Engine Cooling 101

This is my basic primer on things to consider in order to get the most out of your cooling system. There are a bunch of facets to this issue so here goes. Keep in mind this is the Gospel According To Bob so after you read this research it yourself and make you own decisions. There is a lot of misleading info out there. I will try to clear up some of it. Keep in mind that every scenario is different and in my many years of building and working on cars it is readily apparent.

Back To The Basics

The reason here is this... many of the inquires I get is "I need to buy an electric fan for my vehicle because I am overheating. My buddy said that I need this electric fan.". Here are a couple of noteworthy issues that pop up immediately. Chances are you have other issues and beyond that you are not fixing the actual cause. But with that said, we will assume it is a fan issue. Most vehicles have a highly engineered cooling system from the git-go. If you have decided to attempt to out think fluid engineers then you may want to reconsider. If you have built a unique version of the vehicle then there may be some solutions.

You need a fan shroud!

If it had one and you took it off put it back. If it doesn't have one put one on. If the fan and shroud are intact make sure the fan is (A) positioned inside the shroud lip and (B) within 1 inch of it's overall diameter. If it is positioned outside the shroud, the shroud is broken, or you don't have one, you are just beating the air! Most electric units have a built in shroud BUT it will only capture the footprint it has attached to it. You may have to direct the rest of the radiator face through that unit for maximum effect. Look at most late model automobiles. Even with electric fans a full shroud, no gaps!

Here's a simple test to see if YOUR set-up is working like it should. Get a sheet of notebook / printer paper and with the vehicle running place it on the face of the radiator. It should stay in place! If it doesn't you need to fix that. Because that means you don't have a whole lot of air flow through the radiator at idle which is when you need it the most.
Position is everything

The closer you can get to the radiator core the better. 1 to 1 1/2 inches is the best configuration.

Air Flow is Everything

The stock bladed and clutched fans move the best air at idle but disengage at speed. So if your concern is volume while cruising this is a no brainer. You may want to stick with the stock units. Just make sure all the rest of the package is right.

FACT If you do decide to make the change to an aftermarket fan shop carefully. Most stock fans perform better than a lot of aftermarket flex style units. There are exceptions! My research and personal experience substantiates that the Derale HD units outperform the equivalent Imperial HD units, but both are much better than stock. The Flex-a-Lite units are, for the most part, very poor performers. Cute, but crappy!

Blocking off all the exit paths or directing air headed for the radiator is always a benefit. Make it go through the radiator not by it or around it! Those thin black crossmember blades ( little black plastic strips hanging down) that you see under the nose of a lot of new cars can actually account for a 15% airflow input!

Debunking the urban legends

This is where I get to spout off about a variety of what I consider to be "urban legends" that are entangled in the street rod cooling arena. Feel free to read them and make your own decisions!

Urban Legend #1 - An mechanical fan cost HP and an electric fan will recover that HP

Interesting but in my opinion not so. Cool, you have removed the fan, clutch and spacer from the water pump. You gained some HP from the rotating assembly. Now you haveadded an electric fan. Note the key word here "electric". You just transferred the load from the water pump to the alternator. One major element to keep in mind that some fans can use up to 80 amps to start and 30 amps to run. Some more, some less. Unfortunately you may have to upgrade your wiring / alternator in order to make this happen. The cost involved could be major. The HP benefit questionable.

Most factory mechanical fan setups incorporate some sort of clutch unit. Most of them are thermally actuated. What this does is engages the fan at low speeds. Typically at 170 degees for a fan speed of 70-90% of the engine speed. Then it de-clutches at speed. Typically to 20-30% of engine speed. Simple and highly effective. Some recent [2000's] invocations use an electronic version under ECU control.

Urban Legend #2 - Electric fans are better

IMHO not so! At low speed it could be an advantage but at driving speed the fan unit, which is not usually running, actually can cause overheating. Especially if front mounted. Why? Because it is sitting right in the middle of the prime cooling area just hanging out blocking air! Don't believe me? Take a look at any of the flat motor style aftermarket fans. Hold it up and look through it. That's what the radiator sees! Usually about a 50% blockage. In addition you just added more electrical to potentially go bad. If it fails for whatever reason it could be costly!

Urban Legend #3 - Aluminum radiators are better

Yes and No. Aluminum units actually do not pass heat better than equivalent brass/copper types. Especially at idle or low speed. Basic Physics Class 101. But most aftermarket aluminum units compensate by being the oversize so it does cool better overall. But we are in an apples and oranges comparison. The aluminum units cost lots more, have far more problems relating to repair and are very prone to long term vibration/stress cracking along with some corrosion issues if not maintained. Not an advantage on a street car. There are some weight savings but from a cost and durability standpoint really not a good "bang for the buck" deal.

The realistic solution is to get a good quality brass/copper unit with the maximum core rows possible. These are usually listed as heavy duty or extreme units. Another plus is to have the unit recored using a "high fin" field. Basicly it has a higher fins-per-inch count and increases the heat transfer ratio. You end up with the same physical size but more heat passage capacity! Any good radiator shop can do this.

Urban Legend #4 - High Volume / Aluminum Water Pumps are better

Don't count on it! Most aluminum pumps are more for weight savings than performance. And on the street the 10 ponds is, for the most part, negligable. At idle HV pumps can actually not cool as well. It is because they move the coolant so fast through the radiator that it doesn't have time to dump off all the heat it should. At driving speed it is almost a wash. This is probably a poor choice, again cost versus return, not good!

This same theory applies to thinking it's going to run cooler with the thermostat removed. Way not so! It moves way too fast through the radiator. The system actually needs some restriction to slow it through the radiator in order to let the radiator do its' job. Once again a simple look through a thermostat will give you an idea of what is in the fluid flow. At least a 50% restriction, maybe more. Many race cars , while not using thermostats, do use a a simple restrictor washer in place of it to do that.


Reference reading