ATT 6.5 Fuel Mileage Hit with AC on

[Paveltolz]

Admittedly, the ATT mileage increases others have reported haven't fully materialized for my truck. Maybe my driving style and it don't match well but regardless, with the use of the A/C, numbers have reversed from mid to high 16s down to as low as 15.4 mpg.
History: When I first got the truck in 2007 and drove it across the country in January (pretty much OEM everything). I would see a freeway norm of 17 mpg and that had to be on winter blends (WA, ID, WY, UT…you get it). City mileage, after getting to NC was 15.9 to 16.5mpg. Doubtful we had winter blends in the Sand Hill region though.
Four years later...this winter I have conducted 5 test runs for extended periods on the ATT and GM-5 comparing driving conditions and winter to summer fuel blends. I did change computer tunes mid way through the first test period but didn’t see a noticeable change with either the ATT or the GM-5.
ATT numbers to date have been:
16.81 November to December (mixed driving)
15.51 December to February (GM-5, new tune, no change from last year)
16.9 from February to the first part of May (mostly Daily Driving),
16.32 for the rest of May (mostly Daily Driving) and
17.02 this last weekend on a road trip to Vega BUT...,
I anticipated some good numbers with constant freeway speeds, the lower altitudes/denser air but with the A/C on, that didn’t really happen.
Freeway speed driving down and most of the way back was 75mph with AC on for the first four tanks.
Worst tank was descending out of St George UT to Vegas and driving around town; 15.4 mpg. Climbing out of Vegas was 15.8mpg. Best tank was the last one with the A/C off and slowing to the sweet spot of 70 mph (2200rpm) with some back road driving for a while at even lower and very inconsistent speeds; 19.34 WOW.

Still, without the high numbers for that last tank, considered it a fluk, the trip would have been closer to 16.5mpg and I know that since I'm back to my Daily Drive, the numbers will be back down around what I'm seeing after test run 4 which was 16.32 and those were without the A/C being run.

So, regardless of the mpg improvements you've had with your ATT swaps (good on ya) has anyone else seen their ATT equipped trucks take an MPG hit with the A/C on or am I just special?
Another question for the field, has anyone not seen their MPG numbers increase with summer finally coming on this year? Mine seem to be even less from the winter.

 

[ak diesel driver]

if I drive over 65 I take a big hit. I think it's from my lumber rack messing up the airflow

 

[WarWagon]

Anything over 65 hits mileage. The wind load is double at 70 MPH then the load at 60 MPH according to Gear Vendors towing an RV.

AC takes up to 25 HP to run. It also kicks on the engine fan more and that can take ~5 HP.

I imagine towing with sustained boost would net the most MPG gain. ATT could be costing MPG while spooling up where the GM turbo is like putting a spring loaded flapper on the exhaust when the wastegate opens. I would take it as a good sign that the MPG is steady between the turbo's as you are not loosing anything by going to the A-Team, but have room to gain power and MPG.

I just like the power when the A Team spools on the burb and this is hurting my MPG as I lean on it a lot...

 

[Slim Shady]

I have noticed that my fuel mileage is down this year from last year. I suspect something with the fuel blend. Mileage has been consistently down with my Jetta Turbo Diesel usually ran around 45 to 49 in the summer months and around 40 in the winter months. This year fuel mileage was around 36 consistent and improved to 39 when the fuel changed. This is consistently lower than the previous years and nothing changed on the vehicle and the wife drives consistently fast to the tune of 124 miles a day on the car. SO I suspect there may be a fuel BTU issue here, at least this is what I gather because others that I know have reported lower MPG in most of their diesels.

I have a friend that has a fleet and he is religious about that and he is seeing the same thing. I don't know if that is possible but have wondered.

 

[buddy]

I have seen an MPG hit running either turbo over the last year. I never saw an MPG gain with the ATT, but still would use the ATT for its greater power and less engine stress at high speed or with heavy load. I dont put enough consistent highway miles to get a good gauge of either.

Its quite annoying because here in central CA diesel is more expensive that premium gasoline and the efficiency advantage of a diesel daily driver truck are dwindling.

