Of all the issues facing those who build and drive street rods, controlling the operating temperature of the engine can be one of the most challenging. The nature of our hobby is to do things like fill a tiny engine compartment of an old car with a big powerplant with two to three times the displacement and three, four, or more times as much horsepower as the original mill. Of course we're still stuck with the same size radiator that was barely big enough to keep the stocker from boiling over, and then we do things like add air conditioning for comfort and get rid of all the louvers in the hood sides so the car looks smooth. The point is overheating problems are sometimes self-inflicted, so let's take a look at some of the common reasons for it and some simple precautions to keep it from happening.

What are the Risks?
Overheating an engine usually comes with consequences: they may be as simple as occasionally burping out coolant to substantial engine damage. The first sign that an engine is getting hot is often pinging/detonation or running on after the ignition is shut off (fuel with an inadequate octane rating will do this as well). Severe overheating can cause pistons to expand enough to scuff or even seize in the cylinders, rings can lose tension, stick, or literally weld bits of themselves to the cylinder walls, exhaust valves may stick in their guides, heads can warp, head gaskets can develop compression and water leaks, and blocks and heads can crack. In short a good engine can become a great boat anchor if it gets too hot.

How Hot is too Hot?
Internal combustion engines create heat and lots of it. The chemical energy in the fuel is transformed into thermal energy, when the fuel burns the rapidly expanding gases create the pressure to push the pistons, spin the crankshaft, and drive the vehicle down the road. But while heat is part of the process, much of that energy that produced it is wasted. The average gasoline engine is only about 22-28 percent efficient, which means over two-thirds of the available energy produces heat that goes out the tailpipe or is soaked up by the cooling system. Diesels are slightly more efficient, using 32-38 percent of every dollar's worth of fuel purchased, but in both cases there is a lot of wasted energy in the form of heat that must be removed by the engine's cooling system.

To a certain degree (no pun intended), the hotter an engine runs the more efficient it becomes, however, there's a practical limit because the internal components can only tolerate so much heat. While engineers have been tinkering with exotic materials and metallic-ceramic alloys in an attempt to build high-temperature, super-efficient engines, the cost is far too high for everyday applications.

For our purposes there are a number of theories on street rod engine operating temperature. Fuel system guru Henry Olsen, of Ole's Carburetor and Electric, explains that due to the lower volatility of today's fuels, 180 degrees is the minimum engine temperature for efficient combustion. Of course for many of those who grew up with cars that had temperature gauges that actually had numbers on them, having the needle point to anything over 180 degrees made the driver break out in a sweat and meant it was time to look for shade for you and your ride. Today, however, most experts agree 190 degrees is probably the ideal, as it's hot enough to boil off contaminants in the oil and prevent the formation of sludge. And while it makes some people nervous, most engines today are designed to operate within a "normal" temperature range of about 195-220 degrees F.

Choosing a Radiator
As a basic rule of thumb, fit the largest radiator you can cram in the space available-you'll never regret having too much radiator, the same can't be said for having too little.

Radiators can be divided into two basic types: downflow and crossflow. Early cars had downflow radiators, but as automotive stylists made the front sheetmetal lower and wider there wasn't room for a traditional cooler and the change was made to the crossflow design. The reasoning behind that change is easy to comprehend: The longer path through the core exposes the coolant to more area for more effective heat transfer.

One of the newest trends in aftermarket radiator construction is the multi-pass core. With this design the coolant passes through the core one way, then is redirected by internal baffles and flows back through the core in the opposite direction. Multiple pass radiators may use double or triple paths depending on the size of the core.

When shopping for a radiator one of the decisions that will have to be made concerns the material used-you'll have to choose between aluminum and copper/brass. The truth is in terms of cooling the difference is minimal. Appropriately sized, a radiator made from either material will work. The real differences between aluminum and copper brass are these: aluminum is lighter and cheaper, which is why the OEMs use it and copper/brass is easier to repair and is less susceptible to corrosion.