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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old August 21st, 2008, 09:56   #16
tnp
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Thanks David for the support! Much appreciated. While there isn't a throttle, Air Flow is most certainly an adjustable quantity. Anyone who has access to a Vag-Com can certainly see this move up and down. The hotter the engine, it is likely that a "leaner" mixture can be made. That may be a pretty cool graph, actually - injection quantity and airflow vs temp. I think I might try getting some samples this summer and this winter and see what happens. I'll post it in a few months (assuming I remember!)

Don't get me wrong here. I do certainly respect Drivbiwire's thoughts. Newbie "hazing", right? He has made some very valuable insights and comments on many of these posts. But, there is not a need to be terse about this stuff. We're all here for the same thing - to tweak out these machines to performance that only folks like us can get geeked out about!

Back to my Beetle running in the 70's... I agree it is either a bad thermostat, or it was the 71C one that was put on in the factory. I'm going to make a change to the 87C to start here and see how it goes. I was going to go with the 92, but I was thinking that it may start causing the fan to turn on. The fan turns on at 102 and turns off at 95. With a 92, that is getting too close for me. With an 87, I think I'll be pretty safe to keep that off more often than not.

I made a few calls this morning. Here are the part numbers, if anyone is interested:

87C Thermostat: 044-121-113
92C Thermostat: 078-121-113


If everyone doesn't mind here, I'm going to do a mash-up. Let's take these quotes together...

Quote:
Quote by ExtremeTDI
2000 Jetta Tdi gets better fuel ecomomy on the hotter days.
Quote:
Quote by Ronbros
I agree with tnp, I think we can run much higher temps in the cooling jackets, but finding a thermostat that opens at around 150-160C i cannot find ,am considering making one adjustable. NOW, before anyone shouts, coolant will be a nonwater based coolant
Quote:
Quote by DrivBiWire
In respect to coolant temperatures, Hotter is not always better. having seen first hand desert testing with VW, Audi, Mercedes, Skoda and BMW they verify every possible operating temperature in order to validate the predicted operating ranges in real world conditions.
Mashing these all together... These engines do run better when it is warmer, as ExtremeTDI says. There is always a point where it is "too hot", as DrivBiWire says. But, I also agree with Ronbros, that we gotta think about pushing the limit up here and testing it out.

If anyone has access to the fuel maps, we can predict if fuel efficiency will be greater with increasing temperature, and we can also see where "the knee of the curve" is, where it starts getting too hot.

Then, we'd need a hotter thermostat, different fan settings, probably a different coolant ratio (I don't think a different coolant), and then some real world testing.

Well, that's my 2 cents... Or, maybe closer to a dime with all this typing! Cool - I blew almost 15 min here at work and I'm sure it sounds like I'm working hard to the boss...
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Old August 21st, 2008, 15:30   #17
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There is every reason to believe that increasing the thermostatic valve opening temperature will increase engine efficiency. Throughout the history of internal combustion engines we have seen this temperature increase from 160F to 180F to an average of 190F today. Various changes in internal combustion engine design especially metallurgical advances have made these corresponding changes possible. There are industrial engines that have increased from 190F to 205F and measured significant increases in efficiency.

There have been people who have posted about buying 205F thermostatic valves (generally available for Ford products) and adapting for tdi use. I don't think any of them have reported a successful adaptation.

From the standpoint of heat transfer efficiency you want your combustion chamber temperatures as hot as possible. Todays pistons operate at the consistency of butter. Thats why when you lose your timing chain the valves generally will leave their calling card there. Cooling of the combustion chamber occurs a little before the exhaust valve closes, after the hot exhaust gases are expelled. Heating occurs a little before combustion. Hotter coolant temperatures mean hotter combustion chamber temperatures. Hotter gases in the combustion chamber mean more pressure. More pressure pushes down harder on the piston for greater efficiency.

