I just clipped the following from the same page at Allpar. Is everybody this confused?

"You can probably increase performance and gas mileage by switching to a good dual-barrel carb. "

"The 2 bbl also usually gets poorer mileage in my experience."

What I do depends on who's right.

this is one of those things that goes both ways. a good, properly sized, and well tuned two barrel will get better mileage than a one barrel. it will because the two-barrel can meter the air/fuel mixture better. however, it goes the other way when you have a 2-barrel that is too big but works well at high rpm, makes excellent power, but gets terrible mileage.

the trick is to get a carb that is in the middle of both worlds. that is why people get a progressive two barrel that under cruise and light acceleration, there is only one barrrel open. this limits the amount of air going in to the engine which in turn limits the amount of fuel.

the same theory applies to four barrels with mechanical or vacuum secondaries. that's about as far as my knowledge goes.

zedpapa

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1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!

I got slightly better mileage with a Super Six than a Holley 1920, although I'm not sure if the difference was actually meanngful. It didn't hurt mileage, though. But something like a 500 cfm Holley, I could see that hurting mileage.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Thanks. That makes a lot of sense and is what I figured to be the case. And even if sized right, etc., a poorly set up 2 bbl would get worse mileage than a well set up single.

Presumably if you were getting rid of an old carb (some parts fell off mine) and you had a 2 bbl manifold available the BBD would be a candidate.

BTW, I thought the suggestion that the aluminum head be a tad thick to allow for either low or high compression made a lot of sense.

No interest in 4 bbl here. In fact I'm not sure I could make myself keep my "foot out of the carburetor" with two barrels on tap. Thanks.

BTW, anyone, I've seen intake manifolds both with the two holes perpendicular to the engine and parallel to it. Would I be way off if I "voted" in favor of the parallel setup? Probably another mixed bag.

Ross,

Having the carburetor's throttle shaft parallel with the crankshaft helps even out wet flow fuel distribution. With the carb mounted perpendicular to the crank you get wet flow off the throttle plates towards the rear of the engine. With the parallel throttle shaft the wet flow is more evenly distributed end-to-end. When I ran a 4bbl on a Clifford intake I mounted the carb with the primaries toward the left for this very reason. Pontiac did the same on their 4bbl OHC sixes. My end cylinders ran leaner (or was it richer? It's been a while.) than the center cylinders. Most folks with the primaries towards the front have lean front cylinders. I think a Hyper Pak intake would be less sensitive to carb orientation than the Clifford as the direction of flow within the manifold is more similar between the cylinders. The Clifford intake's end cylinders run fore and aft before turning toward the head and the center cylinders have a straighter shot. This difference in runners is much less pronounced on the Hyper Pak intake.

If you can get fuel distribution equal you can run leaner mixtures and get better economy before hitting lean missfire. My Clifford 4bbl combo would get a little bit of lean misfire on cold nights so I had to richen it up a bit. That's one big penalty of an unheated intake. Lean is ok at light load until you get misfire then the economy goes down the toilet.

So I vote for having the throttle shaft parallel to the crank.

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Joshua

So a single carb leaned too much can give poorer mileage as well?

any carburetor improperly tuned or improperly sized for the application, or in poor condition, can give poor mileage.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If you have a 2-stage 2-bbl carb, you'd want the 2 holes perpendicular to the crank............... (which would also have the throttle shafts parallel)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I'm having trouble digesting that. What do you mean by having "the 2 holes perpendicular to the crank." Technically speaking, holes holes cannot be perpendicular but an imaginary line through the centers of the 2 holes can be perpenticular to the crank axis and it is on a "super six" manifold but that imaginary line is also parallel to the throttle shaft which is therefore perpendicular to the crank axis... unless I'm missing something.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

With a single stage 2bbl (BBD), a line running thru the centerline of the 2 holes would be parallel to the throttle shaft.

With a staged 2bbl, a line running thru the centerline of the 2 holes would be perpendicular to the 2 throttle shafts. So the throttle shafts are perpendicular to the imaginary line. The imaginary line is perpendicular to the crank. Thus the throttle shafts would be parallel to crank.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Ahhhh.... now I understand. I was looking at the holes in the manifold and the BBD carb that I have; parallel wasn't computing. I wasn't accurately picturing the layout of a staged 2 bbl such as the Weber 32/36.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

...I might have missed something somewhere along the way, but no one ever seems to take into consideration fuel slosh when discussing the pros and cons of an east-west vs. a north-south carburetor orientation. How it would affect fuel metering during quick accelleration or hard braking likely varies based on carb type (fuel bowl/metering jet layout), but I would think that the BBD would be particularly susceptible to a north-south throttle shaft orientation, as would Carters in general. Then again, I realize the factory mounted 4 barrel carbs (including Carters) in the N-S orientation on the "hyper pack" 6 and crossram V8's, including the Max Wedge and Hemi.

D/W

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If it ain't broke, fix it!

Unless fuel sloshes out and then into the manifold, with a 4bbl carb and single plane manifold, I think it averages out.......... one side goes lean as the other goes richer....... (turns and braking)

With a BBS, the main fuel jet is pretty close to the middle, so turns wouldn't have much of an effect. As the jet is towards the rear, there would be a slight enrichening effect on acceleration (leaning on braking), which in both cases is probably ok.

Not sure how the bowls are arranged on a BBD.........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Oh, heck, that's easy. If your car stalls when you stop short, you've got a Carter BBS or BBD or other carb with a front- or rear-hung float. If your car stalls when you make a hard left or right turn, you've got a Holley 1920 or other carb with a side-hung float.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.