Mileage: 1bbl v 4bbl
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Mileage: 1bbl v 4bbl
I'm thinking of doing a 4bbl conversion on my '78 F150 custom, but I've heard differing opinions on how mileage will be affected.
Theory A: The 1bbl will have superior mileage. Less airflow would mean less fuel being burned in the engine.
Theory B: With a light foot, the 4bbl will get better mileage, as the primaries meter less fuel than the 1bbl. (May or may not be true) Once on the gas hard and the secondaries open, the 4bbl will burn much more fuel than the 1bbl.
Any opinions out there? Both theories make some sort of sense, but I'd like to hear what results people have actually gotten.
Theory A: The 1bbl will have superior mileage. Less airflow would mean less fuel being burned in the engine.
Theory B: With a light foot, the 4bbl will get better mileage, as the primaries meter less fuel than the 1bbl. (May or may not be true) Once on the gas hard and the secondaries open, the 4bbl will burn much more fuel than the 1bbl.
Any opinions out there? Both theories make some sort of sense, but I'd like to hear what results people have actually gotten.
Elder User
Joined: Jul 2004
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From: Northern Virginia, USA
Both theories do make sense, and are correct. The 1BBL YFA just doesn't flow, you're absolutely right, so you can't "pour it on", but it's mediocre at everything, so you won't get a really good sweet-spot. I got 15.5 driving fast on the highway with the YFA, 12.5 doing abusive highway hauling, and probably 15 again city, if I babied it.
A modern 4BBL will be great at metering fuel, and with small primaries, you'll get great cruising mileage. If I putter my Q-JET around and do a lazy 60 on the highway, my mileage seems to be pretty dang good, due to the nickel-sized primaries on the QJET and small primary plenum & runners on the Offy DP manifold. Probably 18 or higher. Don't drive it much anymore, and don't have a GPS in it to calculate trip distances (ODO inaccurate). On the other hand, if I lay into it, the mileage is pathetic, worse than with the YFA, but it sure moves quick.
A surprisingly good alternative, I've heard, is the Motorcraft 21xx carb, usually a 2150 with the 1.08 venturis. Small enough to give good mixture control, and all the CFM you'd need for an exhaust upgrade, and maybe a mild cam. Inexpensive, easy to tune, and they "just run right" from day 1.
A modern 4BBL will be great at metering fuel, and with small primaries, you'll get great cruising mileage. If I putter my Q-JET around and do a lazy 60 on the highway, my mileage seems to be pretty dang good, due to the nickel-sized primaries on the QJET and small primary plenum & runners on the Offy DP manifold. Probably 18 or higher. Don't drive it much anymore, and don't have a GPS in it to calculate trip distances (ODO inaccurate). On the other hand, if I lay into it, the mileage is pathetic, worse than with the YFA, but it sure moves quick.
A surprisingly good alternative, I've heard, is the Motorcraft 21xx carb, usually a 2150 with the 1.08 venturis. Small enough to give good mixture control, and all the CFM you'd need for an exhaust upgrade, and maybe a mild cam. Inexpensive, easy to tune, and they "just run right" from day 1.
Postmaster
Joined: May 2002
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From: Broken Arrow, OK
Anytime you only have one barrel feeding an engine you have a lot of compromises. You can overcome some of them with more barrels. On a 4 barrel you can jet the primaries for mileage and the secondaries for power and get a little bit of the best of both worlds. I used to have a 79 Courier that had a two barrel that the second barrel was vacuum actuated. It was a bitch to work on, but when it was right it was great.
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From: Fraziers Bottom, WV
After I installed the 4bbl and got over "keeping my foot in the carb" and checked the mileage I found that I went from 16 to 18 highway. But I also used EFI manifolds and high flow cat and exhaust. That may have some bearing on the change.
If I pulled the horse trailer I got 11 with each.
Rod
If I pulled the horse trailer I got 11 with each.
Rod
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Joined: Dec 2006
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From: Spokane, WA
What they said.
Small 4v Holley 4160 Models 390-465 cfm. Obsolete 4360 Holley spread bore 450 cfm (Qjet replacement) still available from some rebuilders. AFB 9400 series, 400 cfm...a bit rare but can still be found. Mounted on an Offy DP can give snappy low to mid speed response and mpg if avoiding the "teenager" throttle events.
Small 4v Holley 4160 Models 390-465 cfm. Obsolete 4360 Holley spread bore 450 cfm (Qjet replacement) still available from some rebuilders. AFB 9400 series, 400 cfm...a bit rare but can still be found. Mounted on an Offy DP can give snappy low to mid speed response and mpg if avoiding the "teenager" throttle events.
Logistics Pro
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A surprisingly good alternative, I've heard, is the Motorcraft 21xx carb, usually a 2150 with the 1.08 venturis. Small enough to give good mixture control, and all the CFM you'd need for an exhaust upgrade, and maybe a mild cam. Inexpensive, easy to tune, and they "just run right" from day 1.
Is there a noticeable difference in mileage? NO, but the driveability of the motorcraft is incomparable. The only problem I have is that it sometimes sticks a little above idle. This is only annoying during stop and go traffic.
PS all mileage was calculated using a Garmin Rino GPS, so there is minimal speedo error.
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The 2150 is a 2bbl, correct? I know I pulled an old 2bbl motocraft off of the 1969 302 in my mustang. It's a bit old, but there's nothing wrong with it. Perhaps that'd be a decent candidate for a swap?
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Elder User
Joined: Feb 2004
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From: Indiana
If you want to use a 2150 carb you would need to use a Offy C series or Clifford intake that has a 2 bbl adapter plate. The Offy dual port is a split runner intake and the 2 barrel carb would not mount right.
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Yeah, 2bbl, and good point Ranger. As far as swap material, they weren't going for much on ebay when I looked a while ago. Mine was free because it was on my parts truck, so I don't know what they are worth today.
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So I'm good to go for a 2bbl swap, so long as I use an adaptor plate with the offy or clifford intakes? Since the engine will likely not get modded all that much, (Maybe a low-end cam and exhaust) I'm thinking a 4bbl might be overkill.
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From: Broken Arrow, OK
Another thing about the 2 barrels is that you can helicoil the jet threads to use Holley jets and that will give you a ton more tunability. You an also drill and tap the PVR to accept Holley air bleeds so you can tune the power enrichment separately from the main metering system. That can give you max power without paying a mileage penalty.
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Didn't know about that, but I do know that my 2 to 4bbl adapter had about a 1/8" space on the back where there was a SERIOUS vacuum leak. I ended up adapting the adapter with a 1/8" thick steel plate to compensate for this slight bit of engineering retardedness on (I think) Mr.Gasket's part.
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