparing for air conditioning in the planning stages, lack of insulation, and not allowing for the space required. The evaporator requires a certain amount of space-fuse panels, computers, and speakers can be remotely mounted if necessary.

2. Overcharging system: 134a is very critical on the charge. We all need to keep in mind that 134a is a bubbly milky refrigerant; you will have bubbles in the sight glass. These are dryer and expansion valve systems, so the low side should be around 12-18 lbs.

3. Under-sizing the evaporator to the vehicle-especially on finished vehicles. Customers constantly call stating they only have so much space left, so they'll take whatever will fit. They don't like to hear that they really need to move other components around to get an evaporator large enough to do the job.

Southern Air
The biggest error is the incorrect installation of the water valve. Many people think that the water flow is from the water pump, but it's from the intake manifold or my favorite place, the cylinder head. If you use the port between numbers 7 and 5 on the head, you have one quarter of the heater hose to run and this is the hose you install the water valve on. Our electric four-way valve helps solve this problem.

The second biggest mistake is the over- or undercharging of the system.

Vintage Air
The most common installation error we deal with is improper system service.Modern 134a systems must be properly evacuated and charged to operate at maximum efficiency. Failure to properly charge a system will result in improper operating pressures, unsatisfactory duct temperature, and overall poor system performance.

The second is failure to properly install and insert the thermostat capillary tube. This results in improper coil temperature sensing by the thermostat, which leads to coil freeze-up, reduced airflow from the unit, and poor duct temperature.

Finally, improper condenser installation. Parallel-flow condensers must be mounted with the manifold tanks in a vertical orientation on each side with the larger fitting (#8) at the top, and the smaller fitting (#6) at the bottom. Mounting the condenser on its side leads to oil pooling in the tanks, and mounting the condenser upside down forces the compressor to push liquid refrigerant uphill as it condenses.

RADIATORS

1. What are the pros and cons of copper/brass radiator construction?

Be Cool
Pros: Cost, and they can be recorded if necessary.

CONS: Heavy, less effective heat dissipation, lead retains heat.

Flex-a-lite
Pros: At one time, materials to build this style of radiator were more economical and construction required less labor, as automation could be used to produce and assemble radiators.

CONS: The use of zinc and tin in the solder can create a point of failure once antifreeze-coolant additive packages fail. Without installation of a zinc anode or additional additives to the coolant, the solder, which holds the radiator together, would degenerate, causing the radiator to leak or just come apart. To limit the amount of solder in the assemble process, some radiators are put together under pressure, pinched, or "glued." These methods have not always held up in the long term.

Griffin
Pros: Easy to repair using a solder.

CONS: Heavy, tube construction inhibits maximum heat transfer.

Mattson's
Pros: They are rebuildable, and you can recore them. You can keep it looking original.

CONS: It's hard to stay competitive because the cost has gone up dramatically. The cooling technology in aluminum cores is much more efficient than the copper/brass.

U.S. Radiator
The thermal conductivity or heat transfer rate of copper is 92 percent versus aluminum at 49 percent. However, the copper fin is bonded to the tubes or water passages using lead solder, which is very inefficient and slows the heat-transfer rate to just slightly better than that of aluminum. This can be a disadvantage if the bonding process does not allow the copper fin to touch the brass tube and why not all copper/brass cores of similar design but different manufactures transfer heat equally. Our tests revealed an almost exact temperature drop, aluminum versus copper/brass, at all operating ranges where the core design was the same, with a slight advantage going to the copper/brass unit (even against our own aluminum units).

Walker
All pros. Copper transfers heat better, has twice the tensile strength as aluminum, and resists corrosion.

2. What are the pros and cons of aluminum radiator construction?

Be Cool
Pros: Better heat dissipation, lightweight, aesthetically pleasing, repairable.

CONS: Raw material cost, does not resemble an OE radiator.

Flex-a-lite
Pros: Aluminum radiators dissipate heat more readily than other materials. In the case of the Flex-a-fit radiators, the tanks with the "T" bar mounting and internal fins have shown an increase in heat dissipation by 230 percent. Aluminum radiators can be assembled with welds for strength, and the tanks can be produced with an alloy for additional strength.

CONS: Aluminum radiators are more susceptible to internal damage from electrolysis if coolant additives deteriorate or a scavenger material like zinc is not introduced into the cooling system. Aluminum radiator construction is more prone to fatigue cracking over time than the softer copper or brass material.

Griffin
Pros: Large tube construction allows maximum heat transfer, lightweight.

CONS: Not as easy to repair, must be welded.

Mattson's
Aluminum cost is becoming more competitive compared to copper/brass. Aluminum radiators are much more efficient than copper/brass radiators. We can make a custom aluminum radiator sometimes faster than we can order a brass core, receive it, and put it together. We can also polish or powdercoat an aluminum radiator to make it blend in or stand out.

