Egt - Mpg

We've talked about the necessity of EGT/Boost gauges, beyond protecting the engine from excessive exhaust temperatures, but also as a way to help manage the powertrain more effectively. I've thought for some time about how to illustrate that fact, especially when it comes to fuel economy. During a drive this weekend, I carefully watched the EGT's and its relationship to vehicle speeds, and am offering a rough estimate for a way to use the EGT gauge as a tool to manage the powertrain for better fuel economy....

While the numbers may be massaged over time, a best guess would put 600 degrees EGT at an optimum (while driving not too fast or too slow) exhaust temperature for fuel economy, and which should produce very close to 20 mpg (or more) if that 600 figure is maintained. In my truck, that means about 60 mph. 65 mph pushes EGT to 650-700 degrees. Running 75 or a touch more pushes EGT above those numbers.

We need your help..... We need more data. If you can log a tank of fuel at a constant EGT at a particular speed, let us know your MPG results. More data means we can plot the EGT and MPG, and arrive at a statistical value that could very well become a "rule" - than the current best guess we have now.

Thanks!

Jim

I think I see where your going on this one, but unfortunately the EGT won't be a direct correlation of your MPG, because the regestered EGT is more a function of load and not necessiarly speed. Of course its an indicatior, but one involving many variables. It sort of stands to reason that the smaller amount of fuel burned the lower the EGT, thus better MPG, but how you gonna make use of the data collected without knowing the rest of the story?

What I expect will happen is a general rule of thumb will emerge. In addition, those who report poor fuel economy can compare their EGT readings to what others are seeing for a given speed (along with their fuel economy numbers), which then hints at a solution or at least a partial answer to the question people ask to help explain their poor fuel economy - or why some get above average fuel economy.

If for example, someone reports a low fuel economy, they can also comment on their EGT. Perhaps the topper, winch bumper, ladder rack or vehicle lift they added raised cruising EGT's by 100 degrees, when compared to before the mod. Using the instrumentation we have available to us allows for more information to examine cause & effect. I expect a scatter plot will eventually show a trend to the EGT/MPG data. That trend might then become a general rule. :)

Jim

Makes sense. The more heat in the exhaust, the more 'lost potential power' that is heating the atmosphere, rather than producing cylinder pressure to move the load on the crankshaft.

I'll be testing your 600 degree theory for a full tankful soon, MP. However, I have a feeling that with the loaded gooseneck behind me, the speeds might well be 55-58mph which is about 15mph under the speed that gets me home on time for dinner...

:)

Jim:
It has been my experience that no two EGT system will read out the same. I have a difference of as much as 50 degrees in certain driving conditions between my SPA and the Attitude readout and much more than that in other conditions. I have a thermocouple for the Attitude that has a fixed depth of penetration into the manifold and therefore cannot be adjusted to the exact center of the exhaust flow through the manifold. On the other hand the SPA can be adjusted to center. With that said I still don't have an exact match. Sometimes they will match but for the most part the difference is noticable. It gets down to which one to believe. In the airplane business the EGT systems are all calibrated to the same length of wireing within the A/C. When an instrument is in the shop the test equipment is the standard by which the instrument is calibrated. Therefore all the systems in a given model of A/C will read within as little as 15 degrees and not more than 25 between positions. I guess what I'm getting at is that most folks only have one gage and who is to say that it is accurate. At a given speed say 60 one may read as much as 100 degrees different from the next guys. Yes, I would say that your plots will be all over the map. I'm probably being to nit picky here but you know me.
Denny

Mark, you make a very valid comment about heat lost. Years ago I ran road racing karts in WKA & IKF events, ever ran a few in your state. We ran not only exhaust gas temp's but I alson ran a thermocouple under neath the spark plug to get a reading on the cylinder head temperature. The leaner the gas mixture the hotter the cylinder ran presumibly due to the long burn effect of combustion. After considerable carniage I learned exactly where the point was that one melted the tops of the pistons off and stayed just below that to get to the checkered flag. Now I'm wondering if you could unscrew a glow plug and put a thermocouple underneath it, reinstall it and monitor your cylinder head temperature at the same time as the exhaust temp.

