Thermostat Change for Better MPG???

ExtremeTDI

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2000 Jetta TDI
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.
 

Variant TDI

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I'm not brave enough to install a higher temp thermostat... however... I have blocked the center vents in my lower bumper with foam pipe insulation to aid in cold weather warmup.

I'm not sure I would do it in Texhas, but I leave my blocking on year round, because at worst in horrible traffic on 100 degree days, my scangauge says the highest coolant temp I've seen is 204 F. Typcially it's 182.
 

EddyKilowatt

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Just guessing, but your better economy on warm days is probably, mostly, due to your transmission, differential, and wheel bearings running warmer. Plus your engine starts out warmer and takes less time to get to operating temperature.

There's also the small but noticeable decrease in air density on warm days (ask any of the pilots here about density altitude) which plays directly into the aero drag equation, and thus into your fuel economy.

Get a Scangauge and see what your coolant temp is actually doing in winter (dash gauge doesn't tell you)... if it is not maintaining the 190-ish setpoint then a hotter thermostat won't help anyway, and you are in a position to consider reducing airflow thru the engine compartment, per VariantTDI's suggestion.

Eddy
 

ExtremeTDI

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I do have a scangauge. It's awesome. Right now the temp on the scangauge is holding right around 163F on days ranging from 70F-106F. When you say"190-ish" are speaking about gauge pod temp or scangauge temp. The cars gauge pod temp is holding at round 190F.
 

rotarykid

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ExtremeTDI said:
I do have a scangauge. It's awesome. Right now the temp on the scangauge is holding right around 163F on days ranging from 70F-106F. When you say"190-ish" are speaking about gauge pod temp or scangauge temp. The cars gauge pod temp is holding at round 190F.
Come on this is a German car use the metric temp as that is how the factory thermostat is rated ............

You should have an ~85-87 * C from the factory on A4 TDIs . Which means it should hold just above or at that point under normal loads . If you are seeing temps holding that low ~58 * C your thermostat is bad and sticking open . You are holding below 60 * C and that is really bad for the engine if you do this very long . Running this cold will cause more wear from longer warm up times shortening the engine's life . And lead to premature coking up of valves and other internals . Running this cold can also lead to the radiator cooling fans not operating properly leading an over heat causing engine failure . Replace it now ................

When the thermostat is functioning properly you should reach the set point of it's rating and not fluctuate up or down from that point .

On hotter thermostat improving fuel economy ??,

The answer is yes it will . But with a higher thermostat you must monitor actual temps with something like a scangauge to prevent head gasket failure from over heat . I have a ~91 * C in my 97 Passat TDI and the engine holds around 93-98 * C under normal driving .
 
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david_594

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rotarykid said:
You should have an 85 * C from the factory on A4 TDIs .
87*C was the factory thermostat in my 2000 Jetta.

Swapped it out for a 91* thermostat and havent noticed any real change in anything. Maybe it will make a difference in the colder months, but I am not holding out much hope.
 

EARL97850

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I too do the foam pipe insulation trick. Although I had the intention in the beginning to remove it during the summer months, I didn't and have left it on for the last two years.

My mileage didn't change one way or the other, but it does help in the winter months.

Cya
 

tnp

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I believe there are 4 different thermostats for our AHU and ALH engines. 71C, 80C, 87C, and 92C. The fan switch is then supposed to turn on at 100C or 102C and then turn off at 95C

On my Vag-Com, my max coolant temp always seems to be in the upper 70's. I've tested it now last winter and now this summer. So, I believe I have a 71C. I was actually going to be looking into changing this so that I have the 92C so that my engine runs hotter.

BTW - there is some logic to this. The warmer everything is (fuel, air, coolant), the easier the fuel will atomize and burn, and the peak temp shouldn't be as high. With that understanding, I believe that the factory ECU makes this mixture just a smidge little leaner.

Remember, one of the main things that is limiting the factory ECU fuel maps is emissions output. Otherwise, it would all be super-lean or super-rich, depending if you want power or economy. But, it isn't programmed that way.

It would be great if anyone had some part numbers for these parts. I'm going to make a few calls when I get a chance. If anyone has these P/N's, please post it.

Also, if anyone has done a before/after test from doing this which would validate this theory, that would be great. I'm willing to invest in the possibility if no one has, though...
 

ronbros

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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, boiling point 375-400F(you convert to celsious) higher temps in the chamber,

faster vaporization of the atomized droplets could mean more complete combustion for better mpg, less carbon biuldup or coking, turbo would be snappier, maybe in the future we will not need those DPFs, somehow i feel they are backwards engineering. also i think we have not done enough testing for higher static compression ratios, like 25-30 to1, what i can understand we do not have the proper materials, to build such engines YET! anywho, im about to try the high cooling temp project as we speak!

Thx Ron
 

ronbros

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of course the computor wont like any of these ideas! so you readjust them to suit your needs!

Thx Ron
 

rotarykid

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david_594 said:
87*C was the factory thermostat in my 2000 Jetta.

