8.1 Allison 2500HD Wants Better MPG

[Rinkrat456]

Tomorrow I'm applying credits towards a friend's '02 Silverado 2500HD with the 8.1L and Allison. In the mean time, to familiarize myself with the OS I've been looking at repository tunes. A few questions have come up from the last few public tunes I've looked at...

1. The owner drives 100-110 miles round trip to work every day with this truck and he's tired of 7-8mpg highway. See question 2 for part of the reason why. I'll be dialing in VE and MAF, PE, getting rid of some nasty factory spark table dips and disabling all TQM that applies to reverse, 1st and 2nd gear (he's tired of not being able to back up over a speed bump). I was curious if anyone has an 8.1L tune they're willing to share their DFCO settings and other MPG tips?

2. The tires on this truck are 2" taller than stock...I'd like to calibrate the speedo for the final drive change, but I'm noticing that some Allison tunes don't allow for the gear/tire calculator. I'll have to wait until tomorrow to see what this truck actually allows me to do but is this normal for an Allison not to be able to change the tire/gear?

[n0dih]

Honestly having a L29 454 (4L80E) and having tuned a L18 496 (4L85E), if he is getting 7-8 highway the tune isn't the problem. I get around 13.5-14 highway and 10ish city. Best, 15.1 highway. A friend who had a 8100 Avalanche was getting around 11-12 city and not sure on the highway, but overall he always bested my 454 in mpg. 100k miles difference might help that.... We both had 265 75 16's and 3.73's.

If his tires of 2" taller than stock, is he calculating that into his mpg numbers?

[Rinkrat456]

If his tires of 2" taller than stock, is he calculating that into his mpg numbers?

You bring up a good point. I don't think he's aware of how the 285/75R16's affect the odometer if they're not calibrated for. Either way it's a 7-8% difference which only equates to 8-9mpg.

I know 8.1L's aren't all that efficient but when he told me his gas mileage woes I made a goal for myself to see if I can get him into the 10's or better. From your 8100 experience, do you find the tunes to be pretty far off from the factory or have really timid DFCO properties? Or perhaps I could go the other route and dial in the Allison better for highway? I don't know, I'll have to see when I post up the tune tomorrow afternoon.

[n0dih]

They are usually low on timing, but you have to be VERY careful, if he hooks up a 15k trailer to it and the timing is up too much for that sorta load, pistons will not be so happy....

The bigger issue is the cam installed position. GM (and Ford and Dodge and ....) install the cam usually around 5-10 degrees retarded. Then add in timing chain wear/slop and you have a cam that is so far off the engine never will get good mpg, you'll be 1000 rpm away from torque peak at cruise.

Have him shoot for the intake centerline on the cam to be around 112. Look how small the factory cam is and then look at the torque peak. They are pretty whacked. Best we can tell they put the cam in so late to flatten the powerband to reduce emissions some.

I always look back at the 76 Olds 98 Regency. 2.41 to 2.73 gears, THM400, no TCC, carb, crap "J" heads, aero about as bad as most pickups, and yet it was EPA rated at 12 city and 18 highway. So why can't a modern port EFI, OD, dual exhaust, high flow cat, fast burn headed 454 do better? Gotta be something they did to "hurt" it....

You bring up a good point. I don't think he's aware of how the 285/75R16's affect the odometer if they're not calibrated for. Either way it's a 7-8% difference which only equates to 8-9mpg.

I know 8.1L's aren't all that efficient but when he told me his gas mileage woes I made a goal for myself to see if I can get him into the 10's or better. From your 8100 experience, do you find the tunes to be pretty far off from the factory or have really timid DFCO properties? Or perhaps I could go the other route and dial in the Allison better for highway? I don't know, I'll have to see when I post up the tune tomorrow afternoon.

[Rinkrat456]

Here's the tune. Why can't I calibrate for the tire/gear size?

[Rinkrat456]

Are you guys seeing what I'm seeing in the spark tables of this tune? Do big blocks really idle with 40-44 degrees of advance!?!?!?! Holy cow. I'm tuning this truck starting Friday and probably going thru the weekend if he leaves the vehicle with me. For now it's just research and prep work.