well with my truck ,with preditor program ,60 hp towing at 114 k.m per hour 2150 rpm seems to be the sweet spot average 670 km per 100 litres of fuel.runs at exhaust temp of 600 to 650 post turbo,now same speed empty not towing exhast temp 500 range millege 720 per 100 litres. this summer pulled same weight but at 97 km per hour exhaust at 550 , 510 per 100 litres averaged on three tanks of fuel same brand as well. So to tow slow poor economy too fast the same result exhaust does corelate to fuel economy because if engine temps to low fuel used to build heat not power,my feelings 2100 rpm 600 degree exhaust seems the sweet spot. winter or summer fuel.also 120000 km on my truck on first set of brakes with aprox 50 % left.i dont drive hard ,but dont race just steady . Would love to see j.k tune for towing set to 2100 rpm max torque and h.p .

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Originally Posted by Inspector

Jim:
It has been my experience that no two EGT system will read out the same. I have a difference of as much as 50 degrees in certain driving conditions between my SPA and the Attitude readout and much more than that in other conditions. I have a thermocouple for the Attitude that has a fixed depth of penetration into the manifold and therefore cannot be adjusted to the exact center of the exhaust flow through the manifold. On the other hand the SPA can be adjusted to center. With that said I still don't have an exact match. Sometimes they will match but for the most part the difference is noticable. It gets down to which one to believe. In the airplane business the EGT systems are all calibrated to the same length of wireing within the A/C. When an instrument is in the shop the test equipment is the standard by which the instrument is calibrated. Therefore all the systems in a given model of A/C will read within as little as 15 degrees and not more than 25 between positions. I guess what I'm getting at is that most folks only have one gage and who is to say that it is accurate. At a given speed say 60 one may read as much as 100 degrees different from the next guys. Yes, I would say that your plots will be all over the map. I'm probably being to nit picky here but you know me.
Denny

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I assume this is on the Dmax? If so do you have both probes in the same manifold or on opposing sides? If on opposing sides that will likely explain your situation. All EGT gauges should be sold as a "Kit" in which the probe, thermocouple, and gauge are calibrated for each other. In such cases accuacy isn't a big deal, it's when mixing a matching of parts happens that makes things get out of whatck. Just my .02.
Justin

I would agree that 600-700F EGT is a good indicator of better fuel economy expectations. Earlier this week I went to Salmon, ID in my PSD it was about a 200 mile trip and I was trying for good fuel economy numbers, I kept my RPM under 2000 and EGT's at or below 700F. The fuel economy for that trip was 19.8mpg!! (hand calculated)...

Chris

I'll try your 600 degree theory MP, but I can't ever seem to run through a tank of fuel without towing and/or hotrodding around some. The worst milage I've gotten to date is 16MPG with my '01 and that was today with a 75/25 mix with a trailer in tow for the 75%. Bucking a headwind the Pyro said 900 pretty constantly with spikes up to 1100. On the other hand when I can burn a full tank just commuting to and from work the EGT is usually at 550-650 with spikes up to 1000 on accell I can net 20mpg consistantly.

The 6.5 never was a milage star... I got 19mpg on a couple occasions, but normal commuting was around 13-15mpg. As best as I can remember cruising EGT was around 500 with spikes up to 1200 on accelleration.

