View Poll Results: What's your highway mileage and fuel delivery system?
1 to 3 mpg 0 0%
4 to 6 mpg 0 0%
7 to 9 mpg 7 70.00%
10 to 12 mpg 1 10.00%
13+ 2 20.00%
Fuel Injection 3 30.00%
Carburetion 5 50.00%
Multiple Choice Poll. Voters: 10. You may not vote on this poll

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Old 04-23-2018, 12:34 AM   #1
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1966 26' Overlander
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Gas Mileage of the carbureted vs. fuel injected 454

Hi there,

I have a cousin who runs a small car dealership and has bought a few motorhomes over the years. He keeps them for awhile, fixes a few things, and then eventually sells them. He's not a motorhome dealer but he's driven and used perhaps a few more motorhomes than average. He was telling me that in his experience only that the fuel injected 454s (especially the Vortec 454) usually got about 12 MPG while the carbureted 454s got around 6 MPG on the highway.

Has this been everyone else's experience?
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Old 04-23-2018, 02:37 AM   #2
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I doubt anyone has doubled their MPG in a change from carb to injection, unless the carb was in really bad shape.

I believe my 77 20 foot Argosy with the Qjet and headers "Could get" 10mpg.

My 84 27 foot Airstream came with a Qjet and it usually got about 6 to 8 mpg. I installed a TBI system and maybe I get 1 to 2 mpg better, but it is hard to tell. I dont drive enough miles, on the same route, carrying the same weight to get good numbers.


With the TBI, 12MPG in my rig would be unloaded, all down hill and a good tail wind


No matter what your fuel delivery method, weight plays a large factor in the final number.

If you are going to measure MPG, you cant use the gas gauge because it's accuracy is totally useless for that. You can fill to the top on each fuel stop, but that is a lot of weight to carry. I installed a dip stick in my tank and even parked on level grade, it is only accurate to 2 gallons out of 60, but better than nothing.


My next cheap method for better mileage is to try synthetic oil.
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Old 04-25-2018, 05:43 PM   #3
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Quote:
Originally Posted by WayneG View Post
I doubt anyone has doubled their MPG in a change from carb to injection, unless the carb was in really bad shape.
I'm sure that's right. I don't think FI alone would make that change. Technically speaking the doubling mileage (in his experience) was from the 454 Vortec, which according to him is supposed to have different head designs in addition to fuel injection.

Quote:
Originally Posted by WayneG View Post
No matter what your fuel delivery method, weight plays a large factor in the final number.

If you are going to measure MPG, you cant use the gas gauge because it's accuracy is totally useless for that. You can fill to the top on each fuel stop, but that is a lot of weight to carry. I installed a dip stick in my tank and even parked on level grade, it is only accurate to 2 gallons out of 60, but better than nothing.
All good points. Thanks.
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Old 04-25-2018, 08:23 PM   #4
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1987 34.5' Airstream 345
Menomonee Falls , Wisconsin
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Hey,
I had 2 GMCs. One had a 455, the other had a 403. The 455 got 7 to 8 mpg.The 403 got around 10 mpg. My brother bought the 78 GMC from me. And claimed he got 13 one time. Point is, these old 454 s were basically from the 60s like 1966 I think. If you put aluminum heads on them, did some engine work internally, put on fuel injection, I suppose you might pull 10 to 12 out of these things. We re pulling a lot of weight and wind drag. I'm putting on a FiTech TBI unit. I'll be impressed if I can get 10 mpg out of it. One trip out west will almost pay for itself in gas savings. We ll see. In my experience, 62 mph is the break point for top mileage. My GMCs got there best mileage at that speed. The airstream is a little taller so that may be a factor. Angle of windsheilds on both units seem to be pretty close. My GMCs weighted 11 k loaded
Alignment played a huge factor in the GMCs. That tag axle could also present a milage drainer. I'm going to put my AS on my Aligner this spring to see if the tag is scrubbing. I'm going to bet it is. My years with Hunter engineering, showed me many senerios in alignments tag axles were notoriously out of any kind of normal alignment spec. Sorry, got off on a tangent. Anyway... We ll see if the FTech tbi system is as good as they claim. DJ
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Old 04-26-2018, 06:29 AM   #5
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1986 34.5' Airstream 345
Londonderry , New Hampshire
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I bought a NOS Zemco ZT4 driving computer that I am going to install in my 345.
It measures the MPG continuously as you drive. It should be good at combating
my heavy foot.

I have not completed my restoration project so I am not on the road yet. Has
anyone else installed one of these?
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Old 04-28-2018, 10:15 AM   #6
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Hey.
We got that vacuum gauge in the dash. If you keep the vacuum as high as you can, while driving, that would be your optimum gas milage. The FiTech fuel injection I'm installing has an O2 sensor on the collector of the header,. That is where the precise metering of fuel comes from. I'd be curious to see if the later AS with TBI injection, got better milage than our QUadrjets carbs. 454s were never great on milage, but today we got so many things we can do to enhance the engines capabilities. Headers, mufflers, better heads, fuel injection, radial tires, better alignment capabilities. Overdrive transmissions.I would think we should be able to pull 12-14 out of these units. That's my goal. From my racing friends, they say the heads are where you get the most performance from. I may go that route in the future. But for now, 62 mph should be the sweet spot . I think 22-2400 rpm is the best rpm to run at. I hope that doesn't put me at 55 mph. I may getting old....er but 55 is just to slow for me. DJ
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Old 06-07-2018, 04:48 AM   #7
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Quote:
Originally Posted by Air345Fly View Post
I bought a NOS Zemco ZT4 driving computer that I am going to install in my 345.
It measures the MPG continuously as you drive. It should be good at combating
my heavy foot.

