Swapping in a higher temp thermostat

[Daox]

So, the Paseo look like it needs a new thermostat. I went online and browsed over the selection I have. It seems the stock thermostat is set to 180°F (82°C). Now, I've heard of guys using higher temp thermostats and getting better mileage. So, I continued digging and found I can get a 192°F (89°C) thermostat for the car. Hurray. Now, why does this help? What does it hurt? OEMs must not use 192°F stock for a reason. I'm thinking it probably hurts power output as ignition timing may need to be retarded due to insufficient cooling and hotspot developing. I'm not sure why hotter would be better except that you loose less energy through your coolant system.

[bhazard]

192-195 is stock on most cars Ive come across.

[tasdrouille]

The thermostat on my 2008 starts to open at 177°F and is fully open at 201°F.

[LostCause]

I also do not understand the logic behind running a higher temp. thermostat. Theoretically, wouldn't you be lowering efficiency by increasing the temp. of the cold reservoir? Then again, I'm wondering what the true cold reservoir is...the coolant or the air.

The equation I'm thinking of is the Carnot Theorem: 1 - Tc/Th

I can't really think of any benefits...

- LostCause

[cfg83]

Hello -

I am not an authority, but I have read on forums that the higher temp means that the engine is running more efficiently. The analogy would be a cold engine in the morning that does not become "efficient" until it is all warmed up.

If you are an auto manufacturer, I think that you are engineering with two goals in mind. Efficiency *and* reliability. The "balanced" response is to get the engine to be hot enough to run efficiently, but cool enough to be as far away from the "overheat zone" as possible. If you want reliability, you choose the T-Stat that gives you that compromise.

Also, you are designing the cooling system for the broad majority spectrum of drivers. Most drivers are not the "feather foots" that we are, so the cooling system needs to serve their driving habits, not ours.

As an Ecomodder, we are doing mods that "push" the engine for more fuel efficiency. As a result, we have to monitor the engine's health to a greater degree. We have to be vigilant, just like someone who puts a high-performance mod onto their car.

I have the hotter T-Stat. I used to run at about 180 degrees F. Now I run at 190 degrees F.

CarloSW2

[Daox]

You had to see this comming.

Did it make a noticable difference?

[AXMonster]

IIRC the reason for higher engine temps for economy are:

1) the higher the engine temp the lower the friction (mainly due to design clearances and pumping of fluids),
2) also the better the vapourisation of fuel prior to combustion leads to more complete combustion (a more homogenious mixture)
3) and the lower the differential between combustion flame temperature and piston/cylinder/combustion chamber temperature.

The chamber temperature is an interesting one as the heat released to the metal by the flame (and hence the loss of energy to the coolant) is actually less the higher the engine temperature. This means that more energy is available to expand the gasses in the cylinder and propel the vehicle.

Ideally the engine would run at the same temperature as the combustion flame, so there would be no heat rejection to the coolant and therefore no energy loss & perfect efficiency

[tasdrouille]

Thermal losses accounting for roughly 65% of all the energy spent in city driving, there's a lot to gain in reducing those losses.

[cfg83]

Daox -

Originally Posted by Daox

You had to see this comming.

Did it make a noticable difference?

Good question. I made the switch in my early less-accurate data recording days. ... dig dig dig ... Ok, here are my gaslogs :

1999 Saturn SW2 with switch to hotter T-Stat

BEFORE            
Stock T-Stat    Miles Gal(US)      MPG
08/31/2006     192.00    4.84    39.66
09/03/2006     148.00    3.23    45.82
09/07/2006     187.00    6.53    28.63
09/10/2006     140.00    3.64    38.46
09/13/2006     170.00    4.19    40.57
09/16/2006     202.00    6.85    29.48
09/19/2006     259.00    6.94    37.31
09/22/2006     265.00    7.04    37.64
09/26/2006     288.00    7.15    40.27
09/29/2006     275.00    6.99    39.34
TOTAL         2126.00   57.40    37.04
            
AFTER            
Hotter T-Stat   Miles Gal(US)      MPG         
10/02/2006     219.00    6.90    31.73
10/07/2006     279.00    7.00    39.85
10/11/2006     274.00    7.30    37.53
10/16/2006     305.00    7.25    42.06
10/20/2006     292.00    6.00    48.66(*)
10/23/2006     187.00    6.05    30.90
10/26/2006     232.00    5.72    40.55
10/31/2006     265.00    6.25    42.40
11/02/2006     246.00    6.30    39.04
11/07/2006     260.00    6.10    42.62
TOTAL         2559.00   64.87    39.45

39.45 / 37.04 = 1.0548 => 5.5% increase in MPG.

Soooooo, it appears that I got a +5% increase in MPG. Here are my only modifications during this time :

2006/09/30 - Switched to 192-195 Coolant Thermostat (Napa Part #THM268)
2006/10/28 - Front wheel spoilers (modest aerodynamic upgrade)

(*) Now, here's the rub. On October 19, 2006, I discovered the P&G (Pulse and Glide) driving strategy. This was "mild" engine-on P&G, but my daily runs improved.

Therefore, after October 19th, I cannot say if it was the T-Stat or the modified driving style, .

CarloSW2

[dremd]

Efficiency improvement simple, you are wasting less heat from combustion in to the cylinder walls/ head.

+ other things.

I didn't believe it at first, but it is true.