Hope this helps.
Justin

DD:
That is my point. No two set ups will read the same no mater where they are located. Mine are in oppisite manifolds. I assume that these tests will all be done solo. My truck will be used mostly hooked to my fiver. If I get 12 and I have, I'm tickled pink. Temps are around 900 to a thou in cruise on the flat. Gage difference about 50 degrees. Pulling a fiver is not the time for an economy run. I do my best not to push to hard but the truck has to work. I worry about the price of fuel but I can't let it keep me at home either.
Denny

To better illustrate the idea of establishing a connection (or trend) between EGT and MPG, I've created a theoretical scatter plot.

http://www.thedieselpage.com/images/scatterplot.jpg

Now, you'll see that no two people got the exact same result - they don't have to. Data from those participating simply appear as a dot on the graph. Highs and lows become less important in the final analysis, and errors in calculating fuel economy or EGT calibration will likely average out. What we wind up with is a "trend", as represented by the red line. The more data we have (truck owner participants), the more accurate and representative the data will be.

Once enough data has been collected and plotted, any trend that emerges will become a "General Rule". The purpose of this exercise is to explain the relationship between exhaust temperature and fuel economy, and/or help troubleshoot a low mpg truck by using the EGT gauge. In addition, those who are concerned about fuel costs, and wish to drive their truck for maximum fuel economy will learn how an EGT gauge can be used to increase fuel economy.

Over the next few month's I'll collect data using my truck. Of course, your help will improve the overall accuracy.... :)

Jim

PS. You'll notice in the above scatter plot that the fictional data (chosen to be close to reality) agrees very well with Inspector's towing MPG's and his reported EGT's (950 degrees EGT = 12 MPG).... This could work for anyone, whether driving solo or towing heavy, driving fast or slow.... :)

Ok I can now buy the trend idea. I can see that a splatter charting of EGTs can show trend data. I just didn't grasp the intent.
Carry on. I will be driving solo to St. George Ut. next month and will try to gather some data on that leg of the trip. I will be pulling my fiver back so that will be a different look.
Thanks Jim
Denny

I predict your fictional data curve overlayed on your real data curve will be pretty colse to what you have now, based on my simple understanding of the physics involved. Course when JK gets the most efficient tune......

based on temp ,temps climb back your foot out of it ,keep even temp,i.e load for speed . We know we can pull at 80 mph but at 900 degree exhaust temp and xmph but if we keep at 600 degree temp what will our mpg be much better also less braking load trany wear many things . i have 120.000 k.m on my truck ,will push it to blow away rice burners or other diesels but usually drive steady and even . Some what when i used to drive gas truck and used vacume gage for economy. M.P has point and his scatter plot does show it steady ,consistant ,moderate driving will return best mpg. But with a duramax it is hard to do as it is so much fun to push the pedal to the floor,kinda brings back some youth. With todays fuel prices i can only play a litle ,so i drive smooth,even.at 600 degree temp and 2050 rpm which seems to be the sweet spot for my 2002. But no 2 trucks are the same my budy has 2005 and 2150 works for his truck.

Are you wanting to limit this to the Dmax, or is 6.5 data useful too?

My snowmobile trip pulling 2 place trailer egt's 750 to 800...14-15 mpg. Same trip running solo egt's 600.........18-19mpg. The chart looks accurate. This is with stock exhaust. When I open up the exhaust people claim 100 - 200 degree drop in egt. Will I get better mileage ? I will find out when the new exhaust goes on.

MP first time I have read this thread. After looking at the chart you have it is very close to what I see. All the way up the line it is very close. I'm making a 700 mile trip one way on Sunday and will try to keep and even speed and see what I get. On the return I will be in a hurry and will push it a little harder.

Greg

Wow, you guys are really into this!

Quick question. I'm planning to install a pyrometer tonight. I plan to install the probe by the tip of the downpipe just below the v-clamp where it connects up to the manifold. Is that a good location? Next, when located there, what are safe EGT levels. I pull about a 14,000# trailer with corsa straight through exhaust, edge ez and boost stick. I'd like to see if I can safely run on the EZ's highest level.