U.S. Radiator
Because of their weight and durability, copper/brass radiators have been around a long time and are easily disassembled and reassembled for cleaning purposes. Not the case with aluminum unless speaking of the OE version that comes with crimp-mounted plastic tanks.

Walker
All cons. Aluminum cannot handle vibrations as well as copper; it does not transfer heat as well as copper or resist corrosion.

3. What do you recommend for minimum/ maximum engine-operating temperatures?

Be Cool
Depends on application (i.e., electronic fuel injection/carbureted). Most engines operate at 180 to 210 degrees Fahrenheit. This temperature range is suitable for most street applications.

Flex-a-lite
The thermostat or restrictor plate that is installed in the engine determines engine-operating temperatures. Some fluctuations in temp will occur due to load and outside ambient temperature changes.

Griffin
Today's late-model fuel-injected engines are designed to run at least 10 degrees hotter than earlier-model carbureted V-8s. A safe range is 185 to 210 degrees at idle and 185 to 200 degrees while moving. You want the engine to get hot enough to evaporate any moisture that may have accumulated inside of the oil system.

Mattson's
The engine manufacturer always suggests an operating temperature. The late-model engines are running hotter and hotter. What used to be hot is now considered too cold in some applications. Some systems with fuel injection do not operate correctly at lower temperatures. We always suggest engine manufacturer guidelines.

U.S. RadiatorMost hobbyists aren't concerned with fuel efficiency, so our recommendation would be 175 to 195 degrees. Higher operating temps will burn fuel more efficiently, but the increase in operating pressure and metal distortion can easily create problems over time.

Walker
Minimum and maximum recommended engine temperature range varies. Engines up to the mid-'80s normally had a range of 180- to 200-degrees operating temperature. The late '80s to the present have a range of 195 to 220 degrees Fahrenheit.

4. Do you recommend internal or external automatic transmission coolers?

Be Cool
Internal transmission coolers are sufficient for normal driving conditions. External transmission coolers are needed for towing, race conditions, or when the addition of heat to the radiator is not desired.

Flex-a-lite
Not all radiators are built with internal transmission and engine oil coolers. In these applications, the use of the external oil cooler would be required. In most cases, the external oil cooler will excel over the smaller internal coolers.

In extreme cold-weather areas of the country (approximately 20 degrees Fahrenheit and below), the use of the internal oil cooler would be necessary to keep the oil heated, allowing it to flow more readily. It can then be run through an auxiliary cooler mounted in front of the radiator. Most new-style, synthetic fluids will still flow under the above temp. The use of an external cooler with this style fluid would then be OK.

Griffin
If the stock converter is used, then an internal transmission cooler is fine. If a high-stall converter is used, then we recommend the use of an external cooler. The external cooler can be used in conjunction with the transmission cooler in the radiator.

Mattson's
Transmission coolers can do a terrific job inside the radiator. You can always add an external cooler if you have a special application like a high-stall or towing application. The one issue with a cooler is that it is in front of the radiator so it's an airflow obstruction. There is added heat blowing through the radiator, and the transmission cooler inside the tank creates additional heat inside the radiator. The difference is probably so minimal that it doesn't matter; however, being in the radiator provides for a cleaner look and a chance to add an additional cooler if needed. If you get down to it, an under-the-car cooler with a fan on it would be ultimate, except for the extra wiring and road debris that may affect it.

U.S. Radiator
External transmission coolers are preferred to keep unnecessary heat out of theradiator.

Walker
A proper-sized internal cooler works well for normal driving. A proper-sized external cooler working in conjunction with the internal cooler is a must for cars pulling trailers or performance transmissions with high-rpm stall speeds.

5. How does radiator core thickness affect cooling?

Be Cool
Increasing core thickness slightly increases cooling capacity. It should be noted that increasing frontal area is a more efficient means of increasing cooling.

Flex-a-lite
In vehicles with the belt-driven fan option, the thickness would not be as much of a factor in cooling as the electric would. Electric fans, when installed, will require a minimal static load, blockage, in front of it. In the event a "multicore" radiator is used, it may cause a reduction in the cfm that the electric fan can pull through the core. Too much static load will reduce the effective cfm to a point where it will not cool the application.

Griffin
Cfm is too low with a thick core (more than 2.25 inches) and the fan, so you then run the risk of the radiator not cooling properly at idle.

Mattson's
The air will have a hard time cooling off the radiator if the radiator is too thick. Some of the old continuous fin-type cores were great for the dirt roads and minimal traffic a long time ago.

U.S. Radiator
An increase in thickness over a stock application allows for greater fin bond surface and therefore greater temperature drop. When going from a two-row to a four-row, for example, you double the fin bond or heat transfer points. However, the increase isn't a one-to-one because the transfer efficiency of the trailing rows is adversely affected by the increase in air temperature from the previous rows and the decrease in air velocity caused by the increased thickness.