 

[Acesneights1]

I have noticed that my fuel mileage is down this year from last year. I suspect something with the fuel blend. Mileage has been consistently down with my Jetta Turbo Diesel usually ran around 45 to 49 in the summer months and around 40 in the winter months. This year fuel mileage was around 36 consistent and improved to 39 when the fuel changed. This is consistently lower than the previous years and nothing changed on the vehicle and the wife drives consistently fast to the tune of 124 miles a day on the car. SO I suspect there may be a fuel BTU issue here, at least this is what I gather because others that I know have reported lower MPG in most of their diesels.

I have a friend that has a fleet and he is religious about that and he is seeing the same thing. I don't know if that is possible but have wondered.

I have not received any bulletins about a change in the blends other than Winter/Summer.

 

[buddy]

I wonder if people are distributing B5 or whatever and not saying, since that would have lower BTU but charging us the same

 

[Slim Shady]

Aces who sends you the bulletins, about fuel blends?

 

[Paveltolz]

Anything over 65 hits mileage. The wind load is double at 70 MPH then the load at 60 MPH according to Gear Vendors towing an RV.
AC takes up to 25 HP to run. It also kicks on the engine fan more and that can take ~5 HP.
I imagine towing with sustained boost would net the most MPG gain. ATT could be costing MPG while spooling up where the GM turbo is like putting a spring loaded flapper on the exhaust when the wastegate opens. I would take it as a good sign that the MPG is steady between the turbo's as you are not loosing anything by going to the A-Team, but have room to gain power and MPG.
I just like the power when the A Team spools on the burb and this is hurting my MPG as I lean on it a lot...

When I'm floating down the road and the A/C starts running and I can feel the truck shudder like it is suddenly missing on a couple of cylinders. Try to gently accelerate a bit by say, 5 or 10 mph turns into a labor as well. I did finally notice the fan kicking in too. Always wondered what everyone was talking about but now I know. Howls or moans like a large pop bottle with the wind blowing over the top of it.
I told the wife, with three long trips this summer, it may not be a bad thing $$ wise to suffer the heat a bit. Its either that or drive the Passat Gasser. Uhg.

Turbo's. I guess the best way to describe my results would be "love/hate" for them both. Love 'em for their strengths and hate for the weaknesses. I'll let you know what shakes out on the road trip next week. I do like the way the ATT pulls when it gets up on its power band. That is a hard thing to give up but I'll see what daily driver numbers look like now that my commute seems to be taking a hit with the slower speeds and shorter runs.

Aces who sends you the bulletins, about fuel blends?

That would be great info to be able to get your hands on. I know Crank69 works the petroleum industry too and it would be good to get his take / input on what may or may not be going on.

I was wondering if they had changed the blends later or at all. We've had the cooler weather extending further into the year here but it hasn't been COLD, even by local standards, for a while. I mean, touching on 0 Celcius at night isn't like sustained sub zero for days on end.
As for the B5, my neighbor was speculating that "they" have kept the blending going to increase profits but i have to tell you, I have no idea how that would work out to anyone's benefit as I don't know what costs are realized or saved in the winter blending department. But it would explain the non summer bump in mileage we seem to be having across the board from the Midwest to Cali. Still, if that were the case, I would think the hue and cry from the Long Haulers would have materialized by now or have my daughter's wedding plans caused me to miss yet another important bit of news?

 

[ak diesel driver]

tell your daugter congrats Paul

 

[Acesneights1]

Aces who sends you the bulletins, about fuel blends?

Global Petroleum and Sprague Energy. I am no longer an Exxon Mobil retailer. If a product was to be changed, I as a reseller would have to be notified and I can assure you Distributors such as them would not mess around with mixing fuels at all. Now, retailers, possibly of middle eastern decent...That I cannot speak for.

 

[Acesneights1]

I wish I had saved the stuff Exxon Mobil sent me when Low Sulfur diesel came about and the damage studies showed it would do to older engines but the govt mandate didn't care.

