Thermostat Replacement Testing

Description & Theories

Many performance enthusiasts have wondered, made claims and tossed conjectures about the performance impact from replacing the factory 195ºF thermostat with a cooler value thermostat on the GM 3800 Liter V6 (Naturally Aspirated L36 and Supercharged L67). The purpose of replacing the thermostat with a cooler one is to reduce the heat in the engine. This will theoretically provide a number of benefits. First, a cooler engine means the intake system will run cooler and thus not heat up incoming intake air as much. This affect increases the air density and result in increasing power output. Secondly, the reduced engine operating temperature should reduce combustion chamber temperatures. This affect should reduce the susceptibility of the compressed air/fuel charge from detonation. Reducing the onset of detonation will reduce the Knock Retard (KR) calculated by the Powertrain Control Module (PCM). Consequently, the PCM will provide greater spark advance timing. Greater spark advance timing typically produces more power output, to a point. Despite these theories that would indicate a cooler running engine should be able to produce more power, there is a point of contention that suggests the contrary. When an engine's combustion chamber and cylinder walls are cooler, then more heat is lost from the combustion process to the cooler combustion surfaces and results in less of the combustion energy being converted to mechanical energy. In scientific terms, this is called the overall engine efficiency. Namely, how efficient the engine is in converting the energy released by combusting fuel into mechanical energy. The cooler an engine is, the less efficient they become. The question then is, will the benefits from a cooler running engine out weigh the drawbacks of a cooler running engine.

Test Setup

To help resolve these theories and conjectures, I obtained a test vehicle (Pontiac Grand Prix GT, L36 engine) with a complete factory powertrain system. No performance modifications were done to the vehicle other than swapping out the thermostat for this test. The testing would involve replacing the factory 195ºF thermostat with a modified 180ºF thermostat obtained from ZZPerformance. The 180ºF thermostat was modified by drilling four small hoses around the thermostat body to permit a small amount of coolant flow through the cooling system even when the thermostat is closed. This modification results in reducing the thermostat's opening reaction time when the engine is relatively cool to begin with and is then challenged with prolonged wide open throttle runs. Without a drilled thermostat, the temperature of the cylinder heads can become quite elevated before the thermostat has a chance to open and establish coolant flow through the heads to cool them down.

Testing Performed

Prior to the thermostat swap, I collected performance data to baseline the car's performance. After this data was collected, I changed out the thermostat and repeated the performance tests. The weather conditions for the day were hot and humid. During the factory thermostat testing, the ambient air temperature was 93ºF, humidity of 72% and a barometer of 30.02 "of Hg. The engine was thoroughly heat soaked and running at stable temperature by the PCM's Engine Coolant Temperature values. A total of 24 passes were used to collect all the performance data, with 4 replications for any one parameter. The results for replicated runs of each parameter were averaged together in the final analysis. The test conditions with the 180ºF thermostat installed were nearly identical, the ambient air temperature crept to 95ºF while the humidity and a barometer remained unchanged. The GT with the L36 engine was running on 87 octane and had ~3,000 miles on the engine.


As can be seen in the charts below, the gains in peak HP and Torque were ~2.5 WHP and 3.7 ft-lbs respectively. Also noteworthy is the torque and resulting HP curves were benefited throughout the entire RPM range. This dynamometer data would suggest running a cooler thermostat does produce a net benefit to producing more power. Looking at the volumetric data indicates almost a 1% increase in volumetric efficiency and total volumetric flow rate. This increase corroborates the increase in HP and Torque. What is also interesting is the slight increase in overall spark advance through reduction in Knock Retard. About 2º of advance was picked up. This increase in spark advance and the slight improvement in volumetric air flow is what produce the small but significant performance gains. My conclusion is this modification is beneficial to producing more power.

While these results were obtained on a completely stock GT with an L36 engine, they are representative of the gains to be made on more significantly modified L36 engines as well as L67 engines. The more modified the L36, the more problematic KR and engine heat become. I would estimate proportional increase would be observed on an L67 supercharged engine. If this is the case, replacing the stock 195ºF thermostat on an L67 engine with a 180ºF unit would extrapolate to a gain of 3 Peak HP and 4.5 ft-lbs Peak Torque. While these gains are not huge, they are significant and provide an edge over cars still running with a stock thermostat.

Points of consideration

There are some other issues that do arise with running a cooler than stock thermostat. To begin with, all OBD-II computer controlled engines (all vehicles manufactured from ~1996 and on) monitor the Engine Coolant Temperature (ECT) with a temperature sensor. If the engine fails to reach the programmed lower temperature set point, then the PCM will set a Diagnostic Trouble Code (DTC) in the computer's memory and illuminate the Service Engine Soon (SES) light in the instrument dash panel. For most GM vehicles, this is typically around 170ºF. Furthermore, all the while the PCM sees the engine running below this set point, it considers the engine not warmed up and the PCM runs in what is called Open Loop Operation. This open loop condition has the engine running with preset fuel injection and spark advance values that are not optimal for performance and economy. Consequently, running continuously in open loop condition is a bad thing. Therefore, if one would conclude that running a 180ºF thermostat is good and running a 160ºF should be better, may find themselves with setting off the SES light and not achieving the performance and/or fuel economy they other wise would have.

Another issue that arises with the use of cooler thermostats is how the PCM uses the cooling fans behind the radiator to keep the engine cool. Nearly all contemporary cars now use electric fans to pull air through the radiator to cool the engine coolant. Only when the vehicle is at higher speeds (typically above 35 MPH) does the vehicle's motion naturally force air through the radiators and removes the need for the fans. Problems can even occur at higher speeds when drafting behind another vehicle and your car experiences less frontal air pressure. At lower speeds or in those special situations, the fans are necessary to keep the engine cool. However, these fans are controlled by the PCM and do not run all the time. Furthermore, the PCM typically controls the fans at two different speeds, Low Speed and High Speed. For many GM vehicles, the PCM will turn the fans to Low speed when the PCM sees the ECT reach ~215ºF and switches to High speed when the engine temperature reaches ~225ºF. Consequently, having a 180ºF thermostat will NOT have the engine running at ~180ºF when you are traveling below 35 MPH or in stop-n-go traffic. In this condition, the thermostat will be wide open, coolant racing through the radiator but without any airflow through the radiator, the coolant is not cooled down. Thus, the engine begins to superheat over the thermostat set temperature. To counteract this difficulty, a cooler thermostat must be complemented with a manual override cooling fan switch. With a cooling fan switch, you can command the fans to run when the PCM would otherwise have them off. With the manual fan switch activating the fans, you will be able to maintain the engine at the thermostat set temperature regardless if the car is standing still or traveling in stop-n-go traffic. Enthusiasts who enjoy trips to the local drag strip further take advantage of a cooling fan switch by allowing them to rapidly cool down their engines between passes down the track.