Walker
The key to all cooling is determined by the prime cooling area. The prime cooling area is the frontal core area and the prime cooling area of the core is the first 1 inch. This is the area where the radiator fin's design and the air management package are critical. Doubling the thickness of a radiator results in only 20 percent more cooling efficiency.

6. How does fin count affect cooling?

Be Cool
Increasing fin count will increase the capacity to reject heat, up to a point. If the fin spacing becomes too close, the airflow can become restricted and may reduce cooling capacity. OE radiators use 18 to 20 fins per inch. Be Cool has proven that its 14-fin-per-inch design is the most efficient for air travel with the least resistance.

Flex-a-lite
Theoretically, the more fins per square inch a radiator has, the better it will cool. More fins will provide more surface area to dissipate heat. Reality is that the number of fins and the angle of the fins can affect the ability of the cooling fan to move the air through it.

Griffin
Same effect-you can run a thin core with a high fin count, or you can run a thicker core with lower fin count.

Mattson's
The type of fin and the number of fins per inch determine the efficiency. The type or too many or few fins affect the proper efficiency.

U.S. Radiator
We've found that core design, and not material, had the greatest effect on temperature drop. While all radiator cores might look the same, they perform vastly different based on tube spacing and fins per inch. Heat transfer points where temperature is actually allowed to leave the radiator are where the fin is bonded to the tube. The more transfer points, the greater the temperature drop. A '60s core, for example, had a 1/2-inch tube spacing (i.e., 1/2-inch fin between the tubes), and by going from a two-row radiator to a four-row core design, we were able to double the heat transfer points, which resulted in a 15 to 20 percent increase in temperature drop without changing the other variables (airflow, coolant flow). In the '80s, the Japanese came out with a core design in response to the need to downsize that has become the standard and was efficient enough to allow the re-introduction of aluminum (a less efficient heat transfer material) at the OE level. By changing the tube spacing to 3/8-inch, a design referred to as High Efficiency in the industry, more tubes or water passages and fin were allowed across the face of a core with a specific width in inches. The design was simple enough but proved to be very efficient in that more heat transfer points created greater temperature drops inlet to outlet.

Walker
Fin count (number of fins in a given inch within the core area) affects the number of times a fin-to-tube contact is made. The more fins per inch you have, the better the heat transfer (up to a point). Too high of a fin count will cause an air blockage, limiting the core's cooling performance.

7. Do you recommend mechanical or electric fans?

Be Cool
Electric fans are preferred because sufficient airflow is maintained at idling or low ground speeds. Airflow supplied by a mechanical fan changes with engine rpm and can be insufficient at low-engine rpm, and also uses up horsepower.

Flex-a-lite
With most cars and light trucks, the installation of an electric fan is a great choice to free up horsepower and increase fuel mileage. If the vehicle is to be used under "extreme" conditions, such as towing or when using a four-core radiator, the use of the original belt-driven fan may be a better option.

Griffin
If the car still has a properly operating mechanical clutch fan, it will pull more cfm than an electric fan, and consideration should be given to keeping it. If the mechanical fan clutch is not working properly, then it should be replaced or an electric fan can be used. Keep in mind that the only time a fan should be necessary is when the car is standing still. If an electric fan will keep the car cool while at idle, then an electric fan is fine. Today's electric fans are producing more cfm than ever, and 3,000 cfm is common.

Mattson's
The engine-driven fan is great for higher speeds. We recommend one if you have a vehicle that does a lot of towing and driving at highway speed. It's a good idea to use the engine fan. If a vehicle does a lot of idling, cruising, or driving slow, the electric fan on a shroud is the way to go.

U.S. Radiator
Why rely on another operating system (that being electrical) if you don't have to? Mechanical fans turn when the motor turns. However, we highly recommend a shroud properly fitted to the fan and radiator. The only time you really depend on a fan is at idle or low speed where there is little or no air flowing through the grille. Shrouds are necessary to maximize the amount of ambient air being pulled through the grille and radiator. Proper fan and shroud alignment should have the leading edge of the fan 1/3 in, and trailing edge 2/3 out. The airflow off the back of the fan deflects at about a 45-degree angle when set this way. When the blade extends farther into the shroud, the air off the back of the blade flows straight back into the block and decreases the airflow efficiency by about 15 percent.

If an electrical fan is the only way to go, place it on a shroud that covers the entire core. We often see an electric fan attached directly to the core and the only thing this does is waste the rest of the core surface when you need it the most. A 16-inch electric fan attached to a core only cools a 16-inch circular section of that core.

