Tstats and Engine Temperature

By SRT-Tom · Jul 21, 2020 ·
  1. SRT-Tom
    As we all know, the main purpose of a thermostat (tstat) is to maintain your Challenger’s engine at the ideal operating temperature and help produce fewer emissions. First, let’s talk about how a tstat functions and then why racers install low temperature ones.

    Basically, the thermostat starts at a closed position when you start the engine to help it reach operating temperature. While the engine is running, the coolant temperature slowly rises, making the thermostat begin to open. The opening allows warmer coolant in the engine to flow into the radiator. In turn, the water pump pushes lower-temperature coolant from the radiator into the engine.

    When the lower-temperature coolant reaches the thermostat, the warmer fluid begins to cool, closing the thermostat. During engine operation, the thermostat actually never fully closes or opens, but gradually nears either state to control coolant flow. This allows the engine to operate at a specific temperature controlled by the thermostat. Note the valve position (#3) in the following diagram.

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    The engine has to reach a specified temperature before the computer will go into what is called closed loop mode. This is where the computer adjusts the fuel mixture from the signals it receives from the oxygen sensor. Your engine must be allowed to run in closed loop mode.

    So, if the engine doesn’t reach this specified temperature the computer won’t go into closed loop mode. But, instead, it will stay in the open loop mode. This will cause the computer to deliver more gas because it thinks the engine is still cold.

    Here are some basic tstat facts:

    Functions of a Thermostat
    • To accelerate engine warm-up
    • To regulate the engine’s operating temperature
    Advantages of a Working Thermostat
    • Helps fuel economy
    • Reduces engine wear
    • Diminishes emissions and blow-by
    • Improves cold weather driveability
    • Provides adequate heater output
    • Helps with overheating
    Signs of Thermostat Problems
    • Higher Than Normal Engine Temperature
    • Lower Than Normal Engine Temperature
    • Fluctuating Engine Temperature (changing erratically)
    • Poor Engine Performance
    • Engine Takes A Long Time To Warm Up
    • Engine Overheating
    • Popping-Boiling Noises Coming From Your Heater
    Most Thermostat Problems Happen in One of Two Ways
    1. If the thermostat becomes stuck in the open position, there is continuous flow of coolant into the radiator causing the engine to run cold. Over-cooled engines run inefficiently, which leads to increased fuel consumption and higher emission levels and engine parts enduring more wear. In addition, the car’s interior will not heat up properly- a big problem in the winter..
    2. If the thermostat becomes stuck in the closed position, the circulation of the coolant is blocked so the coolant cannot get to the radiator to be cooled which causes the engine to overheat.
    When to Replace?

    If your engine has overheated for any reason, replace your thermostat. Thermostats are cheap and can be purchased at your local auto parts store.

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    Low Temperature Tstats

    Now, let’s get to the topic of low temperature tstats. Before we discuss this topic, you should be aware that the SRT engineers do not recommend changing to a 180°F thermostat due to emissions regulations.

    Generally, if your engine is still running on the stock tuning a switch to a lower temperature tstat is not recommended. Simply stated, a cooler engine does not necessarily make more horsepower. Sure, you may lose a few degrees of incoming air temperature and see some small gains but, the primarily advantages are the prevention of heat soak and protection against engine-damaging detonation. Remember the knocking sound you heard when trying to accelerate an overheating vehicle? That is detonation and this can quickly destroy a high horsepower engine being pushed to maximum performance. But aside from possibly saving your engine, there are a couple ways that a low temperature tstat can actually help make more power.

    An engine at 180° F can run more timing advance safely than one running at 205° F, so tuning, along with a low temperature tstat, can make some significant gains. If you're using a handheld programmer, chip or custom tune, then a 180°F thermostat is a good performance mod. Running a cooler engine has been the abc of high performance since the early days of hot-rodding. Those guys were pulling their tstats out or drilling holes in them because they knew they could run more timing by doing so.

    One drawback about very low temperature tstats is fuel economy. If the engine does not reach a high enough temperature for the PCM to leave warm-up mode, gas mileage may drop around 1 mpg. This is the trade-off made for the improved throttle response of a more effective engine. . That is why very low 160°F tstats are not suited for the street. It seems that around 180°F is the best balance overall.

    Another concern, with tstats under 170°F, is oil temperature and viscosity. If the engine oil does not get hot enough to burn off certain contaminants, then the oil's effective life will be reduced. In extreme cold weather environments, to be safe, you should change your oil more frequently or swap in the OEM thermostat for the colder months.

    With the 180°F tstat, you need a small window for the fans to turn off after the temp cools down, so you always operate above the tstat’s threshold. This is where a programmer, like the Diablo Predator, comes in. You need to reprogram the fan control settings to come on accordingly with the new tstat- generally, a 20°F drop. Otherwise, the fan will not come on until the original programmed temperature is reached. The low temperature tstat will allow you to keep your Challenger running cooler and from getting heat soak due to excessive idling, city driving or racing. This will also keep your engine from losing power and throttle response. Recommended settings to use are as follows:

    All HS Fans (High Speed) AC On & AC Off: 194°F
    All LS Fans (Low Speed) AC On & AC Off: 189°F
    All MS Fans (Medium Speed) AC On & AC Off: 192°F

    If you decide to go with a 180°F tstat, a Jet T-Stat is a good choice. The Jet comes with a gasket and most others don't. Also, it is almost identical in design to OEM and flows more than most other brands. Some of the others that are being sold require you to trim the stat a bit to make it fit right.

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