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dropping the thermostat 10deg

15 September 2000
Hillsburito, OR
One of my compadres ownes a vette. A typical mod they do is drop the thermostat 10deg to pick up some extra HP. I can't remember how much HP increase it gives but I think it is 10 HP. Anyone run lower temps?
10 degrees isn't a big change, but I suspect it will cause the carb chip to look deep inside itself for the proper code to run at this engine temp. (Small joke). Proper fuel vaporization is temp dependant. Over the years, I have noticed that 60-70 degrees air temp with about 40%+ relative humidity seems to give an engine more pep. The water vapor breaks down into atoms and the extra oxygen adds a little zing. For a better explanation, I'd have to get out my dusty thermodynamics book...

Heck, you guys up there in rain land know all about this...

I'd leave the themostat alone.
Well back in my mustang days (highschool) I remember that the first mod that most people would do to was to replace the thermostat. Not a huge difference if I remember correctly but this mod with a few other cheap mod's together would significantly increase performance. Mind you the 5.0 wasn't built with as high tolerances as an NSX motor but I would think a cooler running NSX should have an edge (however small) over a hotter running one.
Hang on, let me get this straight, are you guys telling me that by getting a different Thermostat you can fool the ECU into thinking the engine is cooler? Does that really change anything? I cant seem to understand how "fooling" the ECU will do anything as the engine's actualy conditions arent any different. And if the ECU acts different at lower temperature (and makes more horsepower) why dont they make it react that way all the time regardless of temp?? Sorry if this is a dumb question but this just does NOT make sense to me.
As NSXMAN pointed out the thermodynamic principles of the otto cycle produce the greatest efficiency at high compresion ratios high combustion temp and low exhaust/head temp. Aside from increasing compresion ratio and displacement the only way to get more power is to reduce exhaust/head temp. Long story short: cold exhaust = more power.

Oh, by the way the computer has nothing to do with this horsepower gain. He is just joking around.

Of course you would have to program in the extra fuel flow for cold temp and bypass the o2 closed loop sensor from leaning the sytem out and robbing all the power(bigger and better joke

When you design a car for use in cold weather you need a heater ie a thermostat running at hooter temp. Also depending on internal engine design the coeficient of thermal expansion of the piston may differ from the coeificient of thermal expansion odf the cylynder walls and are designed to run at a specific temp. My guess is this probably doesn't matter much for a 10-20deg drop. Most people do not need their NSX with heater.
Ok, I can understand the Higher compression/high temp part, but it was my understanding that cold exhaust was not advantageous. Hence the "Ceramic coated" Headers, and "Header insulator" thingies. The hotter the exhaust, the less dense the exhaust, Which makes the exhaust easier and lighter to transport from header to exit, thereby creating more power via less backpressure..right? Or am I off in La La land?
No the ceramic coating helps act as an insulator keeping the exhaust cooler thus producing more power. The turbo intercooler works on the same principle. When the input charge is compressed the temp increases and power is lost. By cooling the intake charge power is increased. And as any pilot can tell you a normally aspirated engine produces more power at cold temperatures and low altitude.

Back pressure on the exhaust is another topic. It simply reduces the efficiency of the combustion. It is the primary use of headers.
Whoa whoa hang on, why on EARTH would Exhaust temp get LOWER if you insulate it from colder air. yes Colder intake air generates more power, hence the "Cold Air induction" kits you see everywhere, but you cant keep exhaust colder by insulating it from the ambient temperature which is several hundred degree's lower than the exhaust temperature's...
All insulation will do is help keep the exhaust gas HOT..not COLD...right??
This is all nuts. I have to believe that Honda's engineering department knows what the heck they were doing to get the best overall performance out of the car.

The funny thing is, these same types of arguments are made in the Porsche world about my 996. Please. Some guy in a garage in Boise knows more about these things than the engineers in Zuffenhausen?

I'm sure the same arguments are made in the BMW circles, the Corvette circles, etc.

As with most mods, you may get more HP/Torque in one area, but you're robbing it from somewhere else. Net result: no improved performance.

The thing I find funny, is that people are saying COLD INTAKE is best, and now COLD EXHAUST is best. SO... I'll just leave my engine turned off all the time and feel good about myself that my stone cold NSX in my garage is faster than a warm one being driven on the highway.

Go do some dyno runs with these homebrew "racing mods". But if you do, PLEASE do them on the same car, same dyno, same day, same environmental conditions, etc., ad nauseum.

Flame away.

91 NSX Black/Ivory

Your wearing me down man! Metal holds heat. Cast iron manifolds hold heat. They have hotter exhaust exit temps than headers. Now if you have the best headers possible ie ceramic then your exhasust does not hold heat ie. cold.

Basic thermodynamic efficiency formula for the otto cycle:

n = Wnet/qin = 1-qout/qin = 1-(T4-T1)/(T3-T2)

where n is efficiency, q is heat and T is the temperature at each of the four cycle points. Point 1-2 is compression, point 2-3 is heat addition (combustion), point 3-4 isentropic expansion (power stroke), point 4-1 heat rejection (exhaust). To maximize power you need to maximize efficiency. To maximize efficiency you need to minimize delta T (T4-T1). T1 is input temperature (temp of ambient air) T4 is temp of exhaust.