[n0dih]

But at what airflow?

IIRC my BBC 454 idles around 6-7 gm/sec on the MAF at 600 rpm. Knowing the idle condition I would be very surprised if it actually idled over 26 degrees.

The 454 is pretty different, it uses a MAP/RPM table for timing, open throttle and another for closed throttle. Mine only idles around 25 degrees.

[Rinkrat456]

Hopefully I'll be able to tell @ what airflow on Friday. I'm actually going to tune this truck twice, once without a trailer and again with a single car trailer hauling a 5000lb off-road buggy. I want to see how the tune reacts under load and find a safe happy medium.

The owner wants to continue using "Up to 10% ethanol" 87 octane fuel in this truck and Minnesota's ethanol levels vary from 5-8% most of the time. I'm leaning towards changing the stoich value to 14.271777....which is the value Flex fuel vehicles have in the 6.25% ethanol column. Hopefully that will rescale everything accordingly, but I'm going to shoot for a commanded PE AFR of 12.54 with E10 in the upper RPM's.

Since this is more of a towing/daily driven truck the emphasis will be on fuel savings and low end reliability, so I kind of want to put some restrictions on power enrichment enable to come on only when needed. I'd like PE to come on based off engine load instead of TPS or RPM related. I'd rather rely on the Allison to downshift rather than more fuel to be added. For example, towing a single car trailer w/tube buggy on the highway going 70mph, the Allison's 5th gear and 3.73's should land his RPM's in the 1900-2000rpm range. I'll have to log to see where the common MAP values are at while cruising, but for argument's sake say they're at 50kpa and the driver encounters a slight incline raising MAP to 60kpa. If I can set the PE enable MAP to say...65-70kpa it should give enough headroom to not enter PE.

If more throttle is applied to maintain 70mph (steeper incline or passing a grandma going 50 in the slow lane) I'd rather the Allison downshift from 5th to 4th to bring RPM's up to 2700-2800 instead of entering PE mode and increasing engine power as well as lugging a heavy trailer in OD. Does that make sense or am I off in my thinking here? I'll have to figure out what settings allow PE to come on only when he's hot rodding the truck and really wanting a smoke show.

And how in the heck do I change the new tire size in the Allison?

[n0dih]

On the L18 496 I did, for some slight improved mpg I put in the L29 454 shift tables, it was welcome on the L18, didn't rev as high when not towing, the owner liked it better like that.

[Rinkrat456]

The 7.4L came with an Allison?

[LEDbetter]

Rinkrat, how did the tuning go on the 8.1? I will be tuning one soon, but it'll be the first one I've done.

[n0dih]

L18's had the Allison A1000 as an option in the GMT800's in some models (3500 only I think) and the L21 454 came only with the Allison A500 (4 speed). A very undesired transmission, 30k lb gross weight capability, but only a max input torque of around 400 lb-ft's before parts breakage occurs.

The 7.4L came with an Allison?

[fatguy]

I'm curious also. Just ordered my wideband and am trying to figure out what needs to be done. I want to set mine for a fuel miser tune, then when the tow/haul is pressed, a towing tune. Do you know if this is possible?

[kingtuner]

HLOOO
PLEASE I NEED HELP FOR SIEERA OR SELVERADO 2500HD _3500HD 2012
I NEED SPEED CUT AND ADGUST TUNER
THE VCM HP TUNER NO READE FAIL THES TRUK
I NEED READE AM NO 23 TRUCK FOR THES PLEASE HELP
[email protected]

[bigred6974]

I'm curious also. Just ordered my wideband and am trying to figure out what needs to be done. I want to set mine for a fuel miser tune, then when the tow/haul is pressed, a towing tune. Do you know if this is possible?

I'd like to know if this is possible too. I've got an 03 8.1l 4x4 crew with 285s and would like a fuel econ tune / tow tune with the tow haul button being the switch. Don't know if this is possible, I'm new to this and haven't messed with tuning.