Walker
Mechanical fans with fan shrouds work well. The main problem with a mechanical fan is that you should never use less than 17 inches in diameter and never without a fan shroud. We have found the engine will take longer to recover with a mechanical fan rather than with an electric fan when you have to make a sudden slow-down or stop in extreme temperatures at interstate speeds.

Electric fans work well if they are properly sized. As a rule, you should never use less than a 16-inch-diameter blade. Hook design blades are more efficient and are quieter than a paint stir stick design.

8. How much pressure should the system be under?

Be Cool
It depends on the application. Under normal street conditions, the cooling-system operating pressure will be under the rating of the radiator cap. All Be Cool radiators are designed to function with a 13-psi radiator cap. If the cap allows the radiator to vent frequently, there is another problem with the cooling system. The radiator may be too small, the electric fan may not have the correct flow rate, or the water pump may be incorrectly sized.

Flex-a-lite
In older vehicles without a coolant-recovery system, the pressure can be as low as 7 lbs. The system on these older cars would expand, forcing the coolant out and then leave a cavity in the top tank after they cooled. In vehicles with a coolant-recovery system, the pressure can be about 16 lbs.

Griffin
Up to a 17lb cap can be used for street use. Circle-track racers run between 22-29 lbs. For higher pressures, such as Nextel Cup or Busch Series cars, Griffin manufactures the Survivor Series, which will handle 50-60 lbs.

Mattson's
It's always best to use the factory recommendation; however, there are sometimes no recommendations available to the customer for the modifications to old cars with late-model engines. The radiator cap has a spring that will lift up and allow the fluid to escape through to the overflow. The different-pound caps will lift up at different temperatures. A 13lb cap is most common, with a 16lb cap raising the boiling point even higher. A 7lb cap is common in the older vehicles. If a higher-pressure cap is used on the wrong system, it may make the radiator tank come apart.

U.S. Radiator
By increasing the pressure by 1 lb, we increase the boiling point by 3 degrees; so, by running a 12lb cap, our water won't boil until it gets to 248 degrees. An engine that wants to run at 248 degrees will open that cap up long before it gets that hot.

Walker
Pressure systems have been around for quite sometime. Older systems started at a 4lb rating; 7lb systems were introduced in the '50s, followed by 15-17lb systems in the '60s, and today's engines run up to as much as 20 lbs. As a general rule of thumb in the street-rodding world, a 15-17lb system works well.

9. How is proper radiator size determined?

Be Cool
One would need to know the required number of Btu/min that needs to be rejected by the radiator. With a given air and coolant speed, select a core that will reject the required amount of heat with a factor of safety. Experience also plays a large role.

Flex-a-lite
Radiator size is determined by the requirements of the available mounting area, the powertrain, and how and where the application will be used. The correct radiator can be determined once these factors are reviewed.

Griffin
There are many factors to consider when sizing a radiator. Not all radiator sizing is based strictly upon the horsepower that the engine is producing, but also the type of fuel the car is running, how much grille area we have to work with, and under what type of driving conditions the car will be used. For most street applications, a good rule of thumb is 1 square inch of core surface per horsepower for a two-row 1-inch tube aluminum radiator.

Mattson's
We try and put the largest radiator in the available area.

U.S. Radiator
There are formulas to determine appropriate radiator size based on engine heat output (operating Btus) and radiator heat transfer rates (also stated in Btus), but my recommendation to a hobbyist is to put in the most efficient radiator that fits up to a four-row copper/brass or two-row aluminum core.

Walker
Unfortunately, being a performance radiator designer and manufacture, we have to use the area given to us (the square inches of the frontal radiator area) by the original automobile manufacture or the custom-car builders. From there, we will need the engine specifications, its modifications, and also its determined air-management package to be used. It is good to remember that the air-management package is just as important as the coolant-management package in their designs and ability to work together.

10. Do you recommend recovery tanks?

Be Cool
Yes, all the coolant that is vented from the radiator by the overflow should be reclaimed by a recovery tank. Recovery tanks allow the cooling system to replenish and helps to ensure that the radiator is always full.

Flex-a-lite
The requirement of a coolant-recovery system is not only environmentally sound, but a good idea to maintain a constant level in the cooling system. Most coolant is not environmentally friendly. It is a pollutant, and therefore should not be released to the ground. Maintaining a constant level in the cooling system will allow it to function better and prevent air from entering the system that could bring with it contaminates that might cause a failure in the cooling system in the future.

Griffin
Absolutely.

Mattson's
We recommend a recovery system on everything we do today.

U.S. Radiator
Rule of thumb is to buy the largest one that will fit in the engine compartment and still look cool. Check the recovery tank often and you'll know the size is inadequate if you find it empty.

Walker
It's a must in the performance field, and it's an advantage but not a cure-all in the street rod world. If you have the correct cooling system design, don't waste your money (I have never needed one).