Okay so I did 1000 miles over the last few days, and all but a few of it was 10* or colder (down to -13* a couple times). For the interstate driving to the hunting grounds I had all 4 flaps open and IAT's ran about 50-75* at 80 mph, depending on boost. The lower the boost the lower the IAT. Mileage wasn't great at 14.0, but it was COLD and windy. For the next 500 miles I was just cruising around and idling a lot, so all 4 flaps were closed. IAT's were never over 65*, and sometimes in the 30's and 40's if it was cold enough out and we were going slow (again boost is a major factor in temp rise). There was one pass between the hotel and the unit and it was 4 miles up and 4 miles down, with about a 1,000 foot elevation rise. The temp on the hill was between 0 and 10 every time. I drove it at 60-65 in cruise with all 4 flaps closed and the IAT's were 100-130* by the top. Flat cruise at 70 with all 4 closed at 0* was about a 70* IAT, but it rises quick with boost. For the drive home the ambient started at 7* and got up to 27*, I had 2 flaps closed for the whole drive and IAT's were 65-85*, very good. We also had no wind so mileage was better, probably 16 but haven't filled up yet).

So the last time I ran 4 open at 80 and had temps over 100 I must have had a serious head-wind and not realized it.

I will no longer remove my winter front for any winter driving, unless towing makes the IAT's over 100* on flat cruise.

The winter front did a great job helping keep the motor warm in the cold temps and really helped keep the heat in when idling and when shut down for short periods of time.

So, what do you think? Was mileage better at a higher IAT?

And, what kind of gauge did you use?

My mileage was a little lower than doing the run last year, but the conditions were a little different, I had a very strong head wind this year. Boost seemed to be lower so I don't think it was working as hard.

I am using a SGII to monitor IAT's.

It is my understanding that the colder you can make the intake air temps, the more efficient your engine will run with regards to air density and fuel mix. The statement about running more efficient with higher temps applies to the cooling system and its effect on the cylinders.

The engine can make more power, but several people have done tests and the engine gets better economy the warmer the intake air is, to a point.

I guess colder air=denser air, but when you get to a certain point, your contradicting one of the 3 elements required for combustion (heat), and since compression is not one of those elements, you can only offset it so much.

I will no longer remove my winter front for any winter driving, unless towing makes the IAT's over 100* on flat cruise.

Close enough to what other big truck drivers (and the shop) told me when I started. A winter front would be a no-brainer I would think. We simply had to make sure the fan airflow wasn't restricted (circular opening corresponding to fan diameter). The farther the coolant temp drops from "normal" the worse FE will be.


It is my understanding that the colder you can make the intake air temps, the more efficient your engine will run with regards to air density and fuel mix.

For a quarter-miler, absolutely.

IAT control is where it's at. In the 1960's, as emissions laws were coming into focus the carmakers learned that fuel economy/performance was best balanced with constant air temp control under cruise. For the gas motors this was around 115F for best atomization.

Still true today, but computer control makes a wider range of air temps acceptable. Warm, not hot, still seems to be best for economy (except the 0.0001% of time at WOT). (See the many posts on ECOMODDER about IAT).

As always diesels are different. But there is bound to be a fairly narrow temp/humidity range. I have seen it stated as 80F and low humidity, ambient, relative (another contributor). I "tend" to agree having covered the same route for nearly 16,000 miles over about one year, a 600-mile commute. The highs and lows weren't that different, but the cooler weather (below 60F) was more likely to start to see my numbers trend downward. Slight, but noticeable (temps under 35F simply not encountered in that time).

It would be nice to see comments from CUMMINS on this subject (ideals of temp/humidity/altitude, and the tuning range as it were). How much is attributable to engine ideals versus other contributing factors.

I made a 435-mile roundtrip to Houston today (with three stops once there) and showed just under 26-mpg, total (55 mph at 1,650-rpm; 38 mph average) once I refueled. Temps ranged from just above 50F to 70F. Truck was just under 8k, estimated. Same time last year I didn't see above 24 in cooler temps with a slightly lighter truck (and no stops). Not an exact comparison, but FE was slightly better summer through Xmas, and again past February (winter end, here).

Diesel engines (and cooling systems) are so efficient at shedding heat that I tend to think that the vehicle is "comfortable" when I am . . . but it handles the heat somewhat better. So, a cold headwind is rough, in diesel-speak. I've had to run slower in bad weather in a big truck to keep coolant temps sane. (It's a lot easier to control IAT for gasoline motors).

The optimum temp range for all components, fluids, etc, is really rather narrow. Fastest warm-up, never idle (any reason), and avoid temp/pressure spikes to keep near factory-designated point for longest life and lowest fuel use. That IAT has an ideal doesn't seem like a stretch, nor does a lower limit cutoff.

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if i remember correctly per the cummins troubleshooting manual for a 2006 ISL intake temps below 60 degrees while at operating temp are not desirable, the same probably goes for a ISB. A couple years ago it reached -30*F actual air temp. this is when i had my 99, i had cardboard behind the grill and everything. The Edge monitor was showing intake temps at 40 degrees and the truck ran real loud (like all VP44 trucks sound like when you first start them cold)

Thanks!!