Egt - Mpg

[More Power]

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

[SoTxPollock]

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?

[More Power]

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

[Mark Rinker]

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...

[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

[SoTxPollock]

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.

[kerryw]

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 .

[dieseldummy]

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

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

[dieseldummy]

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