Home » Public Forums » GMCnet » TBI vs Carb MPG Question (It was asked if TBI will pay in saved gasoline)
| TBI vs Carb MPG Question [message #104408] |
Wed, 27 October 2010 14:06  |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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There was another thread in which the question about saved gas $$ was going to make TBI pay for itself. Sorry the findings here are so long.
First, I am sure there is not answer. Second, GMCs (or any motorhome) are not the best project to take on if you don't want to spend any money.
That being said, I now quote Gene Fisher, "GMCs will get 8 to 10 mpg depending on how you drive."
I love my TBI. Runs good, and behaves at 8,000 feet. However, there are some realistic factors that must be considered. The coaches weigh 12,000 pounds. The coaches, although streamlined, push a lot of air out of the way. The faster you go the more air you push and it takes energy to push air.
We recently did a 4+ week trip to Du Quoin, with lots of great stops in between. On the way to Du Quoin, we stopped at the Van Winkles where Randy and I did many many tests, looking for the best MPG. We tested two coaches, one with with 3.07 gears and a 455 and one with 3.70 gears and a 403. We both had TBI, with the more modern EBL computer that allows us to quickly control spark and fuel mixture. Both coaches have one air conditioner and a storage pod. Most tests were related to spark advance. Some fuel mixtures during Lean Cruise were tested, and as we reported at Du Quoin, 16.4 to 1 Fuel to Air was about the best. We find both coaches run in lean cruise a great deal of the time. Spark was another kettle of fish.
I am reporting on an '74 Eleganza SE, with a 455 and 3.07 gears, driving about 62 to 65 most of the time. I do not have switch pitch. The engine and trans are what I call Ho-Hum. Relatively fresh according to the docs when I got the coach, but the suppliers were not name brand.
On the way to Randy's (CA to MO, 1700 miles) I used my spark tables, as best I could tweak them. We had one day of good tail wind and two and a half of steady head winds. MPG was 8.95. This was at the pump gas divided into odometer miles. Some legs were of course great, but with the fill problems of GMCs, I find it best to report an average.
At Randy's I adopted a slightly more aggressive spark and recorded fuel usage from there to Rimrock, AZ. From Rimrock to home, we towed a trailer, so I am not including those miles. From Du Quoin, we had some wind most of the time. All across Kansas, it was buffeting the nose of the coach. In New Mexico, on the return trip, we spent a day and a half using the coach like a car, puttering around some old haunts at 5 to 8 thousand feet. The rest of the driving was "on the road". In this leg there is an elevation increase of a about 2K with _lots_ of ups and downs. MPG was 8.77.
Part of the Towing (U-Haul 9 foot trailer, full) portion of the trip, MPG was 9.5 in two tanks. That mileage drops in elevation of almost 2K feet, with some ups and downs in the middle.
Would I have done better with a Quadrajet? I have no idea. Was I getting worse mileage before the TBI? I switched as soon as possible upon purchase of this coach. Is a TBI going to get you 14 MPG at 63.3 MPH? I doubt it. Remember, a gas engine is not the most efficient and you have to push air and drag weight.
TBI is great. More modern. I like the EBL instant MPG (except in a strong headwind). But, Gene said, "You are going to get 8 to 10 mpg, depending on how you drive." If you are getting way worse, without towing a barn, something is amiss with the old Q-Jet, TBI or your spark system.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
[Updated on: Wed, 27 October 2010 14:07] Report message to a moderator |
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| Re: TBI vs Carb MPG Question [message #104474 is a reply to message #104472] |
Thu, 28 October 2010 09:05  |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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| Jim Galbavy wrote on Thu, 28 October 2010 09:33 |
George,
Like Gene says "there is no difference in MPG whether you are driving with FI or a Carb. 8 - 10 mpg is just that. What matters is your driving style habits. What I do like about a carb is just start up and drive. I don't have to keep tweaking the system. When I first picked up ANNIE six years ago, I was getting 11 to 12 mpg and was afraid I was going to burn a piston so I riched the mix and am currently getting about 10 to 10 1/2 depending on where I'm driving. She seems to like I-95 from Virginia to Florida.
jim galbavy
'73 X-CL ANNIE
currently at Misty Mountain CG, Crozet, Va
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Jim: You might try 75 sometime. Come see us......
C. Boyd
76 Crestmont
East Tennessee
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| Re: TBI vs Carb MPG Question [message #104478 is a reply to message #104472] |
Thu, 28 October 2010 09:36 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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Senior Member |
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| Jim Galbavy wrote on Thu, 28 October 2010 06:33 |
George,
Like Gene says "there is no difference in MPG whether you are driving with FI or a Carb. 8 - 10 mpg is just that. What matters is your driving style habits. What I do like about a carb is just start up and drive. I don't have to keep tweaking the system. When I first picked up ANNIE six years ago, I was getting 11 to 12 mpg and was afraid I was going to burn a piston so I riched the mix and am currently getting about 10 to 10 1/2 depending on where I'm driving. She seems to like I-95 from Virginia to Florida.
jim galbavy
'73 X-CL ANNIE
currently at Misty Mountain CG, Crozet, Va
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Sounds great. I am not selling TBI. I don't have to tweak TBI. I tweak because I can. I am tweaking all parts of our GMC. Right now I have about four projects. This is one of the most useful toys I have ever had. We can't wait to get in the rig and go. Every time I can "improve" what Detroit did years ago, even a tiny bit, it is fun. Nothing but fun.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: TBI vs Carb MPG Question [message #104544 is a reply to message #104519] |
Thu, 28 October 2010 22:20 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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Senior Member |
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| Jim Galbavy wrote on Thu, 28 October 2010 17:41 |
George,
Not a problem. These old motorhomes serve many wants and needs.
Some like to work/play with them. Me? I like to drive and live in it (like last nite and tonite). The Tidewater Crabs are holding a rally at Misty Mountain CG this weekend and I chose to show up 2 days early and enjoy the mountains. During the week I have them all to myself.
Chuck,
It could happen. I'm in the process of slowly moving to Florida
over the winter. ANNIE & I know I-75 quite well and love the drive. I'll have to take you and Kate up on it.
jim galbavy
'73 X-CL ANNIE
Chesterfield, Va / Lake Mary, Fl
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Jim: you could stop by after your rally and hang out till we go to Pensacola for Blue Angles rally Nov.11-14. We could run down together.
C. Boyd
76 Crestmont
East Tennessee
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| Re: TBI vs Carb MPG Question [message #104869 is a reply to message #104544] |
Tue, 02 November 2010 15:20 |
Jim Galbavy
Messages: 1443
Registered: August 2007
Karma: 7
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Senior Member |
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Chuck,
I will be leaving on Thursday for Orlando and have to be in West Palm Beach on Saturday. I won't be able to make the Dixielanders' rally but will be moving ANNIE to JimB's lot for the winter on 19 NOV. My daughter has never taken a trip in ANNIE so she asked if she could do the trip with me. ANNIE ran just fine on last week's trip. .... here is hoping it stays that way. Looks like I won't hook up with you until Tampa in Jan.
jim
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