Sometimes the overheating problem experienced by our hot rod has nothing to do with the engine actually overheating—maybe it's the gauges. To get some input on the proper installation of gauges and what might be some of the pitfalls befalling a temperature gauge we visited Classic Instruments. We found head gauge guy John McLeod under the hood of his '09 Mopar Challenger Drag Pak car (apparently you can break one of these 6.1L Hemis in the quarter-mile!). McLeod turned us over to the capable hands of Jake Bernthal who had the answers and even some questions we didn't know we needed answers to.
One of the most common questions revolves around, "My temp gauge is reading too high and I know my engine and cooling system are fine. What do I do?" Before you "assume" (that word will always get you into trouble) that the gauge is at fault, Bernthal recommended we check four common causes for faulty gauge accuracy.
Bad gauge ground. Ensure the gauge has a good chassis ground wire attached to its terminal. A bad ground will cause the gauge to use the sender as its ground point and cause the gauge to read inaccurately high.
The sender probe is not completely submerged in the engine's coolant. This is caused when thread adapters are used to convert the sender thread size to the intake manifold or head thread size. The adapter keeps the probe from getting all the way into the coolant. Bernthal informed us that Classic Instruments offers temperature senders with 1/8-, 1/4-, and 1/2-inch NPT standards as well as a 12mm thread for use on GM LS engines. This variation allows you to customize to your applications needs.
Air pockets in the cooling system. This will cause the temperature gauge to read high and then come back down when the thermostat opens and pushes the air pocket past the sender probe. The gauge pointer will continue to rise and fall as the air pocket moves past the sender.
Close proximity to external heat sources. This occurs mostly when the sender is installed in the engine head where it is close to the exhaust. The high temperature of the exhaust can artificially raise the reading of the temperature gauge by 10-15 degrees. If possible, mount the temperature sender in the intake manifold or try shielding it from other heat sources.
Multiple devices connected to the same Classic Instruments temperature sender. Classic Instruments temperature senders will only work with one temperature gauge at a time. Connecting multiple devices will cause both devices to operate incorrectly. For example, if you would like to use two temperature gauges but just one sender you would need to install a switch to isolate one gauge connected at any one time.
Multiple temperature senders connected to one gauge. This happens a lot of times with Flathead Ford engines that could have temperature senders installed on each side. In order to monitor temperature on both sides with one gauge, a switch would need to be installed to switch from one sender to the other (or two gauges could be used, one for each sender). Connecting both senders to the gauge will lower the effective resistance and cause the gauge to read inaccurately high.
Connecting warning lights to a Classic Instruments temperature sender. Classic Instruments temperature senders operated differently than senders designed to turn on a warning light. If a light were connected to the Classic temperature sender, it would most likely not work correctly and would also cause the gauge to be inaccurate.
Pulleys & Water Pump
Another area that can cause a bit of consternation to any hot rod as it can have an impact on both the cooling and charging system are over-driven or under-driven pulleys. The philosophy behind these under-driven pulleys was to help slow down the alternator at excessive rpm encountered during racing. While it works ideally in race applications, the reality is you want standard ratio pulleys on the street.
We happen to need new pulleys for the small-block Chevy that was dropped into our '34 sedan so we visited Bill's Hot Rod Company and selected a matched set of stock black steel pulleys. We weren't interested in the benefits of under-driven pulleys as we wanted to make sure the alternator was producing enough rpm to charge the battery properly and spin the water pump properly to maintain proper cooling.
For the basic street rod you want an alternator pulley ratio between 2:1 to 3:1 as this will ensure a good charging system performance and not over-spin the alternator bearings. The 2:1 alternator ratio will provide approximately 5 to 10 amps output at idle. You find your alternator pulley ratio by dividing the diameter of the crankshaft pulley by the diameter of the alternator pulley. Multiply that number times the engine rpm and that will give you the alternator rpm. (For example, a 6-inch crankshaft pulley and a 2-inch alternator pulley equal a pulley ratio of 3:1; at an idle of 800 rpm multiplied by 3 equals 2,400 alternator rpm.)
Another item recommended by Armstrong of U.S. Radiator is the use of an aftermarket high-performance water pump. On our small-block Chevy we opted for a FlowKooler (PN 1668) high flow water pump (20 percent greater flow over other performance water pumps). The objective here is to move more coolant through the radiator at lower speeds (i.e. traffic) because of the lessened airflow in these types of driving conditions. The result of the efforts of the FlowKooler water pump is the reduction of up to 30 degrees of engine temps, increased pressure in the engine, which helps to reduce ping and improves flow efficiency.
It should also be noted that the FlowKooler water pump through its patented impeller system helps to present the onset of cavitation; which is the formation of vapor bubbles in the coolant. This occurs when the pressure of the coolant falls below its vapor pressure, which in turn lessens cooling efficiencies.
The cooling system is much like the electrics on a street rod—it looks like black magic but it doesn't have to be. A little understanding of the basics and knowing what bugaboos to look for will keep you and your car's cooling system out of trouble.