There are some earlier post on the location and placement of the probes some where back in the forum. Probably the most popular and best place is in the rear "side" of the passenger manifold. The earlier post has a very good step by step procedure, and if I remember correctly some diagrams also. I have a probe in both banks each with it's own gauge "same type",and they only read the same at a very light cruise. I think the restriction in the left manifold may account for some of my higher readings on the drivers side "left" and faster response to WOT and pulls by that gauge. I would think that a correlative reading between both the boost psi and EGT might yield a fairly accurate optimun fuel efficency indicator.
I notice that I seem to get the best milage when I don't push the boost up any more than I have to for whatever driving conditions exist. Kinda the opposite of a gas engine getting the better fuel economy with the highest vacuum reading you can maintain.

I just returned from an approx. 1400 mi. trip from KY to SC and back. I averaged approx. 900F on the pyro and hand calculated an average of 11.1 mpg. I was pulling a ~9000 lb. Coachmen TT over the Appalachian Mts. BTW, I was seeing about 600F EGT downhill and about 1200F EGT up fairly steep grades. I kept speed fairly constant from 55 - 60 mph.

I hope this is helpful to MP's scatter plot.

A round trip last Friday of 450 miles on I-90 between MT & ID. Some legs were at 75-80 mph, and some were at 65-70. Averaged approximately 650-700 F EGT (900 going up, 300 going down, 700 on the flatter portions), and hand calculated 19.35 mpg for the trip. :)

Jim

I have found it is nearly impossible to maintain a constant exhaust temp unless the terrain is perfectly flat, and where do you find that?

I will try to see what exhaust temps are while not towing on the flat. Towing my trailer, I can't keep temps under 750 degrees unless I go 50-55, and even then, going up hills the temp always clears 1000 degrees. I do notice that as soon as the truck downshifts out of 5th gear, the temps start dropping even as rpm and turbo boost goes up. Does that mean I would use less fuel on a hill in a lower gear? Even in high gear my temps don't exceed 1250 degrees unless the hill is really long. I do wish I could manually select 4th gear somehow.

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Originally Posted by Philsauto

......I do wish I could manually select 4th gear somehow.

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You can. Either keep the speed under the threshold in T/H, or install an OD lockout. Just a momentary switch and a lead into the PCM connector. I think Kennedy still has the procedure on his site. If not, it's here in the forums, somewhere. If you are interested in the high idle, do them at the same time. Same procedure, different pin.

My question is did you`all install the probe pre-turbo or post turbo :confused:
Pre-turbo can/will be 200-500 + degrees higher,depending if you are empty or towing.
Compaired to post-turbo/after turbo when probe is installed on down pipe.

I think most here have installed the pyrometer pre-turbo. Most people indicate their placement in their signatures. I know the guage faces were different depending the placement of the pyrometer. IIRC, the pre-turbo guage was color-coded about 300F higher than the post-turbo.

Jim M.

Yes I read that as well,the one`s that posted higher temps 900-1200 was pre-turbo but also several stated 600.
I have not installed a gauge on my GMC as of yet,but my last truck I monitored pre and post turbo.
Mainly to prove to a local shop that there can be more than 200 degrees difference between the two locations as most shops think.

What happens when you add a grill cover for the winter? if it adds 200 deg from lack of air flow does that raise it from 600-700 to 800-900 or do you have to slow down evenmore to achieve the same MPG without the cover. Inquiring minds just have to know

it is not wise to add any covers to block air flow to the intercooler or the cooling system you can risk overheating and engine to trans damadge be careful. you can also get erratic maf sesnor reading causing injectors to not work correctly and or ses lights

Quote:

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Originally Posted by mastertech

it is not wise to add any covers to block air flow to the intercooler or the cooling system you can risk overheating and engine to trans damadge be careful. you can also get erratic maf sesnor reading causing injectors to not work correctly and or ses lights

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Maybe if you are driving in 'warmer' weather. Those of us that live in the areas that actually HAVE a winter, use them safely.

Adding the winter cover in cold weather shouldn't really change your egts... However I HAVE noticed that I get better fuel mileage with my cover on when the temps are in the 30s.