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
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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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