Cooler thermostats

Pretty much every hop-up guide for every car out there lists switching to a cooler thermostat as one of the first mods you should do to increase performance. The reasoning behind it is that engine will run cooler, and the cooler the engine the more power it will make. This is true to a certain extent, but it's not as simple as that. In modern cars I think it's a bad idea to run a cooler thermostat without doing a little homework first.

First of all, most new cars have a much more sophisticated method for regulating heat than they did back in the day when engine temperature was solely controlled by the thermostat and and engine-turned fan. Most modern cars are cooled by electric fans regulated by a thermostatically controlled switch, or by the engine's management computer (or quite often both.) In a switched system, the fans will only come on when the coolant reaches a certain preset temperature, so even if you open the thermostat earlier, there will be no fan-driven airflow to cool the hot coolant in the radiator until it gets hot enough to trigger the fan (well over 200 degrees usually.) So in effect, your cooler t-stat is doing nothing unless you add an adjustable switch to bring the engagement point down. In computer controlled system, there is a "fan on" and "fan off" setting, where the "fan on" setting may be 210 degrees, meaning that the computer will turn the fan on when it sees 210 degrees and then turn it off again when it sees it drop back below say, 190. In this type of system, you would have to have to reprogram your vehicle's computer to lower that range or again, it would have no effect. Auto manufacturers spend a lot of time getting the balance just right on that, so you will have to as well. If you set it too low (too close to the t-stat opening point) your fan will be on constantly, which can be annoying and it takes away some margin of control(more on that later.)

Secondly, modern engines are designed to run optimally at a pre-set temperature. Most cars will have basically a "warm-up" mode where it runs a certain tune until it reaches a pre-determined operating temperature, and then it switches to another tune (fuel tables) or will switch from "open-loop" mode (running on a pre-defined set of parameters) to a "closed loop" mode (taking real-time feedback from o2 sensors, etc. and using that info to adjust the tune). (Here's a great article by Mike Wesley that goes into more detail on this specifically on the EEC-IV Ford computer.) If you do not allow the engine to reach the temperature required to make that switch, you will never leave that "warm-up" mode and your car will not run efficiently. One of the nice things about modern EFI setups is their adaptability. You can make all sorts of mods, and they will adapt (to a certain extent) to keep the desired A/F ratio even though you have increased the airflow into your engine. If your car never reaches the closed-loop (adaptive) mode and relies on the pre-programmed fuel tables, your A/F could be way off and you nullify your gains unless you've reprogrammed your ECU. Additionally, without the engine running optimally, you will get worse fuel economy. The engine's emission systems are designed to operate at those pre-set temps, so your car will potentially put out a lot more emissions at lower temps, which will make it much harder to pass a smog test. Some folks don't care too much about emissions (yet) but for them, this problem will manifest itself by giving the car that "fat" or "rich" smell that stings the eyes a little bit. That's no good either. I had a 454SS pickup that I was told had a 160 degree t-stat when I bought it. It was sluggish, the exhaust smell was terrible and the truck got around 8MPG. Luckily, a month or two later, the thermostat stuck on me and the truck started to overheat. I swapped in a stock t-stat (195, IIRC) and it ran like a different truck! It smelled better, ran more smoothly and got almost 10MPG! (I know that still sucks, but that's almost a 25% gain!)

Earlier I mentioned control. The thermostat, fans and radiator work in conjunction to keep your coolant in a certain temperature range. If you lower your thermostat (so it's basically always open at normal running temps) and always have your fan on, the only thing controlling your temperature is the efficiency of your radiator. If it's a hot day, or you're climbing a lot of hills with the A/C on in summertime, additional demands are placed on your cooling system. The open t-stat might start letting the water flow through the system so fast that the radiator does not get a chance to cool it down below 160 degrees and so "hot" coolant starts getting re-introduced into the engine where more heat is added. This creates a heat-soak situation and the system will lose its ability to control the temperature. A stock higher-rated t-stat combined with a longer-range window of fan operation (computer controlled) will hold the coolant in the radiator longer and cool more efficiently. I had this happen on my 83 Mustang GT, and later again on a 95 Mustang GTS that came with a 160 degree t-stat. Both times the problem was solved by switching to a warmer t-stat.

So will a cooler thermostat make more power? Maybe. If done correctly and tweaked to match your combo, sure, but it's not a simple "plug-and-play" operation anymore!