Under emergency conditions people have operated internal combustion engines up to 250F and more coolant temperature and some of these engines survived the ordeal just fine. However, this is most certainly inviting disaster. How close to disaster do you want to operate? A lot of engineers feel we could increase efficiency by going to 215F but this could possibly generate more warranty issues from people who are less than conscientious in the care of their vehicles and no one wants to go there.
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Old August 23rd, 2008, 23:10   #18
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Nicklockard tried a hotter t-stat last summer. I'll draw his attention to this thread. I dont seem to remember he saw any improvements though.
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Old August 24th, 2008, 00:00   #19
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Quote:
Originally Posted by ExtremeTDI
I know that my 2000 Jetta Tdi gets better fuel ecomomy on the hotter days. Would it be recommended to install a thermostat that runs hotter during the cooler months of the year to increase cold weather mpg? Furthermore, would be recommended to install a thermostat that runs a little hotter during the hotter months of the year. It seems that, within reason, the hotter the engine gets the better the mpg. What are your thoughts? THanks.
I tried this with a 91C thermostat. Unfortunately I bought the NAPA aftermarket branded version which didn't regulate very tightly. Actual temperatures ranged from 89-100C, versus the stock Volkswagen OEM 87C unit which regulates tightly from 86-92C.

My experiment was in summer time and I didn't have a lot of baseline data to compare it to, but I'd say whatever gain there was was very tiny. Also, you have to consider that at higher coolant temperatures, parts lifespans of various plastic parts in the engine bay will be reduced. Whatever fuel economy gain is to be had is offset by having to replace all sorts of plastic doodads. I had multiple failures due to the increased heat. Granted, this was a higher miles car to begin with, so they may have failed no matter what, but I think the heat didn't help.

If you want an easier way to experiment that won't have this side effect, try just removing the intake air snorkel from behind the headlight leading to the airbox. It's well known that engines ingesting warmer air get improved fuel economy. And it costs nothing and doesn't have the unintended consequences that come with changing the t-stat.

That's my $0.02

Edit addition: I also remembered that the ECU pulls back injection timing (retards it) with warmer air, coolant, and fuel temps, which decreases fuel economy. AFAIK, it's especially sensitive to fuel temperatures with coolant temps coming up second and air temps bringing up the rear. So, you may want to have your ECU remapped at the same time to get the full benefit.

Also, what RiceEater says is 100% true. Warmer combustion is more efficient without a doubt, but unless the car is specifically engineered for it, there are unintended side-consequences such as reduced parts lifetimes to deal with.

Last edited by nicklockard; August 24th, 2008 at 00:08.
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Old August 24th, 2008, 15:07   #20
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Quote:
Originally Posted by nicklockard
.....
If you want an easier way to experiment that won't have this side effect, try just removing the intake air snorkel from behind the headlight leading to the airbox. It's well known that engines ingesting warmer air get improved fuel economy. And it costs nothing and doesn't have the unintended consequences that come with changing the t-stat.
.....
Anyone else have experience with this mod? From what I know, the warmer air coming into the intake will decrease hp. Anyone have real-world numbers for mileage with and without this mod?
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Old August 24th, 2008, 15:18   #21
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Yes it will decrease HP by a tiny amount, not enough to notice unless you go to the drag strip.
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Old August 24th, 2008, 17:27   #22
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Is there a noticeable increase in mileage by removing the snorkel?
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Old August 24th, 2008, 23:42   #23
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It can't hurt anything. Try it. Tune for it, and let us know
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Old August 27th, 2008, 15:27   #24
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Bump for any data on the intake air snorkel removal mod. I'm going to try to make a few minutes tomorrow to remove it and I'll let you know what the Scangauge says.
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Old August 28th, 2008, 11:26   #25
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Interesting thread here! My scangauge water temp NEVER goes above 180F (normal is around 176-178). It's never very hot where I live (average temps summer are 70's or LOW 80's). Very rarely do I venture into hot temps for any length of time. I have to wonder how accurate the temp reading my scangauge is (the sending unit could very well be off). But just for fun, I JUST ordered an 87C thermostat (which is 188F for those of us NOT on the metric system). It will be very interesting to see if my scangauge temps go up accordingly, AND if I will see any increase in mileage (my primary goal). I NEVER would have thought of an increase in engine temp as a way to MAYBE gain an extra mpg or 2...worth a shot! I'm no engineer here, but the description of oil drag and such makes a lot of sense...if I can run my engine even 10 deg warmer and glean even a tiny mileage increase, it will be well worth it (as long as I don't start doing damage to other parts...but I don't forsee that with only the 87C thermostat). And if my scangauge is reporting accuratly, then either I have a lower temp thermostat installed (bought the car used, so can't vouch for whats installed at the moment) or it is not opening at the correct temps. Either way, the 15$ for a new thermostat is a cheap way to experiment. THANKS for the idea, and keep the great ideas/debates coming! I LOVE this place!!!!
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Old August 28th, 2008, 12:40   #26
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Quote:
Originally Posted by ExtremeTDI
Bump for any data on the intake air snorkel removal mod. I'm going to try to make a few minutes tomorrow to remove it and I'll let you know what the Scangauge says.
My air snorkel has been off for a long time... On hot days the car is very doggy, especially in stop and go traffic... Highway economy has never really changed though I do get better highway mileage on warm days versus colder ones.. There's a noticeable difference from a 45 degree night to a 90 degree day..