Swapped it out for a 91* thermostat and haven't noticed any real change in anything. Maybe it will make a difference in the colder months, but I am not holding out much hope.
I'm guess they have changed a little since the older diesels I worked on day in & out . If memory serves they used to be stamped with 83 , 85 , 87 , 89 , 91 & 95 * C .

I guessed at the ~85 * C number off of what I have seen on a scangaugeII . Mostly 83-89 * C on the display of a ALH 5 spd man TDI , that is where I got that number i haven't actually had one out of an ALH in hand yet .
 

Drivbiwire

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tnp said:
I believe... BTW - there is some logic to this. The warmer everything is (fuel, air, coolant), the easier the fuel will atomize and burn, and the peak temp shouldn't be as high.
Given the small amount of fuel burned thanks to a diesels efficiency, higher fuel temps the lower the density of the fuel THUS the ECU has to increase the QUANTITY of fuel injected to compensate for the heat: translation 0 net improvement.

Same thing with air, the ECU reads actual air volume by way of the MAF sensor, IAT (measured after the turbo and intercooler) to calculate actual air entering the cylinders. Hotter air translates again into lower density and HIGHER EGT's translated HIGHER NOx emissions! The ECU then starts cutting back on fueling, retards injection timing (lowers peak combustion temp and pressure) as well as boost to keep the engine within emissions limits as well as to keep the turbo from exceeding it's flow limits, translation: Hotter coolant will result in lower power, If the system is tricked you will not get an improvement, instead you will get a hotter running engine, period.

You can argue that warmer fuel reduces the auto ignition period (Igntion delay) however the nano-seconds in difference in all practicality cannot be measured and is more easily compensated for thru injection timing with greater accuracy accross the engines operating range.

With that understanding, I believe that the factory ECU makes this mixture just a smidge little leaner.

Remember, one of the main things that is limiting the factory ECU fuel maps is emissions output. Otherwise, it would all be super-lean or super-rich, depending if you want power or economy. But, it isn't programmed that way.
Please I want to hear about fuel air ratios in a diesel! Tell us how does it make the fuel air ratio richer and leaner???? This I gotta hear! Does "Diesel Cycle" ring a bell? or are you still confused about how the Otto-cycle works?

It would be great if anyone had some part numbers for these parts. I'm going to make a few calls when I get a chance. If anyone has these P/N's, please post it.
Before you start hacking on the engine first take the time to understand how it works in the first place.

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.

Increasing the coolant temperature will have essentially one result, it increases coolant temperature! Sure it could in extreme climates increase the cooling systems reserve thermal capacity BUT I don't think anybody here is running at 100% load in 145F temperatures with the A/C running at full speed!

If you are running at 100% load in 145F temps then by all means increase the coolant temperature, aside from that you would be pissing in the wind in respect to gaining anything.

DB
 
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david_594

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Drivbiwire said:
Please I want to hear about fuel air ratios in a diesel! Tell us how does it make the fuel air ratio richer and leaner???? This I gotta hear! Does "Diesel Cycle" ring a bell? or are you still confused about how the Otto-cycle works?
It makes it richer by injecting more fuel for a given volume of air. It makes it leaner by injecting less for a given volume of air.

Smartass answer for a smartass comment. Sure, ALH's dont have a throttle body to control airflow at sub ambient pressures, but the ECU does have control over requested boost. It also have control over requested fuel. Combine the two and who would have guessed.... but the ECU has control over the air/fuel ratio.

Your a smart guy, you know it can be done. Why do you feel the need to give him such a hard time about it?
 

Ski in NC

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The OP recorded temps around 60-70C on an engine that should? have a 87C thermostat. Sounds like t-stat is opening early or hanging open. If it was my car, I'd get a factory 87 and put it in.

One bit missing in this thread is that with a cooler engine, there is more parasitic loss simply due to lube oil viscosity. When oil is cooler, the friction due to viscosity is higher. Every bearing and piston skirt drags an oil film. The friction is by definition related to the viscosity. Friction is paid for with fuel.

Of course too thin an oil, by grade or temp, is no good. Too thin a film can have high friction. Or worse, metal to metal contact. There is a sweet spot there. I'd take faith in mfr's spec temp and viscosity.

I switched from 5w-40 to 5w-30 505.00/505.01 on my TDI. Last few tanks show what might be a 2mpg improvement, but the diffences in the data definitely fall into the category of "statistical noise".

-Eric
 

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 by ExtremeTDI
2000 Jetta Tdi gets better fuel ecomomy on the hotter days.
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 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... :)
 

RiceEater

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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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oldpoopie

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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.
 

nicklockard

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ExtremeTDI said:
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.
 
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ExtremeTDI

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nicklockard said:
.....
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?
 

nicklockard

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Yes it will decrease HP by a tiny amount, not enough to notice unless you go to the drag strip.
 

ExtremeTDI

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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.
 

mchapek

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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!!!!
 

TDIJetta99

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ExtremeTDI said:
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..
 

david_594

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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.
 

nicklockard

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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!!!!
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?
 

shizzler

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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.
 

corrado tdi

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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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