Fuel economy of 7.4L (454) powered 3/4 ton suburban

[oil pan 4]

A 2012 2wd 3/4 ton chevy suburban with modern fuel injected 6.0L with cylinder deactivation and overdrive transmission with converter lockup is rated to get 10MPG city and 16MPG highway.

You would think after 26 year, the addition of fuel injection, an overdrive transmission with lockup converter, cylinder deactivation and a 1.4L smaller engine and some actual aerodynamic considerations the fuel economy would be better a lot better.

This is what I have. An obscenely over sized engine that dates back to the 1960s, carburetor technology from the 1970s, ignition system from the 80s, no aerodynamics to speak of and using a cheap parlor trick where I cut off the engine and roll for a little ways.
The most modern piece of technology is my AEM wide band O2 meter.
(that AEM wide band O2 is worth its weight in gold if you want to tune a carb)

Now I don't ever plan on taking this pig in its current 7.4L, 3-speed with 4.11 gear configuration on a road trip and finding out what its highway MPG is.
I would be surprised if it got 15MPG on the highway.

[kafer65]

My neighbor's v-10 Dodge duelly normally made 8 mpg. I drove it to Joplin and back over two days with a car and car carrier and got over 11. I think you've done a fabulous job of wrangling that engine into shape. I'd do the gear swap just out of curiosity. If you could keep your highway speed minimal I don't think it would be that bad all things considered. Wish I could get my slant 6 to improve with the 3.55 rearend. I absolutely hate the Holley 1920 that it has on it.

[oil pan 4]

It just depends on what I think I have time for.
I may rebuild the rear end before I swap in my twinturbo diesel and overdrive transmission.
I can tell you I really don't want to run my diesel on 4.11 gears, ever, at all.
With 4.11 gears, the engine would come up to speed faster than what the big lazy turbos could supply for air, unless there was a 12,000lb trailer behind it.

[kafer65]

Yeah, a 4.11 and a diesel would't be fun at all. If you heat wrapped to retain as much heat to the turbines and were able to keep it from kicking down a gear so you could give it a good load for the turbo to spool against could you get it to spool 6 pounds around 1500 rpm or lower? That would give it a nice push off. I drove a mercedes v6 turbodiesel Vinto? 9 passenger van and it was like trying to get a boat to plain out it was so laggy. Seemed like it took a full two to three seconds to get the boost going good.

[oil pan 4]

The good thing about the twin turbo setup is the is almost no lag. Its starting to give a few PSI at 1500rpm. The oversized turbochargers were selected mainly for highway and towing.
Once I devise a way to control the VGT on the holset it should come on the boost a lot faster. I don't have any complaints about lag.

Once the engine comes out and goes on the engine stand its going to spend a month or 2 there getting a lot of things remade. I was in a little bit of a rush to install the T76 and it started to get very cramped and confined. On a stand, with plenty of room to work and lots of time and a tig welder this time I can make beautiful things happen.

I obtained some slightly used 2 inch 316 stainless alloy steam pipe. I will remake the cross over, wrap and seal it this time.
The cross over is not wrapped or anything right now.
The HE351VE discharge to T76 inlet elbow is 100% stainless and not wrapped, new revision will have that wrapped and sealed too.
The banks manifold was coated by me and looks ok. The backwards mounted HMMWV header is not coated or wrapped, but next time it will be.
Since I am going to have my injector pump rebuilt and crank up the fuel. I can already see 20lb of boost as I go back down closer to sea level so I will have to add a waste gate, which will take the form of a fender exit screamer pipe.
When the waste gate opens at or near 20psi I want it to sound like the world is ending.

I sand blasted and ceramic coated the turbine housings, but after a few years its starting to flake off. Not sure if I want to resandblast and recoat or just be lazy and wrap/seal it.

The first 2 or 3 feet 3'' stainless steel down pipe will be wrapped and sealed.

[kafer65]

"When the waste gate opens at or near 20psi I want it to sound like the world is ending."
That cracked me up! That's going to be one sweet ride!

[oil pan 4]

Another 12.75MPG tank. This time I drove it a total of 87 miles.
So with some tuning, EOC and turning it off at stop lights has doubled fuel mileage.

I think I could break 13mpg for sure if I took the 2 spare tires out, removed the small fold down seat and pulled the radiator fan out and tossed it behind the driver seat.
The only thing stopping me from pulling the belt fan is the radiator leaks, if it would stay full of coolant I would try it, even with it getting into the high 80s here.

In the past the belt driven fan has shown its self to be good for +2MPG, at least on the highway.

I can run steadily gain speed and keep the air fuel ratio between 16:1 and 17:1. Cruise at 15:1 to 16:1. If I need to pick up speed a little more rapidly, can push the gas down a little more, open the secondaries a tiny bit and get some 14:1 air fuel mix into the engine. Then when I floor it the air fuel ratio goes to 12:1 to 14:1 depending on out side air temperature. So I have not given up anything there. Save fuel when I don't need 454 power and still have it when I need it.

I never meant to drive this this rig this much. But our VW is requiring that we pay the VW tax again, for the second time in 2 months.
The VW has cost so much in repairs, that if we never got it and I picked up this suburban instead last year and drove it at 6.5mpg I would still be saving money.
The VW tax can be very expensive.

[oil pan 4]

The 454 suburban is getting between 12 and 13 mpg with lean burn and some EOC.
Which is substantially better than a modern aero dynamic, fuel injected, smaller engine suburban with over drive transmission.

[oil pan 4]

Latest tank was about 50% (about 50 miles) highway driving and that bumped fuel mileage up to almost 16mpg.
I originally thought it would only do up to 15 MPG with pure highway driving.
I was able to stay in lean burn (16:1 A/F ratio) pretty much the whole time.

So if you calculate 50% in town, EOC driving that nets 12 to 13mpg and 50% highway driving means I am getting about 20mpg with pure highway driving.
All I did was lean tune the carb and advance the base timing to 12 degrees.
20MPG with a 7.4L, non lockup, non overdrive with 4.10 gears on the highway should not even be possible.
I would be willing to lean tune the cruise a little bit more. Since this is the first micro highway trip, holding a steady load and speed I really wasn't sure what to expect.
I don't consider leaning the carb and advancing the timing as even really trying, this may warrant further investigation.

Wait, didn't some one tell me that an edelbrock 650 carb and an edelbrock Air-gap intake manifold was just about the worse thing I could do for fuel economy?

[mwilliamshs]

I'm glad you're making progress and satisfied with the results. I've enjoyed this thread.

BUT, this attitude:

Originally Posted by oil pan 4

Wait, didn't some one tell me that an edelbrock 650 carb and an edelbrock Air-gap intake manifold was just about the worse thing I could do for fuel economy?

Is pointless. Just because you're happy with what you're getting doesn't mean whoever said that is necessarily wrong.