 

[Acesneights1]

http://www.globalp.com/page.cfm?pageID=6

http://www.spragueenergy.com/pages/content.aspx?p=diesel_specs

 

[Paveltolz]

Thanks AK, Will do when she and "whosits" get back from the Honeymoon. I swear if he even grins I'm gonna... never mind.
Aces, thanks for the info and the links. I wonder what the reason is for the less than hoped for summer mileage numbers? Anyone want to help formulate a conspiracy theory (jk)?
For the record, on the road trip I fueled at a Flying J in Nephi UT (topped up for the run south with 244 miles on the clock, 17.8mpg using fuel from Smith's), Chevron in St George (16.9), Conoco outside Bolder NV (15.4), Chevron in Beaver UT (15.8) and topped off at home after 180 miles, 19.34. I even went back after driving to work and back (7 miles) to make sure I had actually topped off and not got a quick click. Nope, .44 gal is all it would take and I was clicking mightily.

 

[buddy]

Im surprised the AC affects your engine performance that much. I wonder how Heath has the timing set while youre at cruising RPMs and fuel rates. If you had a scanner you could see how turning the AC on and off affects your cruising RPM, timing advance, and fuel rate.

 

[Paveltolz]

Buddy, good idea on the scanner. I'll be visiting Bill on the way to Ft Lewis on Thursday and Friday so will see about checking what might going on. Nice hill to the south to check things out against grades as well. However, I have to assume at this point it is the combination of the AC drawing off power, the frequent fan clutch engagement and trying to make 75mph either against some head wind and/or climbing out of Vegas (1900 to 5500' in about a 175 miles). Then there's the motor...I've never done a compression check and while the motor will pass the "Blow By Test", it has a fair amount of puffing action going on.
I'll know better driving the first couple of legs heading out to Boise as they are pretty flat and I could get some very consistent numbers. That is as long as the winds are minimal. They can be pretty brutal out there on the flats of Utah and Idaho.

 

[schiker]

AC takes up to 25 HP to run. It also kicks on the engine fan more and that can take ~5 HP.

Its been a long while since I did the calculation - 20 yrs ago but thought an avg car sized compressor takes about 7ish hp to run???? But at 20 yrs ago my memory aint worth bettin on. That 25 hp seems awful high could it be cummulative loading: alternator draw to run duct circulation fan and radiator fan too????

Now 7 hp doesn't seem like much but its right off the crankshaft and jumps in and out pretty fast. I can feel it load the engine while driving. I sometimes pretend its passing gear to turn off the compressor.

Think how much gas a 7 hp lawnmower uses yeah you would probably see a dip in mpg. As far as one turbo vs another can't say. I can see the non wastegated ATT having a bit more slippage of turbine gasses at low backpressure that the GM might benefit from a bit of extra fuel burn to produce beneficial HP???? the ATT misses maybe?????? But thinks its something else. Buddy might be on to something. With the ATT might be lower IAT and less timing under just a marginal load vs the GMX ????

 

[WarWagon]

25 HP may be high, but, a Hybrid car needs 7KW (9 HP) as listed below for peak cooling. A bigger pickup or Suburban with more glass, airspace, and rear air combined with the old design high drag compressor should be in the ballpark.
http://www.nrel.gov/docs/fy00osti/28960.pdf

 

[Acesneights1]

Depends on which compressor and the cfm rating. The newer Sanden junk run IIRC 6 cfm(maybe 8 ?) and don't draw as much HP as the old 10 cfm Harrisons but damn, can't beat a HArrison for cooling. Specially the old Long ones. In the old days you had to turn the a/c to climb a hill...:rof:

 

[iamdave0887]

Depends on which compressor and the cfm rating. The newer Sanden junk run IIRC 6 cfm(maybe 8 ?) and don't draw as much HP as the old 10 cfm Harrisons but damn, can't beat a HArrison for cooling. Specially the old Long ones. In the old days you had to turn the a/c to climb a hill...:rof:

I"ll agree with ya there. The late 70s A6 i've got in my blazer looks to be all original and i'll get some pretty cold air from the vents. I'm sure that compressor is tired after 30+ years though. I have another one here somewheres if this one goes belly up on me.