I'd keep the snorkel in, I just need to find it...


My coolant temp according to vag-com sits in the 93-96*C range.. I changed the thermostat a while ago because it was staying around 60*C.. I don't remember what the application was for the thermostat I used, but t wasn't for my car.. I remember had to take the little dingle ball off the thermostat and seal off the hole for it to work properly..
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Old August 16th, 2010, 13:40   #27
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A two year followup on this.

I never saw any improvement in fuel economy with the 91 degree thermostat. The only change was that on hotter days when shutting down, the engine would be hot enough to keep the cooling fans on for an additional minute or 2 after shut down.

I'm going to be swapping an 87 degree thermostat back in.
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Old August 16th, 2010, 15:03   #28
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Quote:
Originally Posted by mchapek View Post
Interesting thread here! My scangauge water temp NEVER goes above 180F (normal is around 176-178). It's never very hot where I live (average temps summer are 70's or LOW 80's). Very rarely do I venture into hot temps for any length of time. I have to wonder how accurate the temp reading my scangauge is (the sending unit could very well be off). But just for fun, I JUST ordered an 87C thermostat (which is 188F for those of us NOT on the metric system). It will be very interesting to see if my scangauge temps go up accordingly, AND if I will see any increase in mileage (my primary goal). I NEVER would have thought of an increase in engine temp as a way to MAYBE gain an extra mpg or 2...worth a shot! I'm no engineer here, but the description of oil drag and such makes a lot of sense...if I can run my engine even 10 deg warmer and glean even a tiny mileage increase, it will be well worth it (as long as I don't start doing damage to other parts...but I don't forsee that with only the 87C thermostat). And if my scangauge is reporting accuratly, then either I have a lower temp thermostat installed (bought the car used, so can't vouch for whats installed at the moment) or it is not opening at the correct temps. Either way, the 15$ for a new thermostat is a cheap way to experiment. THANKS for the idea, and keep the great ideas/debates coming! I LOVE this place!!!!
Never saw this post before, but yeah, see this formula that's in my sig:

Constant Speed Road Load Power(v, others) =1/2*rho(T, rH)*CdA*cross product*(Vtrue air)^3 + Crr*M*g*Vroad + parasitic_load(Tengine, oil_visc, other)
Red is for Vector, Bold is for things you can change.

Tengine affects oil viscosity and heat rejection (heat lost to coolant instead of creating pressure in the cylinder).

David, I'm not surprised at your results. Been a while since I've seen you post. Working as a Vet?
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Old August 17th, 2010, 06:29   #29
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My T-stat was cracking open around 178F, no good. So a couple years ago (must have been after this thread, though, or I would have chimed in) I bought a Stant 195F unit. Turns out (per my scan gauge) it doesn't open until 204-206 F! With my grill blocks in, I consistently cruise in the 207-211 range and hit 215 on WOT blasts or up hills. I am completely comfortable with this.

An additional benefit is that it bumped my summertime cruising oil temperatures from ~200F up to 220+ per my VDO oil temp gauge in the sump (pan).

I am pretty sure I saw a *slight* bump in economy from the change (i.e. ~0.5 mpg). Within measurement error? Probably. My fans never run after shutting the car down though.

The theory is pretty sound though. Anyone whose coolant is running under 180F should definitely swap the t-stat.
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Old August 18th, 2010, 20:58   #30
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Take a look at this:



I am experimenting with running a 1.8T a lot warmer than stock as a part of a residual heat project. I have a 96 degree Celsius high flow thermostat in the engine along with Evans NPG+ coolant and nearly zero water. So far so good. I had to fill up today and got 34.44mpg with 350 miles travelled and 90 miles of that was city driving with the rest being 80mph highway miles. I usually get 30-31mpg with the same mix of driving. So far when running the engine hotter I have not had a tank below 31 mpg. The engine is chipped with a 2.5" exhaust and decent front mount intercooler. It dynoed at 205whp and 205 tq 7 years and 60k miles ago.

The project is aimed at increasing efficiency through running the motor hotter and having more residual heat after short trips, especially in winter.
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