I have not completed my restoration project so I am not on the road yet. Has
anyone else installed one of these?
Check with KeyAir, know he's quite knowledgeable about Zemco. I've purchased two units actually, one as a backup Amazing that you can purchase these units essentially brand new, 30 years later.

Of course, I haven't installed mine yet to see if it actually works. Oh well, it's another project on the ever growing list of things to do on the Airstream.
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Old 06-09-2018, 04:36 AM   #8
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Gas Mileage of the carbureted vs. fuel injected 454

Another consideration is that these 454s came from and era when performance and mileage were sacrificed for emissions. Lower compression, cam profiles, ignition timing, and richer mixtures to reduce combustion temperatures reduced NOx which was the environmental bad boy at the time. The unburnt hydrocarbons were burnt off with air injection and catalytic converters. If you have an engine with air injection, its there to for that reason. Gains from throttle body injection systems are minimal. Significant gains are usually the result of replacing a poor functioning carburetor with a TBI system.

The newer engines have the potential of significant improvements in performance and mileage. A lot of engineering goes into the complete system. Just swapping engines can be disappointing if the existing transmission shift patterns, final gearing, and the load on the system (weight) are not considered.
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Old 06-09-2018, 06:23 AM   #9
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On the air injection system, if you are installing a TBI and presumably O2 sensor, you need to dump the air injectors like I did:


oxygen in the exhaust before the O2 sensor will screw up the mixture calculated by the ecm.
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Old 06-09-2018, 07:53 AM   #10
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Since nowhere in this thread does anyone specifically talk about fuel type, so I voted 13+ for my fuel injected turbo diesel.........and thats towing a toad......up hill.

Cheers
Sidekick Tony

PS Kota will be a little less than me, as his toad is double the weight of mine.
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Old 06-09-2018, 08:57 AM   #11
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Originally Posted by Isuzusweet View Post
Since nowhere in this thread does anyone specifically talk about fuel type, so I voted 13+ for my fuel injected turbo diesel.........and thats towing a toad......up hill.

Cheers
Sidekick Tony

PS Kota will be a little less than me, as his toad is double the weight of mine.


We get 12-13 toadless. 10-11 with the jeep behind doing the speed limit on the expressways.
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Old 06-10-2018, 05:45 AM   #12
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The real advantage of EFI is:

1) Cold Engine Start

2) Altitude Compensation

Thus: Engine Life, and Diveability.

Comprehensive mechanical & vacuum timing advance is necessary to get the most from them. A custom curve. Same for intake temps: air temp nearest engine inlet needs to be maintained at 115F for optimal combustion. (Only need colder air in WOT). Optimization of steady state fuel burn.

Fuel economy is about use. Planned stops done in advance, for instance. 60-mph is the aerodynamic wall. Above that, down it rapidly goes.

Number of acceleration & deceleration events on a trip leg (should be tallied). Same for steering course corrections, as steering will be the biggest improvement after tire & tread design, then fuel & spark.

IOW, the driver-related inputs still mean more.

Someone serious about FE would completely rebuild the front end to eliminate play (slop) in the system. Brake drag and alignment are next.

Records are mandatory. Every gallon & every mile. Tank by tank is essentially meaningless. Its the long term average that matters.

An engine hour meter is a good aid. Knowing the average speed of a trip is distance divided by engine run time. Its not at all the same as But I drove at 56-mph! as that exclamation doesnt account for idle time, time in side street traffic, etc.

Below 27-mph is bad. 30-35-mph is desirable.

Maximizing steady state is the key.

Hands off the wheel and it tracks straight in a long four-count. On best possible tires. After that Id concern myself with fuel/spark upgrades.

.
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Old 06-10-2018, 06:55 AM   #13
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Quote:
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Hands off the wheel and it tracks straight in a long four-count. On best possible tires. After that Id concern myself with fuel/spark upgrades.
.

Not possible with my boat! Straight is the last place it goes. Not due to alignment, but it is due to road tilt, any breeze and yes, the steering linkage.


Hands off my wheel and I will be a aluminum sculpture on the side of the road in 15 seconds or less.
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Old 06-10-2018, 08:34 AM   #14
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Quote:
Originally Posted by Isuzusweet View Post
Since nowhere in this thread does anyone specifically talk about fuel type, so I voted 13+ for my fuel injected turbo diesel.........and thats towing a toad......up hill.

Cheers
Sidekick Tony

PS Kota will be a little less than me, as his toad is double the weight of mine.
Same for me, but Bella weighing 7000lb, having a small block rather than big block V8, and a UK gallon being 4.5lts all help that!
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