I am told the vette's dyno out at 10hp more for a 10degree drop. Has anyone dyno'd an NSX on thermostat alone? I would guess the gain must only be 5hp or less. So, would I spend $1500 on headers for a 5hp gain or $5 for a new thermostat... It is basically free HP for an NSX which is hard to come by. Although it is not much HP

All insulation will do is help keep the exhaust gas HOT..not COLD...right?? [/B][/QUOTE]
Hey erobbins I'm not arguing with Scott at all, its just that what he shows in his efficiency equation goes against everything I've been told about engine efficieny. I am ALWAYS interested in learning more, and it seems to me as if Scott knows something I dont.

So Scott, according to your equation to acheive maximum efficieny, you want T1-T4 to be as small a number of possible, thus proving that you want as little difference in intake temp, and exhaust temp as possible. Now about those Ceramic Headers, they are usuallyy made of Stainless Steel, and are coated with Ceramic in order to insulate it.

If Cast Iron Manifolds HOLD heat, that means they are absorbing heat from the exhaust and then bleeding it off to the surrounding air consequently making it COLDER. If they headers are coated with Ceramic, the Ceramic does not transmit any of the heat OUTWARD, Keeping the exhaust temperature at roughly the same temperature as when they exited from the combustion chamber. I am definitely not a car expert, but I think you have it backwards (you probably are thinking the same thing about me) Does anyone else have any input on this??

Hey tell me what you think of this, I found it in the NSX FAQ here...read the part that says "Piping" and tell me what you think..
Hi all,

Just my comments from a chem eng background. I'll admit up front that the details from all the thermo textbooks have bled from my memory in recent years.

Cold intake air is best because it is more dense, resulting in two benefits:
1. you can pack more mass into the same cylinder volume prior to compression.
2. the amount of air that flows into the cylinder is a balance of delta P and frictional losses (hence low-restriction intakes and forced induction makes more power). Cooler exhaust would also benefit from this, if you actually had some way of effectively cooling it (fins, anyone??)

Scott's equation looks correct from what little I remember, but that's simply a measure of efficiency. If you ever figure out a way to magically impart a bunch of that heat energy from the exhaust to your crankshaft, you will indeed have a more efficient engine. Same applies to a steam turbine. Doesn't mean it's necessarily possible. Gasoline engines are horribly inefficient... something like 30% efficiency if I remember right.

When I first read this topic header, I thought it was going to be about dropping the water coolant temperature (ie. get a thermostat that thinks the coolant is 10 deg hotter than it really is, so it goes like stink to cool the engine further than normal). If you can cool the block overall, you might hope to slightly improve efficiency at the expense of the heat rejection from the radiator due to the combination of cooler cylinders (hence cooler intake and exhaust). But you'd also be cooling the cylinder on the expansion stroke, which is not desirable. Anyone know how coolant temperature is controlled? Now that I'm thinking about it, I thought radiator fans are on-off devices. Are water pumps variable speed or stroke? Wow, more questions.

My 2 bits, anyway. Interesting stuff...

Let me try to explain how I understand this differently. If you replace your thermostat with a lower temperature thermostat the entire engine will run at a lower temperature beacause the water flowing around the cylinders and through the head will be at a lower temp. The thermostat keeps the engine at this constant temp 180deg? (i think) for the NSX. Now if the entire engine is colder, it can not heat up the intake charge as much as it used to. Therefore, it is like operating at a lower temp (like a mini-intercooler). At colder temperatures the air is denser and you get a better combustion because you can put in more gas to mix with more air producing more power. From my understanding the heat in an engine comes from two primary places, the head where combustion occurs and the exhaust. In the head when the explosion happens it creates heat. Part of this heat is absorbed in the head, part of this heat is absorbed in the exhaust, and the rest is pumped to the atmosphere. The exhaust gas temperature is not what we are trying to cool. The exhaust system must be cool. To keep the exhaust cool you need an insulator so it does not conduct heat.

Anyway I am no thermo expert. We need someone who knows this better than I. What I would like to see is some dyno numbers to see if it produces some gains. Otherwise I might be full of hot air. Pitifull joke!

I am just going to slap on 160 deg thermostats and see what happens.
Did a little reading. The FAQ section on exhaust does say hot exhaust gas is better... that doesn't make sense from a hydraulics point of view. If you gotta move a certain mass of exhaust gas, changing the temperature will change the velocity and lower velocity means less frictional pressure drop (which is the objective in making the pipe diameter bigger). For a gas travelling through a pipe, low pressure drop is a good thing b/c it means you can put more flow through for the same pressure differential -- but it sounds like there's some other part to exhaust theory I'm missing. Maybe because it's repetitive pulse rather than steady flow.
So either the FAQ is wrong, OR there is soemthing between the Thermodynamics Equation and "actual" use that we are missing. It seems like basically what we have found is that on paper it is more efficient to have an overall COLDER engine, whereas in reality there are other factor's not considered (exhaust backpressure, Restrictive intakes etc etc) that may in the end make it more desireable to have your engine running at less than the most mathematically efficient manner? But that STILL does not explain why we dont use say for example Copper or Silver Headers with extensive Heat Fins. That would cool off the exhaust alot quicker than Ceramic Coated Stainless Steel headers. And for the Amount CT and RM charge for those Header's you'd expect them to be made of solid Gold. (Or at least have heat sink fins on them..)