I recently picked up a 2005 Sierra with a forged 370 and a Livernois tubular turbo kit.

A few specs on the truck/turbo kit:

PTE 71HPS HP billet wheel non-ball bearing
Tial 38mm Waste Gate
50mm Blow Off Valve
60# Mototrons
Stock intake manifold
4" intake with K&N cone filter
Full 3" exhaust from downpipe back. Magnaflow 3" in/out muffler
Speed Inc TU1 Cam 225/225 .581" .581" 113LSA
3200 rpm stall converter
Forged 370, stock heads with a valve job.
No boost controller, only wastegate controlling boost.

From what I understand, truck was tuned at around 14psi at Speed Inc. It currently is running about 8-9 psi on the same tune (supposed to be 8psi spring but not 100% positive on that). I believe it has a HPTuners 2 bar SD tune on it. There is no MAF. Boost doesn't seem to creep, but I haven't taken it much over 80mph at WOT. Boost gauge does show the occasional 10psi high, but I believe that is happening during the shift. The truck has a Autometer wideband on it. It's 14.6~14.7 at cruise. Light throttle that stays in vacuum is 14.7. Once I hit about 1" of vacuum, AFR goes to 10.2 and lower. WOT is typically 10.2~10.5.

I've put about 2,500 miles on the truck. I'm typically averaging between 8 and 9 mpg with city/highway. The BEST I have achieved is 9.7mpg. From what I've been reading, I shouldn't have that low of MPG with this setup. I've made sure to baby the truck, drive it nice, stay out of boost and I still can't break 10mpg. The only thing that I can think of is low vacuum causing poor MPG but I'm having a hard time figuring out what low vacuum would be considered. If I recall correctly (haven't driven the truck in a few days), there is about 10" of vacuum at cruise. Not sure if the wastegate may be to small or if there is another underlying problem with the turbo/tune.

Any suggestions would be greatly appreciated. I can post HPTuner logs and the tune if it would help. The truck is also equipped with an EGT gauge if that would help diagnose anything.

**EDIT**: Adding a link to the tune: Turbo Sierra HPTuners Tune

 

Axle ratio & tire size? Does the transmission have OD?

 

4.10 gears, 305/40/22 tires, 4L80E

 

Are you goin off what the cluster says or hand calculating the mileage? That setup I would think would get in the mid teens.. Def shouldn't be single digits I wouldn't think

 

Hand calculating. Cluster reports about 12mpg. I can barely pull 200 miles a tank if I really stretch it (filling at about 1/4 tank because twin intank pumps).

I would expect 14-16mpg with the setup at the current boost levels.

 

I'm assuming that you do alot of stop & go driving. I'm also gonna guess that your converter is on the loose side of things.

With the heavy wheel/tire package, 4.10's, and loose converter, all on top of bigger cubes, it sounds about right for a SD tune. There's no closed loop.

When I tune SD, I target 14.5 to 15.1 for the "low load" or "cruise" sections of the VE table. That should help a little. But a tighter(more efficient) converter would also help alot. What brand 'verter is in there?

 

The converter is a Precision Industries 9.5 5 disc.

 

With a 2 bar tune it should not be at 10.xx AFR at 1" of vacuum so that is not helping. It should be at 13:1 or close to it at that point.

Also you can get some fuel economy into the boost trucks by utilizing the spark/pe adder and not zeroing it out. Keep it lean while accelerating out of boost while being safe with the timing at the same time. There is a good reason GM utilizes this.

Even at 1-4 psi it should still be in the low 12s not 10s for AFR, sounds like a dyno only tune.

 

Why no CL? You can run CL in SD...I do...

When I tune for boost I like to only slightly richen it at low boost; 12.5 for 2-3lbs, then 11 3+

 

Sounds like a tune that was set up too rich to me as well. At no vacuum or boost, I typically target around 12.5, like a NA motor would run. Then let PE take over as the MAP goes over 100.